Cold-adapted Features of Arginine Kinase from the Deep-sea Clam Calyptogena kaikoi

The heterodont clam Calyptogena kaikoi, which inhabits depths exceeding 3,500 m where low ambient temperatures prevail, has an unusual two-domain arginine kinase (AK) with molecular mass of 80 kDa, twice that of typical AKs. The purpose of this work is to investigate the nature of the adaptations of this AK for functioning at low temperatures. Recombinant C. kaikoi AK constructs were expressed, and their two-substrate kinetic constants (k(cat), K-a, and K-ia) were determined at 10 degrees C and 25 degrees C, respectively. When measured at 25 degrees C, the K-ia values were tenfold larger than those for corresponding K-a values, while at 10 degrees C, the K-ia values decreased remarkably, but the K-a values were almost unchanged. The Calyptogena two-domain enzyme has threefold higher catalytic efficiency, calculated by k(cat)/(K-a(ARG).K-ia(ATP)), at 10 degrees C, than that at 25 degrees C, reflecting adaptation for function at reduced ambient temperatures. The activation energy (E-a) and thermodynamic parameters were determined for Calyptogena two-domain enzyme and compared with those of two-domain enzymes from mesophilic Corbicula and Anthopleura. The value for E-a of Calyptogena enzyme were about half of those for mesophilic enzymes, and a larger decrease in entropy was observed in Calyptogena AK reaction. Although large decrease in entropy increases the Delta G(o)double dagger value and consequently lowers the k(cat) value, this is compensated with its lower E-a value thereby minimizing the reduction in its k(cat) value. These thermodynamic properties, together with the kinetic ones, are also present in the separated domain 2 of the Calyptogena two-domain enzyme.