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 basidiomycete Fomitopsis palustris has been proposed on the basis of biochemical analyses of glucose metabolism. There was a strong correlation between glucose consumption and oxalate production. Oxalic acid was found to accumulate in the culture fluid in about 80% of the theoretical yield or about 5-fold, on the basis of the fungal biomass harvested. The results clearly indicate that glucose was not completely oxidized to CO(2) by the tricarboxylic acid (TCA) cycle but converted mainly to oxalate. The determination of the 12 enzymes concerned has revealed the occurrence of the unprecedented metabolic coupling of the TCA and glyoxylate cycles that support oxalate biosynthesis. In this metabolic system, isocitrate lyase (EC ), together with oxaloacetase (EC ), was found to play a pivotal role in yielding oxalate from oxaloacetate via the acetate-recycling routes. Moreover, malate dehydrogenase (EC ), with an extraordinarily high activity among the 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-rotting basidiomycete acquires biochemical energy by oxidizing glucose to oxalate.

Purification and characteristics of a novel cytochrome c dependent glyoxylate dehydrogenase from a wood-destroying fungus Tyromyces palustris.

A new glyoxylate dehydrogenase which catalyzes dehydrogenation of glyoxylate to oxalate in the presence of cytochrome c has been purified as an electrophoretically homogeneous protein from the cell-free extracts of a wood-destroying basidiomycete Tyromyces palustris. The enzymatic reduction of cytochrome c was dependent on glyoxylate which was found to be the best substrate among the compounds tested. The Km value for glyoxylate was determined to be 2.7 mM at the optimal pH (8.0). The UV-visible spectra of the enzyme in oxidized and reduced forms indicate that the enzyme belongs to a family of flavohemoproteins. The flavin nucleotide isolated from the native enzyme by heat denaturation was identified as FMN. The enzyme (Mr 331000) consists of six identical homopolymers (Mr of subunit 59000), which were found to constitute a symmetric octahedral shape by electron-microscopic observation with a negative staining method.