Biochemistry

Life is an aqueous phenomenon, and almost every property of a biomolecule is shaped by its relationship to water. A water molecule is bent, with an H-O-H angle near 104.5 degrees, and oxygen is far more electronegative than hydrogen. The result is a strong permanent dipole: the oxygen carries a partial negative charge and each hydrogen a partial positive charge. Because of this, each water molecule can act as both a donor and an acceptor of hydrogen bonds, engaging up to four neighbors in a fluctuating, three-dimensional network. A single hydrogen bond is weak (roughly 10 to 30 kJ/mol, against about 470 kJ/mol for an O-H covalent bond), but the sheer number of them gives water its high boiling point, high heat capacity, and high surface tension.

Water as a solvent

Water dissolves hydrophilic ("water-loving") substances by replacing the interactions those solutes make with each other. Ionic compounds dissolve because water molecules orient their dipoles around each ion, forming a solvation shell that screens the electrostatic attraction between opposite charges. Polar but uncharged molecules such as sugars and alcohols dissolve because they can hydrogen bond with water. Nonpolar molecules cannot, and this failure has profound consequences.

The hydrophobic effect

When a nonpolar ("hydrophobic") molecule is placed in water, the water cannot hydrogen bond with it, so the surrounding water molecules rearrange into a more ordered, cage-like shell to preserve their own hydrogen bonding. This ordering lowers the entropy of the water. When many nonpolar groups cluster together, the total surface exposed to water shrinks, fewer water molecules are forced into the ordered shell, and the entropy of the system rises. The hydrophobic effect is therefore driven largely by the increase in water's entropy, not by any direct attraction between the nonpolar groups. This single principle explains why lipids form membranes, why detergents form micelles, and why proteins bury their nonpolar side chains in a hydrophobic core.

The four weak interactions

Biological structure is held together not by covalent bonds but by large numbers of weak noncovalent interactions that form and break reversibly at body temperature. Four types dominate:

Interaction	Origin	Approx. strength (kJ/mol)
Hydrogen bond	Sharing of an H between two electronegative atoms	10 to 30
Ionic (salt bridge)	Attraction between full opposite charges	Variable; weakened by water
Van der Waals	Transient induced dipoles between close atoms	~1 to 4 each
Hydrophobic effect	Entropy-driven clustering of nonpolar groups	Collective, context-dependent

Individually feeble, these forces gain specificity and strength through numbers and geometry. A protein-ligand complex or a folded protein is stable because dozens of them act at once, yet flexible because any one can yield. Grasping this balance between weak, reversible forces is the key to understanding nearly every structure in this course.

Water is not chemically inert; it undergoes a slight self-ionization, in which one molecule transfers a proton to another: 2 H₂O reversibly gives H₃O⁺ + OH^-. At 25 degrees C the product of the resulting ion concentrations is a constant, the ion product of water:

K_w = [H⁺][OH^-] = 1.0 × 10^-14

In pure water the two concentrations are equal, so each is 1.0 × 10^-7 M. Because these numbers are tiny and span many orders of magnitude, we use a logarithmic scale.

The pH scale

pH is defined as the negative base-ten logarithm of the hydrogen ion concentration:

pH = -log[H⁺] and similarly pOH = -log[OH^-]

Taking the logarithm of K_w gives the essential relationship pH + pOH = 14 at 25 degrees C. Pure water is neutral at pH 7. A solution with pH below 7 is acidic (more H⁺), and above 7 is basic. Because the scale is logarithmic, each pH unit is a tenfold change: a solution at pH 4 has 100 times the H⁺ concentration of one at pH 6.

Weak acids and pKa

By the Bronsted-Lowry definition, an acid donates a proton and a base accepts one. A strong acid such as HCl ionizes essentially completely, so [H⁺] equals the acid concentration. Most acids in biology are weak acids that ionize only partially, described by an equilibrium constant, the acid dissociation constant K_a. For a generic acid HA reversibly giving H⁺ + A^-:

K_a = [H⁺][A^-] ÷ [HA], and pK_a = -log K_a

A smaller pK_a means a stronger acid (more dissociation). The conjugate pair HA and A^- are inseparable partners: a strong acid has a weak conjugate base, and vice versa.

Worked example

Acetic acid has K_a = 1.8 × 10^-5 (pK_a = 4.76). Estimate the pH of a 0.10 M solution. Let x = [H⁺] that forms. Then K_a = x² ÷ (0.10 - x) ≈ x² ÷ 0.10. So x² = 1.8 × 10^-6, giving x = 1.34 × 10^-3 M. Therefore pH = -log(1.34 × 10^-3) = 2.87. Contrast this with 0.10 M HCl, a strong acid, where [H⁺] = 0.10 M and pH = 1.00. The weak acid is far less acidic at the same concentration because most of it stays protonated.

Cells run thousands of reactions whose rates and equilibria depend on pH, and many produce or consume protons. To keep pH nearly constant, biological fluids contain buffers: mixtures of a weak acid and its conjugate base that resist changes in pH when acid or base is added. A buffer works because it contains a reservoir of both a proton donor (HA) and a proton acceptor (A^-). Added acid is soaked up by A^- (forming HA), and added base is neutralized by HA (forming A^-), so the free [H⁺] barely moves.

Deriving the Henderson-Hasselbalch equation

Start from the acid equilibrium, K_a = [H⁺][A^-] ÷ [HA]. Solve for [H⁺], take the negative logarithm of both sides, and rearrange to obtain the Henderson-Hasselbalch equation:

pH = pK_a + log([A^-] ÷ [HA])

This compact equation is the workhorse of buffer chemistry. Reading it teaches three lessons at once:

• When [A^-] = [HA], the log term is log(1) = 0, so pH = pK_a. A buffer is centered on its pKa.
• Buffering is strongest near the pKa, roughly within one pH unit on either side (a ratio between 1:10 and 10:1), because there both partners are abundant.
• The ratio, not the absolute amount, sets the pH; but the total concentration sets the capacity to absorb added acid or base.

Worked example

You mix 0.20 M acetate (A^-) with 0.10 M acetic acid (HA); acetic acid has pK_a = 4.76. The pH is pH = 4.76 + log(0.20 ÷ 0.10) = 4.76 + log(2.0) = 4.76 + 0.30 = 5.06. If you instead wanted a buffer at exactly pH 4.76, you would use equal concentrations of the two forms.

The bicarbonate buffer

Human blood is held near pH 7.4 mainly by the bicarbonate buffer system, in which dissolved CO₂ forms carbonic acid that dissociates to bicarbonate: CO₂ + H₂O reversibly gives H₂CO₃ reversibly gives H⁺ + HCO₃^-. Its effective pK_a is about 6.1, which seems poorly matched to pH 7.4. What makes it work superbly anyway is that it is an open system: the lungs continuously exhale CO₂ and the kidneys adjust bicarbonate, so the body actively controls both members of the pair. This physiological control lets a buffer operate effectively more than a full pH unit away from its pKa, something a closed buffer in a test tube cannot do.

Proteins are polymers of amino acids, and their astonishing range of functions all traces back to the chemistry of these twenty monomers. Each standard amino acid shares a common core: a central alpha carbon bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group). The side chain is what distinguishes one amino acid from another and gives each its personality.

Stereochemistry and the zwitterion

Except for glycine (whose R group is just H), the alpha carbon has four different groups and is therefore chiral. Proteins are built almost exclusively from the L-stereoisomer. At physiological pH near 7.4, the amino group is protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO^-), so a free amino acid exists as a zwitterion, a molecule bearing both a positive and a negative charge while remaining neutral overall. The pH at which the net charge is zero is the isoelectric point (pI).

Classifying the side chains

Biochemists group the twenty amino acids by the chemical character of the R group, because that character predicts where a residue will sit in a folded protein and how it will function.

Class	Examples	Typical location / role
Nonpolar / hydrophobic	Ala, Val, Leu, Ile, Phe, Met	Buried in the core; van der Waals packing
Polar, uncharged	Ser, Thr, Asn, Gln, Cys, Tyr	Surface or active site; hydrogen bonding
Acidic (negative)	Asp, Glu	Surface; salt bridges, metal binding
Basic (positive)	Lys, Arg, His	Surface; salt bridges, catalysis

Several residues deserve special note. Glycine is the smallest and adds flexibility. Proline, whose side chain loops back to the backbone nitrogen, is rigid and disrupts regular structures. Cysteine can form covalent disulfide bonds that staple parts of a protein together. Histidine, with a pKa near 6, can gain or lose a proton close to physiological pH, which makes it a favorite participant in enzyme catalysis.

The peptide bond

Amino acids link when the carboxyl group of one condenses with the amino group of the next, releasing water and forming a peptide bond (an amide linkage). A chain of these is a polypeptide, with a free amino group at one end (the N-terminus) and a free carboxyl at the other (the C-terminus); by convention the sequence is written N to C. The peptide bond has partial double-bond character due to resonance, so it is planar and cannot rotate freely. Rotation is instead permitted around the two single bonds flanking each alpha carbon, described by the dihedral angles phi and psi. The allowed combinations of phi and psi, mapped on a Ramachandran plot, constrain which secondary structures a backbone can adopt.

A protein's function is inseparable from its three-dimensional shape, and biochemists describe that shape at four hierarchical levels. Each level is built from, and constrained by, the one below it.

Primary structure

The primary structure is the linear sequence of amino acids joined by peptide bonds, read from the N-terminus to the C-terminus. It is specified by the gene and, in a famous principle established by Anfinsen, it contains all the information needed to determine the higher levels of structure. Even a single substitution can be decisive: in sickle-cell hemoglobin, changing one glutamate to a valine (a hydrophobic residue) on the protein surface causes the molecules to aggregate.

Secondary structure

Secondary structure refers to local, regularly repeating folds of the backbone, stabilized entirely by hydrogen bonds between backbone atoms (the carbonyl oxygen of one residue and the amide hydrogen of another). Two motifs dominate:

• The alpha helix is a right-handed coil in which each backbone C=O hydrogen bonds to the N-H four residues ahead (an i to i+4 pattern). There are about 3.6 residues per turn, and the side chains point outward. It is compact and common.
• The beta sheet is formed from extended strands lying side by side, hydrogen bonding to their neighbors. Strands running the same direction form a parallel sheet; opposite directions form an antiparallel sheet. Sheets give a pleated, extended surface.

Turns and loops connect these elements and often contain glycine and proline.

Tertiary structure

Tertiary structure is the full three-dimensional fold of a single polypeptide, the way its helices, sheets, and loops pack together. Unlike secondary structure, it is stabilized mainly by interactions between the side chains: hydrophobic packing in the core (the largest contributor for water-soluble proteins), hydrogen bonds, ionic salt bridges, and covalent disulfide bonds. The overall shape often defines the protein's class, such as a compact globular enzyme or an elongated fibrous structural protein.

Quaternary structure

Many proteins are assembled from more than one polypeptide chain, or subunit. Quaternary structure describes how these subunits associate, using the same weak interactions that stabilize tertiary structure. Hemoglobin, with four subunits (two alpha and two beta), is the classic example: the interaction among subunits lets the binding of oxygen at one site raise the affinity at the others, a cooperative behavior impossible for a single chain. The table summarizes the hierarchy.

Level	What it is	Stabilized by
Primary	Amino acid sequence	Peptide (covalent) bonds
Secondary	Local helices and sheets	Backbone hydrogen bonds
Tertiary	Full 3D fold of one chain	Side-chain interactions, hydrophobic core
Quaternary	Assembly of multiple subunits	Inter-subunit weak interactions

How does a floppy chain of amino acids find its precise, functional shape among an astronomical number of possibilities? Levinthal's paradox makes the puzzle vivid: if a protein sampled every possible conformation randomly, folding would take longer than the age of the universe, yet real proteins fold in microseconds to seconds. The resolution is that folding is not a random search but a guided, downhill process along a folding funnel, in which partially correct structures form quickly and channel the chain toward the native state.

The thermodynamics of folding

The native state is generally the conformation of lowest Gibbs free energy under physiological conditions. The overall stability is set by a competition, and it is worth appreciating how finely balanced it is:

• Folding reduces conformational entropy of the chain (unfavorable), because the ordered native state has far fewer arrangements than the random coil.
• Folding is favored by the hydrophobic effect (burying nonpolar side chains releases ordered water, raising the entropy of the solvent), plus favorable hydrogen bonds, van der Waals contacts, and salt bridges.

These large opposing terms nearly cancel. The net stability of a typical globular protein is only about 20 to 60 kJ/mol, equivalent to a handful of hydrogen bonds. Proteins are therefore only marginally stable, which is biologically useful: it lets them be flexible, regulated, and eventually degraded, but it also means modest stresses can unfold them.

Denaturation

Denaturation is the loss of native structure (and function) without breaking peptide bonds. It can be caused by heat, extremes of pH, or chemical denaturants such as urea, which disrupt the weak interactions. Because Anfinsen showed that many denatured proteins can spontaneously refold when normal conditions are restored, the native fold is encoded in the sequence itself.

Chaperones and misfolding disease

In the crowded cell, nascent chains can misfold or aggregate before they finish folding. Molecular chaperones (such as the Hsp70 and chaperonin families) are proteins that bind exposed hydrophobic patches and give polypeptides a protected environment and repeated chances to fold correctly; they do not dictate the final structure. When folding quality control fails, misfolded proteins can accumulate as insoluble aggregates. This underlies a group of protein-misfolding diseases, including Alzheimer disease and Parkinson disease, and the prion disorders, in which a misfolded protein can even template the misfolding of its normal counterparts. Understanding folding is thus not only a structural question but a medical one.

Nearly every reaction in a cell is catalyzed by an enzyme, a biological catalyst (usually a protein, occasionally an RNA) that speeds a reaction by many orders of magnitude while remaining unchanged at the end. Rate enhancements of 10⁶ to 10¹⁷ are common. Without enzymes, the reactions of metabolism would run far too slowly to sustain life.

Activation energy and the transition state

Every reaction must pass through a high-energy, unstable arrangement called the transition state. The energy barrier from the reactants up to that peak is the activation energy (often written as the free energy of activation). A catalyst works by providing an alternative pathway with a lower activation energy, so a much larger fraction of molecules have enough energy to react. It is essential to see what an enzyme does not do:

• It does not change the reaction's equilibrium or the free-energy difference between reactants and products. It only changes how fast equilibrium is reached, speeding forward and reverse reactions equally.
• It is not consumed; one enzyme molecule turns over many substrate molecules.

The deepest idea in enzymology is that enzymes work chiefly by binding and stabilizing the transition state more tightly than they bind the substrate. Lowering the energy of that peak is what lowers the barrier.

The active site

Catalysis happens in the active site, a small pocket formed by residues brought together by the protein's fold. The molecule that binds and reacts is the substrate. Early models pictured a rigid "lock and key" fit, but the modern induced-fit model recognizes that binding often causes the enzyme and substrate to change shape, closing around the substrate and precisely aligning catalytic groups. The active site provides specificity, so an enzyme selects its substrate from the thousands of molecules in the cell.

Catalytic strategies and cofactors

Enzymes combine several chemical tricks: acid-base catalysis (donating or accepting protons, often via histidine), covalent catalysis (forming a transient covalent bond to the substrate), metal-ion catalysis, and proximity and orientation effects that place reacting groups in exactly the right position. Many enzymes also require a nonprotein helper. A cofactor may be a metal ion (such as Zn²⁺ or Mg²⁺) or a small organic molecule called a coenzyme (such as NAD⁺ or coenzyme A, many derived from vitamins). The protein without its cofactor is an apoenzyme; the complete, active form is the holoenzyme.

Enzyme kinetics is the quantitative study of reaction rates, and it gives us the tools to describe how efficient an enzyme is and how it responds to substrate. Consider the simplest scheme, in which enzyme E binds substrate S to form a complex ES, which then releases product P:

E + S reversibly gives ES, then ES gives E + P

The Michaelis-Menten equation

When we measure the initial velocity (the rate at the very start, before product builds up) at different substrate concentrations, the data trace a hyperbola: velocity rises steeply at low [S] and then levels off, approaching a maximum as the enzyme becomes saturated. This behavior is captured by the Michaelis-Menten equation:

v₀ = (V_max [S]) ÷ (K_m + [S])

It is derived using the steady-state assumption (the concentration of ES stays roughly constant during the measurement, because it forms and breaks down at equal rates). Two parameters define the curve:

• V_max is the maximum velocity, reached when essentially all enzyme is bound as ES (fully saturated).
• K_m, the Michaelis constant, is the substrate concentration at which the velocity is exactly half of V_max. You can confirm this by setting [S] = K_m in the equation: v₀ = V_maxK_m ÷ (2K_m) = V_max/2.

A low K_m means the enzyme reaches half-maximal speed at low substrate, which usually indicates a high apparent affinity for the substrate.

kcat and catalytic efficiency

Dividing V_max by the total enzyme concentration gives the turnover number, k_cat: the number of substrate molecules one enzyme converts to product per second when saturated. To compare how good enzymes are under realistic (unsaturated) conditions, we use the specificity constant, k_cat/K_m. This ratio measures catalytic efficiency; its upper limit is set by how fast enzyme and substrate can diffuse together (about 10⁸ to 10⁹ per molar per second). Enzymes that reach this limit are called catalytically perfect.

Worked example

An enzyme has V_max = 100 µmol/(L·s) and K_m = 2.0 mM. What is v₀ at [S] = 2.0 mM, and at [S] = 6.0 mM? At [S] = K_m = 2.0 mM, v₀ = V_max/2 = 50 µmol/(L·s). At [S] = 6.0 mM, v₀ = 100 × 6.0 ÷ (2.0 + 6.0) = 600 ÷ 8.0 = 75 µmol/(L·s), three-quarters of V_max.

Cells must not only run reactions but control them, and drugs and toxins often act on enzymes. Much of this control works through inhibition, a decrease in enzyme activity caused by a molecule that interferes with catalysis. Reversible inhibitors bind noncovalently and come in three classic types, distinguished by what they bind and by their effect on the kinetic parameters K_m and V_max.

Competitive inhibition

A competitive inhibitor resembles the substrate and binds the free enzyme at the active site, so substrate and inhibitor compete. Because a high enough substrate concentration can outcompete the inhibitor, V_max is unchanged, but the apparent K_m increases (more substrate is needed to reach half-maximal velocity). Many drugs are competitive inhibitors; for example, statins competitively inhibit the enzyme HMG-CoA reductase.

Uncompetitive and noncompetitive inhibition

An uncompetitive inhibitor binds only to the enzyme-substrate complex (ES), not to free enzyme. It lowers both V_max and K_m by the same factor, because trapping ES effectively pulls substrate binding forward. A (pure) noncompetitive inhibitor binds at a site away from the active site, on either E or ES, and reduces the enzyme's turnover without blocking substrate binding; it lowers V_max while leaving K_m unchanged, and cannot be overcome by adding substrate. The table summarizes the diagnostic effects.

Inhibitor type	Binds to	Apparent Km	Vmax
Competitive	Free enzyme (active site)	Increases	Unchanged
Uncompetitive	ES complex only	Decreases	Decreases
Noncompetitive (pure)	E and ES (other site)	Unchanged	Decreases

An irreversible inhibitor forms a stable, often covalent, bond and permanently disables the enzyme; the antibiotic penicillin and the effect of nerve agents on acetylcholinesterase are examples.

Allosteric regulation and feedback

Beyond simple inhibition, key enzymes are tuned by allosteric regulation: a regulatory molecule (an effector) binds at a site distinct from the active site and shifts the enzyme between more-active and less-active conformations. Allosteric enzymes are typically multi-subunit proteins and show a sigmoidal (S-shaped) rather than hyperbolic velocity curve, reflecting cooperative subunit transitions. A central control motif is feedback inhibition, in which the end product of a metabolic pathway allosterically inhibits an enzyme early in that same pathway. This lets the cell stop making a product once it has enough, an elegant and economical form of self-regulation we will see again in the metabolism module. Enzymes can also be regulated covalently, most commonly by reversible phosphorylation, and by activation of inactive precursors called zymogens.

Carbohydrates are the most abundant biomolecules on Earth. Chemically they are polyhydroxy aldehydes or ketones with the approximate empirical formula (CH₂O)_n, which is where the older name "hydrate of carbon" comes from. They serve as fuels, as carbon stores, and as structural materials, and they decorate proteins and lipids as recognition tags.

Monosaccharides

The simplest carbohydrates are monosaccharides (single sugars). They are classified by their number of carbons (a six-carbon sugar is a hexose, a five-carbon sugar a pentose) and by whether they carry an aldehyde group (an aldose) or a ketone group (a ketose). Glucose, the central fuel of metabolism, is an aldohexose. In water, five- and six-carbon sugars do not stay open-chain; they cyclize as the carbonyl reacts with a distant hydroxyl to form a ring. Ring closure creates a new stereocenter at the former carbonyl carbon, the anomeric carbon, giving two forms called alpha and beta anomers that differ only in the orientation of one hydroxyl. This seemingly small distinction has large consequences downstream.

Glycosidic bonds and disaccharides

Two monosaccharides join when a hydroxyl of one reacts with the anomeric carbon of the other, releasing water and forming a glycosidic bond. The result is a disaccharide. Three are worth knowing:

Disaccharide	Components	Linkage
Maltose	Glucose + glucose	alpha-1,4
Sucrose (table sugar)	Glucose + fructose	alpha-1,beta-2
Lactose (milk sugar)	Galactose + glucose	beta-1,4

Polysaccharides

Long chains of monosaccharides are polysaccharides, and the alpha versus beta distinction determines their function. Starch (in plants) and glycogen (in animals) are storage forms of glucose linked by alpha-1,4 bonds with alpha-1,6 branch points; the alpha linkage forms coiled, easily digested chains, and glycogen's heavy branching allows rapid mobilization. Cellulose, the structural material of plant cell walls and the most abundant organic polymer on the planet, is also a glucose polymer, but its beta-1,4 linkages produce straight chains that pack into rigid, hydrogen-bonded fibers. Humans have enzymes to cleave alpha but not beta-1,4 bonds, which is exactly why we can digest starch but not cellulose (dietary fiber). The same monomer, arranged in two geometries, becomes either food or wood.

Lipids are defined not by a common structure but by a shared property: they are largely nonpolar and therefore poorly soluble in water. This diverse group includes energy-storage molecules, the building blocks of membranes, and signaling molecules such as steroid hormones.

Fatty acids and triacylglycerols

A fatty acid is a long hydrocarbon chain ending in a carboxyl group. If the chain has no carbon-carbon double bonds it is saturated (straight, packs tightly, tends to be solid, as in animal fat); if it has one or more double bonds it is unsaturated, and each cis double bond introduces a kink that prevents tight packing, lowering the melting point (as in plant oils). The main storage lipids are triacylglycerols (triglycerides), three fatty acids esterified to a glycerol backbone. Because they are highly reduced and nearly anhydrous, fats store more than twice the energy per gram of carbohydrate, which is why they are the body's long-term fuel reserve.

Phospholipids are amphipathic

The lipids that build membranes are phospholipids. A typical glycerophospholipid has a glycerol backbone carrying two fatty-acid tails and, in place of the third, a phosphate group linked to a polar head group. The result is an amphipathic molecule: a hydrophilic phosphate head and two hydrophobic tails. When placed in water, phospholipids spontaneously arrange so that heads face the water and tails hide from it. At the concentrations found in cells they form a lipid bilayer, two sheets of phospholipids tail-to-tail, with the polar heads on both outer surfaces and the tails sequestered in the interior. This self-assembly is driven by the hydrophobic effect from Module 1; no covalent bonds hold the sheet together, yet it is remarkably stable and self-sealing.

The fluid mosaic model

The accepted picture of biological membranes is the fluid mosaic model. "Fluid" captures that the bilayer is not rigid: individual lipids diffuse rapidly within their own leaflet, and the membrane behaves like a two-dimensional liquid. Its fluidity is tuned by fatty-acid saturation (more unsaturation, more fluid) and, in animals, by cholesterol, which buffers fluidity across temperatures. "Mosaic" captures that proteins are embedded in and attached to the bilayer. Integral membrane proteins span the bilayer (often as hydrophobic alpha helices) and act as transporters, channels, and receptors; peripheral proteins associate loosely with a surface. The bilayer is selectively permeable: small nonpolar molecules cross freely, but ions and large polar molecules require specific transport proteins. This combination of a stable hydrophobic barrier with selective protein gateways is the physical basis of the cell's boundary and of every membrane-enclosed compartment inside it.

Nucleic acids store and transmit genetic information. There are two kinds: DNA (deoxyribonucleic acid), the stable archive of the genome, and RNA (ribonucleic acid), the working copy and, in some cases, a catalyst. Both are polymers of nucleotides.

Nucleotides and the backbone

A nucleotide has three parts: a five-carbon sugar (deoxyribose in DNA, ribose in RNA), one or more phosphate groups, and a nitrogenous base. The bases fall into two chemical families: the double-ring purines, adenine (A) and guanine (G), and the single-ring pyrimidines, cytosine (C), thymine (T, in DNA), and uracil (U, in RNA which uses U in place of T). Nucleotides link when the phosphate on the 5-prime carbon of one sugar bonds to the 3-prime hydroxyl of the next, forming a phosphodiester bond. This creates a sugar-phosphate backbone with a defined direction, running 5-prime to 3-prime. The backbone is negatively charged because of its phosphates, which is why DNA migrates toward the positive electrode in gel electrophoresis.

The double helix and base pairing

In 1953 Watson and Crick, building on Rosalind Franklin's X-ray diffraction data, proposed that DNA is a double helix: two strands wound around a common axis. The strands are antiparallel (one runs 5-prime to 3-prime, the other 3-prime to 5-prime), with the sugar-phosphate backbones on the outside and the bases paired in the interior. The pairing is exquisitely specific, following the rules of complementary base pairing: A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds). A purine always pairs with a pyrimidine, keeping the helix a uniform width. This complementarity is the molecular key to heredity: each strand carries the full information to specify the other, so the molecule can be copied faithfully (semiconservative replication).

The central dogma

The flow of genetic information is summarized by the central dogma of molecular biology:

DNA reversibly gives (replication) DNA, then DNA gives (transcription) RNA, then RNA gives (translation) protein

In transcription, the base sequence of a gene is copied into a messenger RNA. In translation, ribosomes read that mRNA three bases at a time; each three-base codon specifies one amino acid according to the genetic code, and the growing chain folds into a functional protein. Thus the sequence information stored in DNA is ultimately expressed as the amino acid sequence that, as we saw in Module 2, dictates a protein's structure and function. Nucleic acids and proteins are the two great informational polymers of life, and the code connects them.

Bioenergetics is the study of energy flow in living systems. All of metabolism obeys thermodynamics, and the master variable is the Gibbs free energy change, delta G. A reaction proceeds spontaneously (releases usable energy) when delta G is negative (exergonic); it requires an input of energy when delta G is positive (endergonic); and it is at equilibrium when delta G is zero. Crucially, delta G depends on actual concentrations in the cell, not only on the standard-state value, so cells can drive reactions by keeping products low and reactants high.

ATP: the energy currency

The central molecule of cellular energy exchange is adenosine triphosphate (ATP), a nucleotide with a chain of three phosphate groups. Hydrolysis of its terminal phosphate to form ADP and inorganic phosphate (P_i) is strongly exergonic, with a standard free-energy change (delta G-prime-naught) of about -30.5 kJ/mol, and even more negative under real cellular conditions. Several factors make this favorable: the three phosphates carry closely spaced negative charges whose electrostatic repulsion is relieved on hydrolysis, and the products ADP and P_i are stabilized by resonance and solvation. ATP is sometimes loosely called "high-energy," but the useful idea is that its hydrolysis has a large negative delta G that can be harnessed.

Energetic coupling

The reason ATP matters is energetic coupling: the cell links an unfavorable (endergonic) reaction to the favorable hydrolysis of ATP so that the sum of the two has a negative delta G and proceeds. In practice this rarely means simple hydrolysis to waste heat; instead the enzyme transfers a phosphate group from ATP onto a substrate or intermediate, raising that molecule's energy and making the next step favorable. In this way ATP acts as the cell's rechargeable energy currency: it is spent (to ADP + P_i) to power biosynthesis, transport, and motion, and it is regenerated by the catabolic pathways in the rest of this module.

Electron carriers

Extracting energy from fuels is fundamentally a matter of oxidation (loss of electrons), and those electrons must be carried to where they can be used. Two coenzymes do most of this work. NAD⁺ (nicotinamide adenine dinucleotide) accepts two electrons and one proton to become NADH; FAD (flavin adenine dinucleotide) accepts two electrons and two protons to become FADH₂. Both are reduced when they pick up electrons from a fuel molecule and are then reoxidized when they deliver those electrons to the electron transport chain. NADH and FADH₂ are therefore the mobile carriers that connect the oxidation of glucose and fat to the synthesis of most of the cell's ATP. Keep these carriers in view; the payoff of glycolysis and the citric acid cycle is measured largely in the NADH and FADH₂ they produce.

Glycolysis ("sugar splitting") is the central pathway that begins the breakdown of glucose. It is ancient and nearly universal, occurring in the cytosol without requiring oxygen. Over ten enzyme-catalyzed steps, one six-carbon glucose is converted into two three-carbon molecules of pyruvate, and some of the released energy is captured as ATP and NADH.

The two phases

Glycolysis divides neatly into two halves:

• Energy investment phase (steps 1 to 5). The cell first spends energy to prime the sugar. Two ATP are consumed: one to phosphorylate glucose (by hexokinase) and one at the committed, rate-limiting step catalyzed by phosphofructokinase-1 (PFK-1). The six-carbon sugar is then cleaved into two interconvertible three-carbon molecules (glyceraldehyde-3-phosphate).
• Energy payoff phase (steps 6 to 10). Each three-carbon molecule is oxidized and rearranged. Here the cell harvests energy: it reduces NAD⁺ to NADH and generates ATP by substrate-level phosphorylation (direct transfer of a phosphate from a high-energy intermediate to ADP, not requiring the electron transport chain).

The balance sheet

Because the six-carbon sugar is split into two three-carbon units, the payoff-phase events happen twice per glucose. Tallying per glucose:

Item	Gross	Net per glucose
ATP invested	2 used	-2
ATP produced	4 made (2 per triose)	+4
Net ATP		+2
NADH produced		2
Pyruvate produced		2

So the net yield of glycolysis is 2 ATP, 2 NADH, and 2 pyruvate per glucose. This is a modest ATP return, but it is fast and needs no oxygen.

The fate of pyruvate

What happens next depends on oxygen. There is one problem to solve first: glycolysis consumed NAD⁺, and the cell has only a limited supply, so NAD⁺ must be regenerated from NADH or glycolysis will stall.

• Aerobic conditions. When oxygen is present, pyruvate enters the mitochondrion and is oxidized to acetyl-CoA (releasing CO₂ and one NADH per pyruvate), feeding the citric acid cycle. NADH is reoxidized later by the electron transport chain.
• Anaerobic conditions (fermentation). Without oxygen, the cell regenerates NAD⁺ by reducing pyruvate. In muscle and many bacteria, pyruvate is reduced to lactate (lactic acid fermentation); in yeast, it is converted to ethanol and CO₂ (alcoholic fermentation). Fermentation makes no additional ATP, but by restoring NAD⁺ it lets glycolysis, and its 2 ATP per glucose, keep running.

The citric acid cycle (also called the Krebs cycle or tricarboxylic acid cycle) is the hub of aerobic metabolism. Located in the mitochondrial matrix, it completes the oxidation of fuel-derived carbon to CO₂ and, in doing so, harvests high-energy electrons as NADH and FADH₂. It is important to see that the cycle itself makes very little ATP directly; its real output is reduced electron carriers that feed the next stage.

Entry and the cyclic reactions

The fuel that enters the cycle is acetyl-CoA, the two-carbon unit produced from pyruvate (and also from fatty acids and some amino acids). In the first step, its acetyl group condenses with the four-carbon oxaloacetate to form the six-carbon citrate, which gives the cycle its name. Over the following steps, citrate is progressively oxidized and two carbons are released as CO₂, regenerating oxaloacetate so the cycle can turn again. Because oxaloacetate is remade each turn, a single molecule of it can process many acetyl groups; the cycle is catalytic in its intermediates.

Products per turn

Each single turn of the cycle (one acetyl-CoA) yields, per turn:

• 3 NADH (from three oxidation steps that reduce NAD⁺)
• 1 FADH₂ (from one oxidation step that reduces FAD)
• 1 GTP (or ATP, by substrate-level phosphorylation)
• 2 CO₂ released

Since one glucose gives two pyruvate and therefore two acetyl-CoA, the cycle turns twice per glucose, doubling these numbers: 6 NADH, 2 FADH₂, and 2 GTP per glucose from the cycle proper. (The oxidation of pyruvate to acetyl-CoA that precedes the cycle contributes an additional 2 NADH per glucose.) The overwhelming majority of the energy released here is stored not as GTP but as the reducing power of NADH and FADH₂.

An amphibolic hub

The citric acid cycle is not only catabolic (breaking fuel down); it is amphibolic, meaning it serves both breakdown and biosynthesis. Its intermediates are withdrawn as precursors for building amino acids, heme, and glucose, among others. When intermediates are drained for biosynthesis, they must be replenished by anaplerotic ("filling up") reactions, such as the carboxylation of pyruvate to oxaloacetate, so the cycle does not run dry. This dual role places the cycle at the very center of the cell's metabolic map: nearly every major pathway either feeds into it or draws from it.

Glycolysis and the citric acid cycle capture only a small amount of ATP directly; their true harvest is the electrons stored in NADH and FADH₂. Oxidative phosphorylation is where that stored reducing power is finally converted into the bulk of the cell's ATP. It has two linked parts: the electron transport chain, which releases the energy of the electrons, and chemiosmotic ATP synthesis, which uses that energy to make ATP. Both occur at the inner mitochondrial membrane.

The electron transport chain

The electron transport chain (ETC) is a series of protein complexes (Complexes I through IV) embedded in the inner membrane. NADH donates its electrons to Complex I, and FADH₂ donates its electrons to Complex II; from there the electrons pass through mobile carriers (ubiquinone and cytochrome c) and down the chain. The electrons flow "downhill" from carriers of higher energy to those of lower energy, and at the end they are handed to the final electron acceptor, oxygen, which combines with electrons and protons to form water. This is why we breathe: O₂ is the terminal sink for the electrons stripped from our food. As electrons move through Complexes I, III, and IV, the energy released is used to pump protons (H⁺) out of the matrix into the intermembrane space.

Chemiosmosis

Proton pumping builds up a higher H⁺ concentration (and positive charge) outside the matrix, creating an electrochemical gradient called the proton-motive force. This stored energy is released when protons are allowed to flow back into the matrix through a remarkable enzyme, ATP synthase. As protons pass through it, part of the enzyme physically rotates, and this mechanical motion drives the synthesis of ATP from ADP and P_i. Peter Mitchell's chemiosmotic theory, the insight that a proton gradient couples electron transport to ATP synthesis, is one of the great unifying ideas of biochemistry. Note the elegant logic: electron transport and ATP synthesis are not directly connected chemically; they are coupled only through the shared proton gradient. This also explains uncouplers, molecules that let protons leak back across the membrane, collapsing the gradient so that electrons still flow and oxygen is still consumed, but little ATP is made and the energy appears as heat.

The grand total

Now we can tally the complete aerobic oxidation of one glucose. Each NADH that enters at Complex I yields about 2.5 ATP, and each FADH₂ (entering at Complex II) yields about 1.5 ATP, using modern measured ratios rather than the older whole-number estimates. Combining every stage gives an approximate total of about 30 to 32 ATP per glucose, the great majority of it from oxidative phosphorylation. (Older textbooks cite 36 to 38, using rounded ratios; the modern lower figures reflect the true, non-integer proton stoichiometry and the cost of transporting ATP and cytosolic NADH.) Whatever the exact number, the lesson is decisive: aerobic respiration extracts roughly fifteen times more ATP from glucose than glycolysis alone, which is why oxygen-using life can be so energetic.

A cell runs hundreds of reactions at once, many of them opposed: it must not build glucose and break it down simultaneously, or it would waste energy in a futile cycle. This final lesson steps back to see how the pathways of the course are governed as an integrated whole. The guiding principle is that metabolism is regulated to match supply and demand, turning catabolism up when energy is needed and biosynthesis up when building blocks and fuel are plentiful.

Mechanisms of control

Cells regulate metabolism on several timescales and by several means:

• Allosteric regulation. As in Module 3, key regulatory enzymes are switched by effectors that signal the cell's energy state. The classic sensors are the adenine nucleotides: a high ratio of ATP to AMP signals abundant energy and slows catabolism, whereas rising AMP signals energy shortage and speeds it. PFK-1 in glycolysis is a textbook case, inhibited by ATP and activated by AMP.
• Covalent modification. Reversible phosphorylation of enzymes, controlled by hormones, switches pathways on or off within minutes.
• Changes in enzyme amount. Over hours to days, cells adjust how much of an enzyme they make (gene expression), a slower but larger form of control.

Reciprocal regulation

Opposing pathways are usually under reciprocal regulation: the same signal that activates one direction inhibits the other. When glycolysis is switched on, gluconeogenesis (glucose synthesis) is switched off, and vice versa. This is achieved by regulating the enzymes at the steps unique to each direction, so the cell commits to one net flux and avoids a futile cycle. Feedback inhibition by end products, introduced earlier, is another expression of the same economy.

Hormonal integration and fuel logic

In a multicellular animal, metabolism is coordinated across organs by hormones. After a meal (the fed state), high blood glucose triggers insulin, which promotes storage: cells take up glucose, build glycogen, and synthesize fat. During fasting (the fasted state), glucagon (and adrenaline) signals fuel mobilization: the liver breaks down glycogen and makes new glucose, and fat is released and oxidized to spare glucose for the brain. The body's fuels are handled with a clear priority. Excess carbohydrate is stored first as glycogen (a fast but limited reserve) and then converted to fat (a large, energy-dense, long-term reserve). Between meals and during exertion these stores are drawn down in reverse. Seen this way, the individual pathways of this course, glycolysis, the citric acid cycle, oxidative phosphorylation, and their biosynthetic counterparts, are not isolated diagrams but the interconnected machinery of a single, tightly regulated economy that keeps the cell supplied with energy under every condition.