Explain the role of enzymes as biological catalysts and the effect of temperature, pH, substrate concentration, enzyme concentration and inhibitors on enzyme activity

What this dot point is asking

QCAA wants you to define an enzyme, describe how it works, and predict how reaction rate changes when temperature, pH, substrate concentration, enzyme concentration or an inhibitor changes. Stimulus questions almost always present a rate-versus-factor graph and ask you to interpret it.

The answer

Enzymes are biological catalysts: globular proteins (and a few RNAs) that speed up reactions without being consumed. They are essential because cellular reactions would otherwise occur far too slowly at body temperature.

How enzymes work

An enzyme provides an alternative reaction pathway with a lower activation energy. Substrate molecules bind to a specific region of the enzyme (the active site), reaction occurs and product is released.

• Active site. A pocket or cleft with a specific three-dimensional shape and chemistry, formed by the enzyme's tertiary structure.
• Specificity. Each enzyme acts on a small set of substrates whose shape and chemistry complement the active site.
• Induced-fit model. The active site is not a rigid lock; on substrate binding, the enzyme changes shape slightly to bind the substrate more tightly and strain its bonds. This is more accurate than the older lock-and-key model.

The enzyme is unchanged at the end of the reaction and can catalyse many further turnovers.

Factors affecting enzyme activity

Enzyme activity is usually measured as reaction rate (product formed or substrate consumed per unit time).

Temperature

• Below the optimum, rate rises with temperature because molecules collide more frequently and with more energy. A useful rule of thumb is that rate roughly doubles per 10 degrees Celsius increase (Q10 around 2).
• At the optimum, rate is maximal. For human enzymes the optimum is around 37 degrees Celsius; for bacterial enzymes used in PCR (Taq polymerase) it can exceed 70 degrees Celsius.
• Above the optimum, the enzyme denatures: hydrogen and ionic bonds maintaining the tertiary structure break, the active site loses its complementary shape, and rate falls to zero. Denaturation is usually irreversible.

pH

Each enzyme has an optimum pH. Outside a narrow range, the charges on amino acid side chains in the active site change, hydrogen bonds break and the enzyme denatures.

• Pepsin (stomach): optimum pH around 2.
• Most cytosolic enzymes: optimum pH around 7.
• Trypsin (small intestine): optimum pH around 8.

Substrate concentration

At low substrate, rate is proportional to substrate concentration (substrate is limiting). As substrate increases, active sites become occupied more often, and rate plateaus once all active sites are saturated (Vmax). The curve is hyperbolic (Michaelis-Menten kinetics, though QCAA does not require the equation).

Enzyme concentration

At constant excess substrate, rate is proportional to enzyme concentration: more active sites means more substrate molecules converted per unit time. If substrate runs out, the enzyme-concentration curve also plateaus.

Inhibitors

Inhibitors reduce enzyme activity. The two main classes appear on QCAA papers:

• Competitive inhibitors. Structurally resemble the substrate and bind to the active site, blocking substrate entry. Their effect is reversed by raising substrate concentration. Vmax is unchanged; the apparent Km (substrate concentration at half Vmax) increases.
• Non-competitive (allosteric) inhibitors. Bind at a separate site (the allosteric site), changing the enzyme's shape so the active site no longer fits substrate well. Their effect is not reversed by raising substrate concentration; Vmax falls.

A third type, end-product inhibition, is a regulatory mechanism: the product of a pathway acts as a non-competitive inhibitor of an early enzyme, switching off the pathway when end-product is abundant (a negative feedback loop).

Cofactors and coenzymes

Some enzymes need a non-protein helper:

• Cofactors are inorganic ions (Mg2+ for ATPases, Zn2+ for carbonic anhydrase, Fe2+ for catalase).
• Coenzymes are organic molecules, often derived from vitamins (NAD+ from niacin, FAD from riboflavin, coenzyme A from pantothenate). Many shuttle electrons in respiration and photosynthesis.

Common traps

Saying enzymes "are used up" in the reaction. They are not consumed; one enzyme can catalyse thousands of turnovers.

Calling denaturation a reversible "loss of shape". It is usually irreversible: hydrogen and ionic bonds break, and the enzyme cannot refold spontaneously.

Treating pH and temperature curves as identical bell shapes. They look similar, but the underlying mechanism is different. pH disrupts charge-based bonding within the active site; temperature provides energy or breaks the tertiary fold.

Confusing competitive and non-competitive on the substrate axis. Adding more substrate rescues a competitive inhibitor but not a non-competitive one.

Cross-link to Year 12 assessment

Enzyme kinetics underlies homeostasis in Unit 2 (enzymes have narrow optimum ranges, motivating tight temperature and pH regulation), the equilibrium concept in Unit 3 chemistry IAs, and biotechnology applications in Unit 4 IA3 (restriction enzymes, DNA polymerase, ligase used in genetic engineering).

In one sentence

Enzymes are protein catalysts that lower activation energy by binding substrate at a specific active site via induced fit; their rate rises with temperature and substrate concentration, peaks at an optimum temperature and pH, denatures outside that range, and can be reduced by competitive inhibitors at the active site or non-competitive inhibitors at an allosteric site.