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.

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).

Examples in context

Example 1. Cane-toad bufotoxin and Daintree predator enzymes. The cane toad (Rhinella marina) introduced into Queensland in 1935 secretes bufotoxins that inhibit Na+/K+ ATPase in predator cardiac muscle, acting as a non-competitive enzyme inhibitor that binds at an allosteric site and changes pump shape. Research at James Cook University on Daintree quolls has documented behavioural avoidance (taste aversion) and, in some populations, mutations in the enzyme's binding pocket that reduce toxin affinity without abolishing the pump's catalytic activity. The case shows three concepts together: enzymes as biological catalysts, specific inhibition by a small molecule, and evolutionary modification of enzyme shape.

Example 2. Pineapple bromelain in IA1 experiments. Queensland Year 11 students often run an IA1 student experiment on bromelain, the cysteine protease in fresh pineapple grown around Yeppoon and Mareeba. Bromelain digests gelatin (collagen) and is sensitive to temperature and pH. Tubes of jelly with 1 mL fresh pineapple juice at 10, 20, 30, 40, 50 and 60 degrees Celsius typically show fastest gelatin breakdown at 40 to 45 degrees Celsius and no activity at 60 degrees Celsius (denaturation). Tinned pineapple, heat-treated above 80 degrees Celsius, fails to digest the gelatin, providing a clean control that supports the conclusion within the ISMG knowledge and analysis criterion.

Try this

Q1. Explain why an increase in temperature from 20 to 35 degrees Celsius increases enzyme activity but a further increase to 60 degrees Celsius reduces it. [3 marks]

• Cue. More kinetic energy raises collision frequency; above optimum, hydrogen bonds break, active site denatures.

Q2. A student measured the volume of gas produced by catalase from liver in five buffers at pH 4, 6, 7, 8 and 10, recording 2, 14, 22, 8 and 1 mL after 60 seconds. Identify the optimum pH and justify whether the data support classifying catalase as a human intracellular enzyme. [3 marks]

• Cue. Optimum near pH 7. Consistent with cytosolic enzyme; would not survive stomach pH.

Q3. Refer to a competitive and a non-competitive inhibitor of the same enzyme. (a) Distinguish the two modes of inhibition. (b) Predict how each affects Vmax and Km on a substrate-rate graph. (c) A new drug raises Km but leaves Vmax unchanged; classify the inhibition. [2+2+2 marks]

• Cue. (a) Active site vs allosteric. (b) Competitive: Vmax same, Km up. Non-competitive: Vmax down, Km same. (c) Competitive.