Enzymes, Inhibitors and Reaction Rates

An enzyme’s active site binds suitable substrate molecules to form an enzyme–substrate complex. Binding can induce a change in active-site conformation that positions groups, strains bonds or otherwise stabilises the transition state. The activation energy is lowered, so a greater proportion of encounters can lead to reaction at a given temperature.

The enzyme is not used up in the net reaction and does not supply the energy needed for an endergonic process. Specificity depends on the active site’s three-dimensional structure and chemical properties, not simply a rigid hole with the same outline as the substrate. Enzymes act inside cells and in extracellular settings such as digestion.

Increasing temperature initially increases molecular kinetic energy and the frequency of successful encounters. At sufficiently high temperature, disruption of structure reduces active enzyme concentration and rate falls. Low temperature usually slows activity without permanently denaturing the protein. Changing pH alters charges and interactions, which can change substrate binding and active-site conformation.

At fixed enzyme concentration, increasing substrate concentration raises rate until most active sites are occupied and enzyme availability limits the reaction. Increasing enzyme concentration can raise the maximum rate if substrate remains available. Compare initial rates so later substrate depletion and product accumulation do not confuse the effect of the variable being tested.

A competitive inhibitor competes for the active site. Increasing substrate concentration can reduce its effect in the simple reversible model because substrate is more likely to bind. A non-competitive inhibitor binds at a different site and alters the enzyme’s function; adding substrate does not fully restore the original maximum rate in the simplified syllabus model.

For an amount-of-product against time graph, the tangent gradient gives instantaneous rate. An endpoint experiment using a fixed amount of product may estimate relative rate as 1/time, but this is not automatically the same as a measured initial rate. State the measurement and its units, and use RP1 to practise the experimental controls.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.