The hydrogenases and formate dehydrogenases of Escherichia coli

Escherichia coli has the capacity to synthesise three distinct formate dehydrogenase isoenzymes and three hydrogenase isoenzymes. All six are multisubunit, membrane-associated proteins that are functional in the anaerobic metabolism of the organism. One of the formate dehydrogenase isoenzymes is also synthesised in aerobic cells. Two of the formate dehydrogenase enzymes and two hydrogenases have a respiratory function while the formate dehydrogenase and hydrogenase associated with the formate hydrogenlyase pathway are not involved in energy conservation. The three formate dehydrogenases are molybdo-selenoproteins while the three hydrogenases are nickel enzymes; all six enzymes have an abundance of iron-sulfur clusters. These metal requirements alone invoke the necessity for a profusion of ancillary enzymes which are involved in the preparation and incorporation of these cofactors. The characterisation of a large number of pleiotropic mutants unable to synthesise either functionally active formate dehydrogenases or hydrogenases has led to the identification of a number of these enzymes. However, it is apparent that there are many more accessory proteins involved in the biosynthesis of these isoenzymes than originally anticipated. The biochemical function of the vast majority of these enzymes is not understood. Nevertheless, through the construction and study of defined mutants, together with sequence comparisons with homologous proteins from other organisms, it has been possible at least to categorise them with regard to a general requirement for the biosynthesis of all three isoenzymes or whether they have a specific function in the assembly of a particular enzyme. The identification of the structural genes encoding the formate dehydrogenase and hydrogenase isoenzymes has enabled a detailed dissection of how their expression is coordinated to the metabolic requirement for their products. Slowly, a picture is emerging of the extremely complex and involved path of events leading to the regulated synthesis, processing and assembly of catalytically active formate dehydrogenase and hydrogenase isoenzymes. This article aims to review the current state of knowledge regarding the biochemistry, genetics, molecular biology and physiology of these enzymes.