1. Acetate kinase [EC 2. 7. 2. 1] from an thermophile, B. stearothermophilus, was purified and crystallized.
2. This enzyme was shown to be a tetramer of identical subunits which had a molecular weight of about 40, 000. Amino acid analysis showed no SH group. By analyzing the CD spectrum it was deduced that this enzyme is composed of 36% β-structure, 21% α-helix and 43% unordered structure.
3. This enzyme shared many common enzymatic properties with the counterpart from mesophiles, i.e. pH optimum, substrate specificity, requirement of metal ions and essential amino acid residues necessary for the catalytic activity. However, this enzyme was remarkably thermostable.
4. A plot of the reaction velocity against the concentration of acetate, ADP or acetyl phosphate gave a curve of the Michaelis-Menten type. However, such a plot against ATP gave a sigmoid curve, suggesting a homotropic allosteric nature of the enzyme.
5. From the results of chemical modification it was deduced that an amino group and an imidazole group, at least, are involved in the active site of the enzyme.