Enzymes
Biological catalysts that make life possible — from digestion to DNA replication. How enzymes work, what controls their speed, and what happens when they are inhibited.
Primary sources: NCBI Bookshelf: Enzyme Catalysis · Khan Academy AP Biology
01 · Mechanism
Enzymes lower the activation energy.
Every chemical reaction requires an input of energy to proceed — the activation energy (Ea). Enzymes reduce Ea by providing an alternative reaction pathway. They do not change the equilibrium of a reaction or its overall energy change (ΔG); they only increase the rate at which equilibrium is reached.
The enzyme's active site — a cleft or pocket with a specific shape and charge distribution — binds the substrate. In the induced-fit model (the accepted model), the active site adjusts slightly when the substrate binds, placing catalytic residues in optimal positions. The enzyme-substrate complex breaks down to release products and regenerate the free enzyme.
For a quick definition, see our enzyme glossary entry. To compare with inorganic catalysts (e.g. platinum, iron), see our catalyst glossary entry.
02 · Factors affecting activity
Temperature, pH and concentration.
Enzyme activity is measured as the rate of product formation. Three factors most commonly affect it:
- Temperature — each 10 °C rise roughly doubles reaction rate (Q10 ≈ 2) up to the optimum. Above the optimum, the enzyme denatures: hydrogen bonds and hydrophobic interactions break, the tertiary structure is lost, and the active site changes shape irreversibly. For human enzymes, optimum ≈ 37 °C.
- pH — pH affects the ionisation state of amino acid side chains in the active site. Each enzyme has an optimum pH (pepsin: ~2; salivary amylase: ~7; trypsin: ~8). Extreme pH also denatures enzymes.
- Substrate concentration [S] — increasing [S] increases the rate until all enzyme active sites are occupied (Vmax). The Michaelis constant Km is the [S] at which rate = ½ Vmax; a low Km means high affinity for substrate.
03 · Examples in the body and in research
Enzymes are everywhere.
| Enzyme | Location | Substrate | Product |
|---|---|---|---|
| Amylase | Saliva, pancreas | Starch | Maltose |
| Pepsin | Stomach | Proteins | Peptides |
| DNA polymerase | Nucleus | dNTPs + template DNA | New DNA strand |
| Carbonic anhydrase | Red blood cells | CO₂ + H₂O | H₂CO₃ (carbonic acid) |
| Catalase | All cells | H₂O₂ | H₂O + O₂ |
| Rubisco | Chloroplast stroma | CO₂ + RuBP | 3-PGA (Calvin cycle) |
04 · Inhibition
Two ways to slow an enzyme down.
Competitive inhibition — the inhibitor has a similar shape to the substrate and competes for the active site. Increasing substrate concentration overcomes competitive inhibition (Vmax unchanged, Km increases). Example: malonate inhibits succinate dehydrogenase in the Krebs cycle.
Non-competitive inhibition — the inhibitor binds an allosteric site away from the active site, causing a conformational change that reduces catalytic activity. Cannot be overcome by increasing [S] (Vmax decreases, Km unchanged). This is the basis of many drugs: statins non-competitively inhibit HMG-CoA reductase, reducing cholesterol synthesis.
Enzymes are also used as tools in biotechnology — restriction enzymes cut DNA at specific sequences; PETase breaks down plastic. See our plastic-eating enzymes article for a remarkable recent example.
Questions about enzymes
- An enzyme is a biological catalyst that lowers the activation energy of a reaction, increasing the rate without being consumed. The enzyme's active site binds the substrate (lock-and-key or induced-fit model), forming an enzyme-substrate complex. Products are released and the enzyme is free to catalyse another reaction. See our enzyme glossary entry for a concise definition.
- The lock-and-key model treats the active site as a rigid shape matching only a specific substrate. The induced-fit model (now the accepted model) shows that the active site adjusts its shape upon substrate binding, providing a tighter fit and more effective catalysis. Experimental evidence from X-ray crystallography supports induced fit in most enzymes.
- Temperature — higher temperature increases reaction rate (more collisions) up to the optimum (~37 °C for most human enzymes); above this the enzyme denatures.
- pH — affects ionisation of active site residues; each enzyme has an optimum pH (e.g. pepsin ~2, trypsin ~8).
- Substrate concentration — increasing [S] increases rate until all active sites are saturated (Vmax).
- Competitive inhibition: inhibitor has similar shape to substrate; binds reversibly at the active site; overcome by increasing [S]. Non-competitive inhibition: inhibitor binds at an allosteric site; changes enzyme conformation; not overcome by increasing [S]. Many drugs work by enzyme inhibition — e.g. statins inhibit HMG-CoA reductase; aspirin inhibits COX enzymes. See our catalyst glossary entry for comparison with inorganic catalysts.
See enzymes in respiration
Cellular respiration relies on dozens of enzymes at every stage. The cellular respiration guide covers each one.