Protease and α-Amylase Inhibitors of Higher Plants

Numerous proteins in higher plants inhibit enzymes found in plants, animals (including humans) and microorganisms. Some of these inhibitors have endogenous physiological functions in the plants; others appear to have a protective role. Some enzyme inhibitors have nutritional implications in human diets. Protease inhibitors are known for all four types of proteases. The best studied are the serpins (inhibit serine-type proteases) and the cystatins (inhibit the sulfhydryl-type proteases). Based on disulfide bond arrangement, primary amino acid sequences and other factors, the groups can be further subdivided. Some of these protease inhibitors have been bioengineered into plants to protect them (and their seeds) against pests. The α-amylase inhibitors are under intense study. Those from beans have sequence homology with lectins and arcelins also found in beans. Some enzyme inhibitors are multi-headed, inhibiting more than one molecule of the same enzyme, same class of enzyme or different classes of enzymes. The structural diversity, mechanism of inhibition, and importance of these inhibitors are discussed.