Cold adaptation: structural and functional characterizations of psychrophilic and mesophilic acetate kinase

Acetate kinase catalyzes the reversible magnesium-dependent phosphoryl transfer from ATP to acetate to form acetyl phosphate and ADP. Here, we report functional and some structural properties of cold-adapted psychrotrophic enzyme; acetate kinase with those from mesophilic counterpart in Escherichia coli K-12. Recombinant acetate kinase from Shewanella sp. AS-11 (SAK) and E. coli K-12 (EAK) were purified to homogeneity following affinity chromatography and followed by Super Q column chromatography as reported before [44]. Both purified enzymes are shared some of the common properties such as (similar molecular mass, amino acid sequence and similar optimum pH), but characterized shift in the apparent optimum temperature of specific activity to lower temperature as well as by a lower thermal stability compared with EAK. The functional comparisons reveal that SAK is a cold adapted enzyme, having a higher affinity to acetate than EAK. In the acetyl phosphate and ADP-forming direction, the catalytic efficiency (k(cat)/K(m)) for acetate was 8.0 times higher for SAK than EAK at 10 °C. The activity ratio of SAK to EAK was increased with decreasing temperature in both of the forward and backward reactions. Furthermore, the activation energy, enthalpy and entropy in both reaction directions that catalyzed by SAK were lower than those catalyzed by EAK. The model structure of SAK showed the significantly reduced numbers of salt bridges and cation-pi interactions as compared with EAK. These results suggest that weakening of intramolecular electrostatic interactions of SAK is involved in a more flexible structure which is likely to be responsible for its cold adaptation.