The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary structure of the protein.
These include temperature, pH, change in substrate concentration or binding of specific chemicals that regulate its activity.
(1) Temperature and pH : Each enzyme shows its highest activity at a particular temperature and pH called the optimum temperature and optimum pH. Activity declines both below and above the optimum value.
Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat.
Effect of change in: (a) pH (b) Temperature and (c) concentration of substrate on enzyme activity (2) Concentration of substrate : With the increase in the substrate concentration, the velocity of the enzymatic reaction rises at first. The reaction ultimately reaches a maximum velocity (`V_("max")`) which is not exceeded by any further rise in the concentration of the substrate. This is because the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecules.
- The activity of an enzyme is also sensitive to the presence of specific chemicals that bind to the enzyme.
- When the binding of the chemical shuts off enzyme activity, the process is called inhibition and the chemical is called an inhibitor.
- When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor.
- Due to its close structural similarity with the substrate, the inhibitor competes with the substrate for substrate-binding site of the enzyme.
Due to this the substrate cannot bind and as a result, the enzyme action declines, e.g., inhibition of succinic dehydrogenase by the malonate which closely resembles the succinate structure.
- Such competitive inhibitors are often used in the control of bacterial pathogens.
