A physiological role for oxalic acid biosynthesis in the wood-rotting basidiomycete Fomitopsis palustris

A metabolic mechanism for oxalic acid biosynthesis in the wood-rotting basi diomycete Fomitopsis palustris has been proposed on the basis of biochemica l analyses of glucose metabolism. There was a strong correlation between gl ucose consumption and oxalate production. Oxalic acid was found to accumula te in the culture fluid in about 80% of the theoretical yield or about 5-fo ld, on the basis of the fungal biomass harvested. The results clearly indic ate that glucose was not completely oxidized to CO2 by the tricarboxylic ac id (TCA) cycle but converted mainly to oxalate. The determination of the 12 enzymes concerned has revealed the occurrence of the unprecedented metabol ic coupling of the TCA and glyoxylate cycles that support oxalate biosynthe sis. In this metabolic system, isocitrate lyase (EC 4.1.3.1), together with oxaloacetase (EC 3.7.1.1), was found to play a pivotal role in yielding ox alate from oxaloacetate via the acetate-recycling routes. Moreover, malate dehydrogenase (EC 1.1.1.37), with an extraordinarily high activity among th e enzymes tested, was shown to play an important role in generating NADH by oxidation of malate to oxaloacetate. Thus, it is proposed that the wood-ro tting basidiomycete acquires biochemical energy by oxidizing glucose to oxa late.