The primary structure of the major pepsinogen from the gastric mucosa of tuna stomach.

The complete primary structure of the major component of tuna pepsinogens was determined by conventional protein chemistry methods. It was composed of a prosegment of 37 residues and a pepsin moiety of 323 residues, having a relative molecular mass of 39,364. The essential aspartyl residues in the active site and the three disulfide bonds common to other pepsinogens were conserved; however, several unique substitutions and/or deletions characteristic of tuna pepsinogen were found at various positions, especially in the prosegment and subsite regions, as compared with the sequences of other pepsinogens, which may affect the rate of activation of the zymogen, and/or the catalytic function and substrate specificity of the enzyme. Tuna pepsinogen is the least acidic among pepsinogens. The sequence identity between tuna pepsinogen and other pepsinogens ranged from 45 to 52%. A phylogenetic tree based on the primary structures suggested that tuna pepsinogen diverged from the pepsinogen A and prochymosin groups in an early period of pepsinogen evolution.