Two-domain arginine kinases from the clams Solen strictus and Corbicula japonica: exceptional amino acid replacement of the functionally important D(62) by G

Arginine kinases (AKs) isolated from the adductor muscle of the clams Solen strictus and Corbicula japonica have relative molecular masses of 80 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in contrast to the 40 kDa AKs found in Mollusca and Arthropoda. The cDNAs encoding Solen and Corbicula AKs have open reading frames of 2175 nucleotides (724 amino acid protein) and 2172 nucleotides (723 amino acid protein), respectively. The amino acid sequence clearly indicates that Solen and Corbicula AKs have a two-domain structure: the first-domain includes residues 1-363 and the second-domain includes residue 364 to the end. There is approximately 60% inter-domain amino acid identity. It is clear that gene-duplication and subsequent fusion occurred in the immediate ancestor of the clams Solen, Corbicula, and Pseudocardium. During substrate binding, it is proposed that AK undergoes a substrate-induced conformational change and that the hydrogen bond between D(62) and R(193) stabilizes the substrate-bound structure. However, in Solen and Corbicula two-domain AKs, D(62) is replaced by a G, and R(193) by A, S, or D. Consequently, the two-domain AKs can not form the stabilizing hydrogen bond. Nevertheless, the enzyme activity of Corbicula AK is comparable to those of other molluscan 40 kDa AKs. We assumed that the substrate-bound structure of the two-domain AK is stabilized not by the hydrogen bond between D(62) and R(193) but by the bond between H(60) and D(197), characteristic of the unusual two-domain AKs. This explains why D(62) and R(193), which remain highly conserved in other AKs, have undergone amino acid replacements in Solen and Corbicula AKs.