Factors affecting enzyme activity
Enzyme speed depends on substrate and enzyme amount, temperature, pH, activators and inhibitors. Each enzyme works best at its own optimum temperature and pH.
Factors and their effect
| Factor | Effect on rate | Example |
|---|---|---|
| Substrate concentration | Rises, then levels off at Vmax (maximum velocity) when all enzyme is busy; hyperbolic curve | Basis of Km |
| Enzyme concentration | Rises in a straight line, as long as substrate is in excess | Basis of measuring enzyme activity in lab tests |
| Temperature | Rises (roughly doubles per 10 °C) up to the optimum, then falls as heat denatures (unfolds) the protein | Human enzymes work best near 37 °C; blanching destroys enzymes in vegetables |
| pH | Highest at the optimum pH; too acidic or alkaline changes charges at the active site | Pepsin about 2, salivary amylase about 6.8, trypsin about 8 |
| Activators | Ions or molecules that raise activity | Cl− activates salivary amylase; Mg2+ is needed by kinases |
| Product concentration | Build-up of product slows the reaction | Glucose-6-phosphate inhibits hexokinase |
Km in simple words
Km (Michaelis constant) is the substrate concentration at which an enzyme works at half its maximum velocity (½ Vmax). It shows how tightly the enzyme holds its substrate. Low Km = high affinity: the enzyme works well even when little substrate is present. High Km = low affinity. Example: hexokinase (low Km for glucose) keeps working in all tissues between meals, while liver glucokinase (high Km) acts mainly after a meal, when blood glucose is high. The Michaelis–Menten equation links them: v = Vmax[S] / (Km + [S]).
Types of enzyme inhibition
| Type | How it works | Km and Vmax | Examples |
|---|---|---|---|
| Competitive (reversible) | Looks like the substrate and competes for the active site; more substrate overcomes it | Km rises; Vmax unchanged | Malonate on succinate dehydrogenase; statins on HMG-CoA reductase; methotrexate on dihydrofolate reductase |
| Non-competitive (reversible) | Binds a site other than the active site and lowers activity; more substrate does not help | Km unchanged; Vmax falls | Heavy metals such as lead binding –SH groups, e.g. of ferrochelatase (some books list heavy metals as irreversible) |
| Irreversible | Binds permanently, often by a covalent bond; activity returns only when new enzyme is made | Vmax falls | Organophosphate insecticides and nerve gases on acetylcholinesterase; aspirin on cyclooxygenase |
Food processing controls enzymes: blanching and cooking destroy them by heat, refrigeration slows them, and acid (lemon juice) slows browning of cut apple. In digestion, pepsin works in stomach acid and stops in the alkaline small intestine. Statins, used for high cholesterol, are competitive inhibitors of HMG-CoA reductase.
10 marks: “Discuss the factors affecting enzyme activity.” Cover substrate (with Km), enzyme concentration, temperature, pH, activators, product and the three types of inhibition, each with a small graph and an example.
5 marks: “Differentiate competitive and non-competitive inhibition with examples.” Competitive: binds active site, Km rises, Vmax same, overcome by more substrate (malonate). Non-competitive: binds elsewhere, Vmax falls, Km same (lead).
2 marks: “Define Km.” The substrate concentration at which an enzyme works at half its maximum velocity. A low Km means high affinity for the substrate.
Competitive = same seat: it fights for the active site and loses when more substrate arrives. Non-competitive = different seat: it binds elsewhere, so more substrate does not help.