B 7.6Compare competitive inhibition and non-competitive inhibition
Inhibitors Chemicals that are able to modify the activity of an enzyme by binding to it. If the inhibitor attaches to the enzyme by covalent bonds, inhibition is irreversible and usually involves the destruction or permanent modification of the enzyme structure. If it attaches by weak interactions, the
Competitive inhibitors Chemicals that bind reversibly at the active site of the enzyme and they compete with the substrate for the binding position at the active site. Chemical structure similar to that of the substrate. Block active site and make it unavailable to the substrate. Increasing the substrate concentration decreases the effect of competitive inhibition
Effect on the rate of an enzyme- catalyzed reaction Vmax remains the same as there is still a substrate concentration where full activity of the enzyme can be achieved. But as it takes a higher substrate concentration to reach this rate, Km is increased.
Example : The conversion that succinic dehydrogenase carries out is: Succinate Fumarate The reaction is inhibited by malonate ions which have a very similar shape to succinate ions.
Non-competitive inhibition Chemicals that bind reversibly away from the active site of the enzyme Binding causes conformational change in the protein structure that alters the active site, inhibiting the ability to bind to the substrate. Increasing the substrate concentration has no effect as the enzyme’s shape still
Effect on the rate of an enzyme-catalyzed reaction Vmax is decreased and cannot be restored no matter how high the substrate concentration. The value of Km is unchanged because the unhibited enzymes are perfectly functional.
Examples : Silver ions (heavy metal) react with -SH groups in the side groups of cysteine residues in the protein chain: If the cysteine residue is somewhere on the protein chain which affects the way it folds into its tertiary structure, then altering this group could have an effect on the shape of the active site, and so stop the enzyme from working.
B 7.7State and explain the effects of heavy-metal ions, temperature changes and pH changes on enzyme activity
Effects of heavy-metal ions Such as lead, copper, mercury and silver are poisonous, due to their effects on enzymes. When a heavy-metal ion is present at the active site, substitution of a different metal ion for the original ion can cause the enzyme to malfunction and lose its activity. This is particularly evident where heavy metal ions can bind or chelate to the S-H ,sulfhydryl groups in proteins to form a covalent bond with the sulfur atom and
Effect of temperature Increasing the temperature will initially increase the rate of enzyme-catalyzed reactions, as more reactants will possess the minimum activation energy. The optimum temperature for most enzyme is about 40°C. Above this temperature enzymes rapidly become denatured as the weak bonds holding the tertiary structure together break. (Denaturation is the loss of the tertiary structure) Lowering the temperature usually causes deactivation of an enzyme. This prevents the
Effect of pH changes At low or high pH values, the enzyme is irreversibly denatured. Each enzyme works optimally at a particular pH. At different pH values the charges on the amino acid residues change affecting the bonds between them, and so altering the tertiary structure and making the enzyme ineffective. It is considered as a way of controlling their activity as different enzymes has different optimum pH. Eg. pepsin is active in the stomach where pH is