Tuesday, November 4, 2008

Enzyme Inhibition

In competitive inhibition, inhibitor has a shape, charge, size and structure similar to that of the substrate. It therefore competes with the substrate for the active sites to form the enzyme-inhibitor complex (E-I complex) and reduces the number of active sites available for the substrate to bind and form the enzyme-substrate (E-S) complex. If substrate concentration is less than that of competitive inhibitor, it is more likely for the enzyme to collide with the competitive inhibitor and form E-I complex, so rate of product formation will decrease.
Km will increase and there are less number of active sites available for substrate molecules. If substrate concentration is more than that of competitive inhibitor, it is more likely for enzyme to collide with substrate and form E-S complex. The reaction will eventually reach Vmax but only at high substrate concentration. One example of competitive inhibition is where malonate competes with succinate for succinate dehydrogenase.

For non-competitive inhibition, the inhibitors have a structure different from that of the substrate. It binds away from the active site at the allosteric site, inducing conformational change of active site. It prevents substrate from binding to the acgtive site as it is no longer complementary to the shape, orientation, and charge fo the substrate, hence the E-S complex cannot be formed. Km remains constant, affinity does not change. Vmax is lower and there are fewer functional enzymes. for example, cyanide is a non-competitive inhibitor of oxygen to cytochrome oxidase.

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