Biological significance of polymorphism in legume protease inhibitors from the Bowman-Birk family

Naturally occurring protease inhibitors (PI) of the Bowman-Birk type constitute a major PI family in cereal and legume seeds. The family name is derived from the names of the two investigators who characterised the first inhibitor of this type, the Bowman-Birk inhibitor from soybean (BBI). These proteins have the capacity to inhibit one or more of a range of serine proteases, including the digestive enzymes trypsin and chymotrypsin. PI from this family interact with the active sites of serine proteases in a 'canonical', i.e. substrate-like, manner via exposed reactive site loops of conserved conformation within the inhibitor. Multiple BBI variants can be found within and among species. A limited number of amino acids located within the inhibitory domain is responsible for the primary functional and biological activities of BBI-like proteins. However, sequence variation in binding loops, post-translational modifications at the amino- and carboxy-terminal ends, as well as differences in the multimeric nature of the inhibitors may act in combination to influence the functional properties and the physiological role of BBI-like proteins. Recently, BBI and proteins homologous to BBI (BBI-like proteins) have emerged as highly promising cancer chemopreventive agents. BBI has been shown to be capable of preventing or suppressing carcinogenic processes in a wide variety of in vitro and in vivo animal model systems. The potential exploitation of BBI-like proteins in human health-promotion programmes will depend on elucidating in detail the molecular basis for the variation in biological activities among the many variant forms. New knowledge, derived both from the use of synthetic cyclic peptides that mimic the inhibitory loops of BBI-like proteins, and from genomic data pertaining to the structure of BBI gene classes, together facilitate the manipulation, screening and selection of appropriate variants through biotechnology.