The Primary Structure of Bdellin B-3 from the LeechHirudo medicinalis.Bdellin B-3 is a Compact Proteinase Inhibitor of a “Non-classical” Kazal Type. It is present in the Leech in a High Molecular Mass Form

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Abstract
A proteinase inhibitor was isolated from extracts of the leech Hirudo medicinalis by gel filtration and anion exchange chromatography. This inhibitor is similar to the bdellins in that it blocks the activity of trypsin, plasmin and sperm acrosin but has a molecular mass, as estimated by SDS polyacrylamide electrophoresis, of about 20 kDa, whereas the bdellins have molecular masses in the range 5-6 kDa. It is therefore designated as high-molecular mass bdellin B-3 (HMB). The amino-acid sequence of the inhibitor was elucidated as far as position 56. This revealed that the molecule consists of bdelline B-3 moiety, corresponding to the N-terminal 46 residues, which is then extended at the C-terminus by a polypeptide chain of the composition Asx15, Glx25, Gly6, Val, His26-127 and Lys4. It has been formerly concluded from a partial amino-acid sequence than bdellin B-3 is a Kazal-type inhibitor. However, the complete sequence of bdellin B-3, represented by the N-terminal 46 residues of HMB, discloses that bedllin B-3 is a non-classical Kazal-type inhibitor when the number of amino-acid residues between half-cystines are considered. Presuming that formation of disulfide bridges principally follows the same pattern as in classical Kazal-type inhibitors the bedllin B-3 molecule was modeleled based on the known three-dimensional structure of the third ovomucoid domains. This showed that a compact arrangement of the peptide chain of bdellin B-3 is conceivable. The conserved core typically formed in Kazal-type inhibitors by the central .alpha.-helix, an adjacent three-stranded .beta.-sheet and part of the proteinase binding loop is maintained. The unusual distribution of the half-cystines within the polypeptide chain causes a very compact tertiary structure of the inhibitor molecule.