Structural and phylogenetic comparison of three pepsinogens from Pacific bluefin tuna: molecular evolution of fish pepsinogens

The amino acid sequences of three pepsinogens (PG1, PG2 and PG3) of Pacific bluefin tuna (Thunnus orientalis) were deduced by cloning and nucleotide sequencing of the corresponding cDNAs. The amino acid sequences of the pre-forms of PG1, PG2 and PG3 were composed of a signal peptide (16 residues each), a propeptide (41, 37 and 35 residues, respectively) and a pepsin moiety (321, 323 and 332 residues, respectively). Amino acid sequence comparison and phylogenetic analysis indicated that PG1 and PG2 belong to the pepsinogen A family and PG3 to the pepsinogen C family. Homology modeling of the three-dimensional structure suggested that the remarkably high specific activity of PG2 toward hemoglobin, which had been found previously, was partly due to a characteristic deletion of several residues in the S1'-loop region that widens the space of the active site cleft region so as to accommodate protein and larger polypeptide substrates more efficiently. Including the tuna and all other fish pepsinogen sequences available to date, the molecular phylogenetic comparison was made with reference to evolution of fish pepsinogens. It was suggested that functional divergences of pepsinogens (pepsins) occurring in fishes as well as in mammals, correlated with differences in various aspects of fish physiology.