Isocitrate dehydrogenase was purified from Hydrogenobacter thermophilus, and the corresponding gene was cloned and sequenced. The enzyme had similar structural properties to the isocitrate dehydrogenase of Escherichia coli, but differed in its catalytic properties, such as coenzyme specificity, pH dependency and kinetic parameters. Notably, the enzyme catalysed the oxidative decarboxylation of isocitrate, but not the reductive carboxylation of 2-oxoglutarate. The carboxylation reaction required the addition of cell extract and ATP-Mg, suggesting the existence of additional carboxylation factor(s). Further analysis of the carboxylation factor(s) resulted in the purification of two polypeptides. N-terminal amino acid sequencing revealed that the two polypeptides are homologues of pyruvate carboxylase with a biotinylated subunit, but do not catalyse pyruvate carboxylation. Pyruvate carboxylase was also purified, but was not active in stimulating isocitrate dehydrogenase. Isocitrate dehydrogenase, the novel biotin protein, ATP-Mg and NADH were essential for the reductive carboxylation of 2-oxoglutarate. These observations indicate that the novel biotin protein is an ATP-dependent factor, which is involved in the reverse (carboxylating) reaction of isocitrate dehydrogenase.