| Abstract: | Upon resolution of the particulate cell fraction of Veillonella alcalescens by gel chromatography, membranes and ribosomes were clearly resolved. Methylmalonyl-CoA decarboxylase was bound to the membranes and not to ribosomes as reported earlier. Membrane vesicles containing methylmalonyl-CoA decarboxylase were prepared by disrupting V. alcalescens cells with a French pressure chamber. About 64% of the decarboxylase was oriented in these vesicles with the substrate binding site facing to the outside. The vesicles performed a rapid accumulation of Na+ ions in response to the decarboxylation of methylmalonyl-CoA. Decarboxylation and transport were highly uncoupled. The efficiency of the transport was considerably increased if methylmalonyl-CoA decarboxylation was retarded by using a low temperature or by slowly generating the substrate enzymically from propionyl-CoA. Under optimized conditions Na+ was concentrated inside the inverted vesicles eight-times higher than in the incubation medium. Methylmalonyl-CoA decarboxylase was solubilized from the membranes with Triton X-100 and purified about 20-fold by affinity chromatography on monomeric avidin-Sepharose columns. The decarboxylase was specifically activated by Na+ ions (apparent Km approximately equal to 0.6 mM). Whereas (S)-methylmalonyl-CoA was the superior substrate (apparent Km approximately equal to 7 microM), malonyl-CoA was also decarboxylated (apparent Km approximately equal to 35 microM). The decarboxylation of methylmalonyl-CoA yielded CO2 and not HCO-3 as the primary reaction product. Analysis of the purified enzyme by dodecylsulfate gel electrophoresis indicated the presence of four different polypeptides alpha, beta, gamma, delta with Mr 60 000, 33 000, 18 5000 and 14 000. The latter of these polypeptides was clearly visible only after silver staining but not after staining with Coomassie brilliant blue. A low molecular weight polypeptide with similar staining properties was also found in oxaloacetate decarboxylase. Methylmalonyl-CoA decarboxylase contained about 1 mol covalently bound biotin per 125 500 g protein which was localized exclusively in the gamma-subunit. This subunit therefore represents the biotin carboxyl carrier protein of methylmalonyl-CoA decarboxylase. A new very sensitive method for the detection of biotin-containing proteins is described. |
