Zinc-, cobalt- and iron-chelated forms of adenylate kinase from the Gram-negative bacterium Desulfovibrio gigas.

Adenylate kinase (AK) from the sulphate-reducing bacterium Desulfovibrio gigas (AK) has been characterized earlier as a Co(2+)/Zn(2+)-containing enzyme, which is an unusual characteristic for adenylate kinases from Gram-negative bacteria, in which these enzymes are normally devoid of metal ions. AK was overexpressed in E. coli and homogeneous Co(2+)-, Zn(2+)- and Fe(2+)-forms of the enzyme were obtained under in vivo conditions. Their structural stability and spectroscopic and kinetic properties were compared. The thermal denaturation of Co(2+)- and Zn(2+)-forms of AK was studied as a cooperative two-state process, sufficiently reversible at pH 10, which can be correctly interpreted in terms of a simple two-state thermodynamic model. In contrast, the thermally induced denaturation of Fe(2+)-AK is irreversible and strongly dependent upon the scan rate, suggesting that this process is under kinetic control. Practically identical contents of secondary-structure elements were found for all the metal-chelated-forms of AK upon analysis of circular dichroism data, while their tertiary structures were significantly different. The peculiar tertiary structure of Fe(2+)-AK, in contrast to Co(2+)- and Zn(2+)-AK, and the consequent changes in the physico-chemical and enzymatic properties of the enzyme are discussed.