Biochemistry
Enzymes: function, cofactors and inhibition
Almost every reaction keeping you alive would be far too slow at body temperature without a catalyst to speed it up.
What enzymes actually do
Enzymes are biological catalysts, nearly all of them proteins. They work by lowering the activation energy of a reaction — the energy barrier reactants must overcome. Critically, they do not change the reaction's equilibrium position or its overall free energy change; they only change how fast equilibrium is reached.
Lock-and-key vs. induced fit
The older lock-and-key model pictured a rigid active site precisely complementary to the substrate. The accepted induced-fit model holds that the active site is flexible and changes shape slightly on substrate binding, tightening around it and straining the bonds to be broken.
Cofactors and coenzymes
Many enzymes need a non-protein helper to function:
- Cofactors — usually inorganic ions such as Mg2+, Zn2+, or Fe2+. See magnesium and zinc.
- Coenzymes — organic molecules, very often derived from B vitamins: NAD+ from niacin (B3), FAD from riboflavin (B2), coenzyme A from pantothenic acid (B5).
That link is the reason B-vitamin deficiencies produce such broad, systemic symptoms — they disable enzymes across many pathways at once, rather than one specific function.
Inhibition
| Type | Binds to | Effect on Vmax | Effect on Km |
|---|---|---|---|
| Competitive | Active site | Unchanged | Increases |
| Non-competitive | Allosteric site | Decreases | Unchanged |
The logic: a competitive inhibitor can be outcompeted by adding more substrate, so maximum velocity is still reachable — it just takes more substrate, which reads as a higher Km. A non-competitive inhibitor changes the enzyme's shape regardless of substrate concentration, so Vmax falls and no amount of extra substrate restores it. See enzyme kinetics for the graphs.
What denatures enzymes
Temperature and pH extremes disrupt the hydrogen bonds and ionic interactions holding the tertiary structure together. Once the active site's shape is lost, the enzyme cannot bind substrate — and above a certain point, denaturation is irreversible.
Frequently asked questions
How do enzymes speed up reactions?
They lower the activation energy required for a reaction to proceed. They do not change the equilibrium position or the overall free energy change, only the rate at which equilibrium is reached.
What is the difference between a cofactor and a coenzyme?
Cofactors are typically inorganic ions such as magnesium, zinc, or iron. Coenzymes are organic molecules, often derived from B vitamins, such as NAD+ from niacin and FAD from riboflavin.
How do competitive and non-competitive inhibitors differ?
A competitive inhibitor binds the active site, raising Km while leaving Vmax unchanged, and can be overcome with more substrate. A non-competitive inhibitor binds an allosteric site, lowering Vmax while leaving Km unchanged, and cannot be outcompeted.
Keep reading: Enzyme kinetics explained · Levels of protein structure · B vitamins and metabolism