EnzymesEdexcel A-Level Biology B: Revision notes

Enzymes are biological catalysts, and every enzyme is a globular protein: a polypeptide folded into a compact, roughly spherical tertiary structure held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups. Hydrophilic R groups point outwards, which makes most enzymes soluble in water.

The folding creates an active site, a small region with a precise three-dimensional shape and a particular arrangement of R groups. Enzymes catalyse a very wide range of reactions. Intracellular enzymes work inside cells, for example catalase breaking down hydrogen peroxide and the enzymes of respiration. Extracellular enzymes are made in cells and secreted to work outside them, for example amylase in saliva, and pepsin and trypsin in the gut.

Each active site is complementary to one substrate (or a small group of closely related ones), which is why enzymes are specific. The induced fit hypothesis says the active site is not a rigid lock. When the substrate binds, the active site changes shape slightly and moulds around it, forming an enzyme–substrate complex. This improves the fit and strains bonds in the substrate. The products leave, and the enzyme is unchanged.

Every reaction has an activation energy, the minimum energy needed to start it. Enzymes provide an alternative pathway with a lower activation energy, often by holding the substrate in the best position or straining its bonds, so more collisions result in a reaction at body temperature.

Temperature: the rate increases as kinetic energy rises, giving more frequent and more energetic collisions between enzyme and substrate. Above the optimum temperature, hydrogen and ionic bonds break, the tertiary structure and active site change shape and the enzyme is denatured. This is irreversible.

pH: a change from the optimum pH alters the charges on R groups, breaking ionic and hydrogen bonds, so the active site shape changes and the enzyme denatures. Pepsin works at about pH 2 and trypsin at about pH 8.

Substrate concentration: at low concentration the rate is proportional to it, because the active sites are mostly empty. At high concentration all active sites are occupied and the rate levels off at a maximum, as enzyme concentration is now limiting.

Enzyme concentration: with excess substrate, the rate is proportional to enzyme concentration, because there are more active sites.

The initial rate is the rate at the very start of the reaction, measured from the gradient of a tangent to the product (or substrate) curve at time zero. It is used because the substrate concentration is still known and has not fallen, little product has built up, and the enzyme has not yet been affected by changing pH or product inhibition. Comparisons between conditions are then fair.

In the core practical, one factor is varied while others are controlled. For catalase, measure the volume of oxygen collected in a fixed short time. For amylase, time how long it takes for iodine to stop turning blue-black. Rate can be found as volume ÷ time, for example 12 cm³ in 30 s is 0.4 cm³ s⁻¹, or as 1 ÷ time taken. Use buffers for pH, a water bath for temperature, and repeats to find means.

Competitive inhibitors have a shape similar to the substrate and bind to the active site, preventing substrate binding. Increasing substrate concentration reduces the effect, because substrate outcompetes the inhibitor, and the same maximum rate can be reached.

Non-competitive inhibitors bind to a site other than the active site (an allosteric site), changing the tertiary structure and so the shape of the active site. Fewer enzymes work, so the maximum rate is lower, and adding substrate does not overcome it.

End-product inhibition is a form of control in metabolic pathways. The end product binds to the first enzyme in the pathway, away from the active site, and reduces its activity. When the end product is used up it detaches and the enzyme recovers. This stops the cell wasting substrate and energy.