Citrate Synthase

Abstract

This chapter focuses on citrate synthase, which occupies a key position within the central metabolic pathway—the citric acid cycle. Citrate synthase is unique among the eight enzymes of the cycle in being the only one that catalyzes the formation of a carbon–carbon bond. In recent years, molecular weights have been determined for a large number of citrate synthases, and the data are presented in a tabulated form in the chapter. The results amply confirm the division of citrate synthases into two molecular types and support the proposal that gram-negative bacteria possess the “large” enzyme. However, two exceptions have been noted—the citrate synthases from Halobacterium spp. and Thermus aquaticus. These gram-negative organisms both require extreme conditions for growth; the halobacteria grow only in the presence of very high salt concentration (∼4 M) and T. aquaticus has an optimum growth temperature of 70°C. As these organisms are thought to be related to the pseudomonads, it may be that their environmental adaptation involved a changeover from the “large” to the “small” type.