Two distinct arginine kinases in Neocaridina denticulate: psychrophilic and mesophilic enzymes.

The small shrimp Neocaridina denticulata has two types of arginine kinases (AKs): AK1 and AK2. We determined the full kinetic parameters for the forward reaction of the AKs at temperatures between 15 and 35°C. The catalytic efficiencies, determined by kcat/(Ka(ATP)·Kia(ARG)), of the two enzymes showed a marked difference in temperature dependence. The efficiency of AK2 decreased markedly with decreasing temperature, while that of AK1 did not decrease with decreasing temperature, suggesting that AK1 is a cold-adapted enzyme. This unusual characteristic of AK1 was attributable to the remarkable decrease in Kia(ARG) with decreasing temperature, which enlarges its catalytic efficiency. Criterion with the values of Δ(ΔH(o‡))p-m and Δ(TΔS(o‡))p-m also indicate that AK1 is a psychrophilic enzyme and AK2 is mesophilic.