Jt

| Starting compound (b)

Enzyme a

Intermediatea

Enzyme b

Intermediateb

Enzyme c

End product processes (see figure 6.13). Non-competitive, non-reversible inhibitors damage the enzyme permanently so that it can no longer function; the inhibitor acts as an enzyme poison. For example, mercury in the antibacterial compound mercuro-chrome inhibits growth because it oxidizes the S—H groups of the amino acid cysteine in proteins. This converts cysteine to cys-tine, which cannot form the important covalent disulfide bond (S—S). As a result, the protein cannot achieve its proper shape.

Competitive Inhibition

In competitive inhibition, the inhibitor binds to the active site of the enzyme, obstructing access of the substrate to that site (figure 6.14). Generally this occurs because the inhibitor has a chemical structure similar to the normal substrate.

A good example of competitive inhibition is the action of sulfanilamide, one of the sulfa drugs used as an antimicrobial medication. Sulfa drugs inhibit an enzyme in the pathway that bacteria use to synthesize the vitamin folic acid by binding to the active site of the enzyme. The drug does not affect human metabolism because humans cannot synthesize folic acid; it must be provided in the diet. Sulfa drugs have a structure simi lar to para-aminobenzoic acid (PABA), an intermediate in the bacterial pathway for folic acid synthesis. Because of this, they fit into the active site of the enzyme that normally uses PABA as a substrate, preventing the attachment of PABA. The greater the proportion of sulfa molecules relative to PABA molecules, the more likely the active site of the enzyme will be occupied by a sulfa molecule. Once the sulfa is removed, the enzyme functions normally with PABA as the substrate. ■ sulfa drugs, p. 517

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