Bromein, a Bromelain Inhibitor from Pineapple Stem: Structural and Functional Characteristics

Bromelain inhibitor, "bromein", is a proteinase-inhibitor specific to the cysteine proteinase bromelain from pineapple stem. In the stem, eight bromein isoforms are known to exist, and each isoform has a short peptide (light chain) and a long one (heavy chain) with five disulfide bonds. The three-dimensional structure of the sixth isoform (bromein-6) is composed of inhibitory and stabilizing domains, and each domain contains a three-stranded antiparallel β-sheet. The genomic sequence of a bromein precursor encodes three homologous bromein isoform domains, and each isoform domain has a signal peptide, three interchain peptides between the light chain and heavy chain, two interdomain peptides and a propeptide. Interestingly, at the protein level, bromein- 6 appears to share a similar folding and disulfide-bonding connectivity with Bowman-Birk serine proteinase inhibitors and shows weak inhibition toward chymotrypsin and trypsin. However, no significant similarity was found between them at the genomic level. This indicates that they have evolved convergently to possess such a structural similarity. To identify the essential reactive site(s) with bromelain, we investigated the inhibitory activity of 44 kinds of the single/double and insertion/ deletion mutants of bromein-6 towards stem bromelain. As a result, it was shown that both the appropriate positioning and the complete side-chain structure of Leu10 in the light chain are absolutely crucial for the inhibition, with an additional measure of importance for the preceding Pro9. Bromein and stem bromelain coexist in the acidic vacuoles of the stem tissue, and one of the key role of bromein appears to be the regulation of the bromelain activity.