Basic Concepts of Enzyme Action

A patient starts a cholesterol drug and within weeks their muscles ache and a blood marker called CK climbs. The drug is doing exactly what it was designed to do — and that is the problem. To understand why, you need to understand what an enzyme can and cannot do. By the end of this chapter, the muscle pain will make mechanistic sense.

OMP decarboxylase, an enzyme your cells run constantly, speeds its reaction by a factor of about 10¹⁷. Without it, that single step has a half-life of 78 million years — longer than the time since the dinosaurs vanished. With it, the reaction finishes in milliseconds. Enzymes are not gentle nudges. They are the difference between a reaction that effectively never happens and one fast enough to keep you alive.

What an enzyme is, and how we file it

An enzyme is a reusable protein catalyst — with one important exception: some enzymes are RNA (ribozymes), and the ribosome that builds every protein in your body is itself one. Its activity depends on temperature, pH, and often on a helper molecule, and its name usually ends in -ase.

There are seven official classes, sorted by the chemistry they do. Oxidoreductases move electrons. Transferases move a chemical group from one molecule to another. Hydrolases use water to break a bond. Lyases add or remove groups across a double bond. Isomerases rearrange a molecule into its isomer. Ligases join two molecules and spend ATP to do it. Translocases, added to the list in 2018, move ions and molecules across membranes. Each enzyme gets a four-part EC number: hexokinase is EC 2.7.1.1, which reads as transferase, phosphate group, alcohol acceptor, first on the list.

Many enzymes cannot work alone. A cofactor is a small helper required for activity, and it comes in two kinds: a coenzyme is an organic molecule, usually derived from a vitamin (niacin becomes NAD⁺; thiamine becomes TPP), and a metal ion is the inorganic kind (Zn²⁺, Mg²⁺). When a cofactor — organic or metal — binds tightly and permanently rather than coming and going, we call it a prosthetic group; the heme iron buried in catalase is the classic example. The vocabulary that trips students up is the pair underneath this: an enzyme missing its cofactor is an apoenzyme and it does not work; the assembled, working complex is the holoenzyme. Apo means away; holo means whole.

What a reaction will do, before how fast

Now separate two questions that students constantly merge. Will this reaction happen? That is thermodynamics. How fast will it happen? That is kinetics. An enzyme answers only the second, and seeing why requires the first.

The currency of thermodynamics is Gibbs free energy, G — the energy in a system available to do work. What matters for a reaction is the change, ΔG, and its sign tells you the direction. A negative ΔG means the reaction releases free energy and runs on its own: it is exergonic, spontaneous. A positive ΔG means the reaction needs an input of energy to go: it is endergonic. ATP hydrolysis is the textbook exergonic reaction at ΔG°′ = −30.5 kJ/mol; forming glucose-6-phosphate is the textbook endergonic one at +13.8 kJ/mol. When ΔG reaches zero, the reaction sits at equilibrium and goes nowhere net.

One trap worth flagging now: exoTHERMIC and exoGONIC are not the same word. Exothermic is about heat (ΔH); exergonic is about free energy (ΔG). A reaction can release free energy without releasing much heat.

The sign of ΔG also tells you where equilibrium sits, through one of the most useful equations in biochemistry:

ΔG°′ = −RT ln Keq

Keq is just the ratio of products to reactants once the reaction settles. The more negative ΔG°′, the larger Keq, and the further the reaction runs toward products before stopping. Work an example: a reaction with ΔG°′ = −17 kJ/mol has a Keq of about 950 — roughly a thousand-to-one in favor of products. The relationship is logarithmic, so a modest change in free energy moves the equilibrium a lot.

What an enzyme changes — and what it cannot

An enzyme cannot change ΔG, ΔG°, or Keq. The destination of a reaction — whether it favors products, and by how much — is set by thermodynamics, and no catalyst touches it. What an enzyme changes is the route, not the destination: it speeds the reaction’s approach to that same equilibrium.

Think of a mountain between two valleys. The starting valley and the ending valley sit at fixed elevations; that difference is ΔG. The enzyme does not lower either valley. It digs a tunnel through the peak. Travelers move between the same two points far faster, and they still end up exactly where thermodynamics said they would.

The peak the enzyme tunnels through is the activation barrier, and to lower it the enzyme exploits the highest, strangest point on the whole reaction.

The transition state

Between substrate and product, every reaction passes through a transition state (written X‡): a fleeting arrangement of atoms, neither starting material nor product, with bonds half-broken and half-formed. It lives for femtoseconds and sits at the top of the energy hill. The height of that hill above the substrate is the activation energy, ΔG‡, and it sets the rate. Lower the hill and the reaction speeds up.

This is the move enzymes make. The active site — a small cleft built from amino acids pulled together from distant parts of the protein chain — is not shaped to fit the substrate. It is shaped to fit the transition state. When the enzyme grips that high-energy, distorted form most tightly, the energy released by binding (the binding energy) pays down the activation barrier: ΔG‡(catalyzed) = ΔG‡(uncatalyzed) − ΔG(binding).

Emil Fischer guessed at this in 1894 with his “lock and key” — enzyme and substrate fitting like a key in a lock. He was close, and he distrusted his own metaphor almost immediately, warning it could only be confirmed once someone could hold a pure enzyme and see its shape. He was right to worry: a rigid lock would bind the substrate most tightly, and enzymes don’t. In 1958 Daniel Koshland corrected it with induced fit — enzyme and substrate both change shape as they meet, and the complementarity that matters is reached at the transition state. Product fits the reshaped site worse, which is exactly why it lets go.

The evidence: transition-state analogs bind tighter

The cleanest evidence that the active site is built for the transition state comes from a simple comparison. Proline racemase binds its real substrate, L-proline, with a Kd of 3.6 mM. Then chemists made a stable molecule, pyrrole-2-carboxylate, shaped like the planar transition state the enzyme passes through. The enzyme binds that with a Kd of 0.022 mM — 160 times tighter than the substrate it actually works on. The enzyme grips the shape it is built to stabilize.

That principle is also a drug-design strategy, which brings us back to the case we opened with. Statins are transition-state analogs of HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis, and they bind it roughly 10,000 times more tightly than its natural substrate. They shut the pathway down, which lowers cholesterol as intended. However, that same pathway also makes CoQ10 for mitochondrial energy and the isoprenoids muscle cells need. Block it hard enough and muscle mitochondria starve, cells break down, and CK leaks into the blood. The aching muscles were the predictable cost of an enzyme inhibitor doing its job exactly as designed.