A Study of the Mechanism of Acetate Assimilation in Purple Nonsulfur Bacteria Lacking the Glyoxylate Shunt: Enzymes of the Citramalate Cycle in Rhodobacter sphaeroides

Abstract

The mechanism of acetate assimilation by the purple nonsulfur bacterium Rhodobacter sphaeroides, which lacks the glyoxylate shunt, has been studied. In a previous work, proceeding from data on acetate assimilation by Rba. sphaeroides cell suspensions, a suggestion was made regarding the operation, in this bacterium, of the citramalate cycle. This cycle was earlier found in Rhodospirillum rubrum in the form of an anaplerotic reaction sequence that operates during growth on acetate instead of the glyoxylate shunt, which is not present in the latter bacterium. The present work considers the enzymes responsible for acetate assimilation in Rba. sphaeroides. It is shown that this bacterium possesses the key enzymes of the citramalate cycle: citramalate synthase, which catalyzes condensation of acetyl-CoA and pyruvate and, as a result, forms citramalate, and 3-methylmalyl-CoA lyase, which catalyzes the cleavage of 3-methylmalyl-CoA to glyoxylate and propionyl-CoA. The regeneration of pyruvate, which is the acetyl-CoA acceptor in the citramalate cycle, involves propionyl- CoA and occurs via the following reaction sequence: propionyl-CoA (+CO2) a methylmalonyl-CoA a succinyl-CoA a succinate a fumarate malate a oxaloacetate (−CO2) a phosphoenolpyruvate a pyruvate. The independence of the cell growth and the acetate assimilation of CO2 is due to the accumulation of CO2/HCO3− (released during acetate assimilation) in cells to a level sufficient for the effective operation of propionyl-CoA carboxylase.