STRUCTURE OF CHYMOTRYPSIN VARIANT-B FROM ATLANTIC COD, GADUS-MORHUA

The amino-acid sequence of chymotrypsin variant B isolated from the py loric caeca of Atlantic cod has been elucidated. The characterization of the primary structure is based on N-terminal Edman degradation and mass spectrometry of the native protein and enzymatically derived pept ides. Chymotrypsin variant B showed 72% sequence identity with the A-v ariant and 64% and 62%, respectively, with the bovine counterparts A a nd B, all consisting of 245 amino acids. This new sequence contains a higher proportion of charged residues compared with bovine chymotrypsi n but fewer polar hydrogen-bond forming residues which might contribut e to its lower thermostability. It also shares the emerging characteri stics of other fish serine proteinases which have relatively higher me thionine content, including a conserved Met-134 in a loop leading into a domain-connecting strand. The inherent mobility in methionine side- chains may contribute to the maintenance of flexibility at low tempera tures, Several amino-acid sequence differences adjacent to the catalyt ic site are observed in the two cod chymotrypsin variants which also d iffer in kinetic properties. Unlike the mammalian chymotrypsins, which contain several autolysis sites, cod variant B only contains a single autolysis site. The three-dimensional structures of the A- and B-vari ants of cod has been modelled on the known crystal structure of bovine a-chymotrypsin showing almost superimposable structures.