Cloning and characterization of the gene encoding Halobacterium halobium adenylate kinase

Cloning and characterization of adenylate kinase from Chlamydia pneumoniae

Chlamydiae proliferate only within the infected host cells and are thought to be "energy parasites," because they take up ATP from the host cell as an energy source. In the present study, we isolated from Chlamydia pneumoniae the gene encoding adenylate kinase (AK). Using the enzyme produced in Escherichia coli, its properties were characterized. K(m) values for AMP and for ADP of the purified C. pneumoniae AK (AKcpn) were each 330 microm, which is significantly higher than the reported values of other AKs, whereas K(m) for ATP was 24 microm, which was rather lower than others. AKcpn contains 1 g atom of zinc/mol of 24,000-dalton protein. Mass spectrometric analysis of AKcpn and analysis of properties of mutated AKcpn strongly suggested that zinc is associated with four cysteine residues in the LID domain of the enzyme. The apo-AKcpn that lost zinc retained AK activity, although K(m) for AMP of apo-AKcpn increased about 2-fold and V(max) decreased about one-half from that of holo-AKcpn. The apo-AKcpn was more thermolabile and sensitive to trypsin digestion than the holo-AKcpn. Moreover, the recovery in vitro of the AK activity during the renaturation process of the denatured apo-AKcpn was dependent on zinc. A mutated protein in which cysteine residues in the LID domain were substituted by other amino acids lost both zinc and enzyme activity. The mutated protein was more sensitive to protease than the apo-AKcpn. These results indicate that zinc in AKcpn, although not essential for the catalysis, stabilizes the enzyme and probably plays a crucial role in proper folding of the protein. Furthermore, the catalytic properties of AKcpn suggest a distinctive regulatory mechanism in the metabolism compared with AKs in other organisms.

Bioenergetics of the Archaea

In the late 1970s, on the basis of rRNA phylogeny, Archaea (archaebacteria) was identified as a distinct domain of life besides Bacteria (eubacteria) and Eucarya. Though forming a separate domain, Archaea display an enormous diversity of lifestyles and metabolic capabilities. Many archaeal species are adapted to extreme environments with respect to salinity, temperatures around the boiling point of water, and/or extremely alkaline or acidic pH. This has posed the challenge of studying the molecular and mechanistic bases on which these organisms can cope with such adverse conditions. This review considers our cumulative knowledge on archaeal mechanisms of primary energy conservation, in relationship to those of bacteria and eucarya. Although the universal principle of chemiosmotic energy conservation also holds for Archaea, distinct features have been discovered with respect to novel ion-transducing, membrane-residing protein complexes and the use of novel cofactors in bioenergetics of methanogenesis. From aerobically respiring Archaea, unusual electron-transporting supercomplexes could be isolated and functionally resolved, and a proposal on the organization of archaeal electron transport chains has been presented. The unique functions of archaeal rhodopsins as sensory systems and as proton or chloride pumps have been elucidated on the basis of recent structural information on the atomic scale. Whereas components of methanogenesis and of phototrophic energy transduction in halobacteria appear to be unique to Archaea, respiratory complexes and the ATP synthase exhibit some chimeric features with respect to their evolutionary origin. Nevertheless, archaeal ATP synthases are to be considered distinct members of this family of secondary energy transducers. A major challenge to future investigations is the development of archaeal genetic transformation systems, in order to gain access to the regulation of bioenergetic systems and to overproducers of archaeal membrane proteins as a prerequisite for their crystallization.