FOCI OF AMINO-ACID RESIDUE CONSERVATION IN THE 3D STRUCTURES OF THE KUNITZ BPTI PROTEINASE-INHIBITORS - HOW DO VARIANTS FROM SNAKE-VENOM DIFFER

The Kunitz BPTI proteinase inhibitor family is divisible into subgroup s based on source and bioactivity. Variants from snake venoms are of s pecial interest because some show only weak inhibitory activity agains t the common proteinases while others are neurotoxic. We analysed the sequences for each subgrouping in the context of the common chain fold to predict the 3D location of interactive sites. The method used was an enhanced form of the previously devised 'regiovariation analysis.' This revealed the foci in 3D of amino acid side chain conservation in each subgroup. Locally high levels of side-chain conservation extendin g substantially in three dimensions can be associated more with the pr eservation of function than conformation, hence the foci probably reve al the most functionally relevant sites. For the inhibitor variants th at do not originate from snake venom, regiovariation analysis gave an exact prediction of the antiproteinase site revealed by X-ray crystall ography of inhibitor-enzyme complexes. However, this site is not the p rincipal focus of evolutionary conservation in the inhibitors from sna ke venom, and other areas of the molecular surface are more prominent. The neurotoxic variants from snake venom (the dendrotoxins) have the principal focus of conservation near their C-terminal region, so this may be the origin of their special properties.