EnzymesEdexcel International A Level Biology: Revision notes

Enzymes are globular proteins that act as biological catalysts: they speed up metabolic reactions without being used up. Each reaction has an activation energy, the energy needed to start it. Enzymes provide an alternative reaction pathway with a lower activation energy, so more collisions between substrate molecules lead to a reaction at body temperature.

Enzymes do not change the products or supply energy.

The substrate binds to the active site, a small region of the enzyme with a specific 3D shape, forming an enzyme-substrate complex. The enzyme then lowers the activation energy, for example by straining bonds in the substrate, and the products leave.

The shape of the active site is determined by the tertiary structure of the protein, which is determined by its primary structure. Only a substrate with a complementary shape can bind, so enzymes are specific. In the induced-fit idea, the active site changes shape slightly as the substrate binds.

Intracellular enzymes work inside the cell that made them, for example catalase, which breaks down toxic hydrogen peroxide (2H₂O₂ → 2H₂O + O₂).

Extracellular enzymes are secreted and work outside the cell, for example amylase and trypsin in the digestive system. They break large insoluble molecules into small soluble ones that can be absorbed across membranes.

• Temperature: rate rises as molecules gain kinetic energy and collide more often with enough energy; above the optimum the bonds holding the tertiary structure break, the active site changes shape and the enzyme is denatured.
• pH: changes the charges on R groups, breaking ionic and hydrogen bonds, so the active site changes shape. Each enzyme has an optimum pH.
• Enzyme concentration: more enzyme means more active sites, so rate is proportional to enzyme concentration when substrate is in excess.
• Substrate concentration: rate rises, then levels off when all active sites are saturated and enzyme concentration becomes limiting.

Measure the initial rate because substrate concentration falls and products build up as the reaction proceeds. Find the rate as the amount of product formed (or substrate used) divided by time, or as 1/time for a colour change.

Vary one factor, keep the others constant: use a water bath for temperature, buffers for pH, and the same mass of enzyme or volume of solution. Catalase can be measured by collecting oxygen in a gas syringe; amylase by timing the loss of the blue-black colour with iodine. Repeat and calculate a mean; draw a graph of rate against the variable.