Biosynthetic pathways in Oxalobacter formigenes
Alan A. DiSpirito
Oxalate is the only substrate that supports the growth of the gram negative anaerobe, Oxalobacter formigenes. Oxalate is decarboxylated to formate plus CO[subscript]2. A small amount of acetate (0.5-1 mM) is required for biosynthetic reactions. Oxalate is reduced and assimilated into cell biomass by aerobic oxalate-degrading bacteria using either the glycerate pathway or the serine pathway. Oxalate is reduced to 3P-glycerate and assimilated as a C[subscript]3 unit. We detected the enzymatic activities of glycerate pathway but not those of the serine pathway in cell-free extracts of O. formigenes;Four potential sources of carbon for cell biomass are available to O. formigenes, oxalate, acetate, formate and CO[subscript]2. We grew the organism in [superscript]14 C labeled carbon sources and determined the contribution of each of these sources to cell carbon. O. formigenes derived at least 54% of its cell carbon from oxalate and at least 7% from acetate. The only other carbon source utilized was CO[subscript]3. Formate was not incorporated to a significant extent. Carbon from [superscript]14 C-oxalate and [superscript]14 CO[subscript]3 was detected in amino acids derived from [alpha]-ketoglutarate, oxaloacetate, pyruvate, 3P-glycerate and in the aromatic amino acids. Amino acids derived from [alpha]-ketoglutarate, oxaloacetate and pyruvate contained carbon derived from [superscript]14 C-acetate;When O. formigenes was grown on [superscript]13 C-labeled oxalate, acetate or CO[subscript]3,the labeling patterns of the amino acids were consistent with their formation through common biosynthetic pathways. [superscript]13 C from oxalate was detected in the majority of the carbons from all of the amino acids. Approximately 60% of the acetate was incorporated as a C[subscript]2 unit into four amino acids (glutamate, proline, arginine and leucine). The other 40% of the acetate was split and was detected in amino acids derived from oxaloacetate and pyruvate;Enzymatic activities detected in cell-free extracts included: glutamate dehydrogenase, phosphoenolpyruvate carboxylase, pyruvate carboxylase, citrate synthase and isocitrate dehydrogenase. These findings support the [superscript]14 C and [superscript]13 C data which indicate that O. formigenes assimilates acetate into protein using the first third of the TCA pathway and that C[subscript]4 units are formed from C[subscript]3 units by carboxylation of pyruvate or PEP.