Bromelain isoinhibitors from pineapple stem (BIs) are unique doublechain inhibitors and inhibit the cysteine proteinase bromelain competitively. The threedimensional structure was shown to be composed of two distinct domains, each of which is formed by a threestranded antiparallel βsheet. Unexpectedly, BIs were found to share similar folding and disulfidebond connectivities not with the cystatin superfamily, but with BowmanBirk trypsin/chymotrypsin inhibitor (BBI). The structural similarity between them suggests that BIs and BBI have evolved from a common ancestor and differentiated in function during the course of molecular evolution.
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