CATABOLIC AND ANABOLIC ENZYME-ACTIVITIES AND ENERGETICS OF ACETONE METABOLISM OF THE SULFATE-REDUCING BACTERIUM DESULFOCOCCUS-BIACUTUS

Acetone degradation by cell suspensions of Desulfococcus biacutus was CO2, dependent, indicating initiation by a carboxylation reaction, whi le degradation of 3-hydroxybutyrate was not CO2 dependent. Growth on 3 -hydroxybutyrate resulted in acetate accumulation in the medium at a r atio of 1 mol of acetate per mol of substrate degraded. In acetone-gro wn cultures no coenzyme A (CoA) transferase or CoA ligase appeared to be involved in acetone metabolism, and no acetate accumulated in the m edium, suggesting that the carboxylation of acetone and activation to acetoacetyl-CoA may occur without the formation of a free intermediate . Catabolism of 3-hydroxybutyrate occurred after activation by CoA tra nsfer from acetyl-CoA, followed by oxidation to acetoacetyl-CoA. In bo th acetone-grown cells and 3-hydroxybutyrate-grown cells, acetoacetyl- CoA was thiolytically cleaved to two acetyl-CoA residues and further m etabolized through the carbon monoxide dehydrogenase pathway. Comparis on of the growth yields on acetone and 3-hydroxybutyrate suggested an additional energy requirement in the catabolism of acetone. This is po stulated to be the carboxylation reaction (Delta G(0') for the carboxy lation of acetone to acetoacetate, +17.1 kJ.mol(-1)). At the intracell ular acyl-CoA concentrations measured, the net free energy change of a cetone carboxylation and catabolism to two acetyl-CoA residues would b e close to 0 kJ.mol of acetone(-1), if one mol of ATP was invested. In the absence of an energy-utilizing step in this catabolic pathway, th e predicted intracellular acetoacetyl-CoA concentration would be 10(13 ) times lower than that measured. Thus, acetone catabolism to two acet yl-CoA residues must be accompanied by the utilization of the energeti c equivalent of (at least) one ATP molecule. Measurement of enzyme act ivities suggested that assimilation of acetyl-CoA occurred through a m odified citric acid cycle in which isocitrate was cleaved to succinate and glyoxylate. Malate synthase, condensing glyoxylate and acetyl-CoA , acted as an anaplerotic enzyme. Carboxylation of pyruvate or phospho enolpyruvate could not be detected,