The carboxyltransferase activity of the sodium‐ion‐translocating methylmalonyl‐CoA decarboxylase of Veillonella alcalescens

Abstract

Methylmalonyl-CoA decarboxylase of Veillonella alcalescens catalyzed the isotopic exchange between methylmalonyl-CoA and [1-14C]propionyl-CoA or between malonyl-CoA and [114C]acetyl-CoA. The exchange was independent of sodium ions and was abolished by avidin. The enzyme also catalyzed the carboxyl transfer reaction from methylmalonyl-CoA to acetyl-CoA yielding propionyl-CoA and malonyl-CoA, and vice versa. The .beta. subunit was dissociated from methylmalonyl-CoA decarboxylase by prolonged washing of the enzyme while bound via its biotin prosthetic group to monomeric avidin-Sepharose. The .beta.-chain-depleted enzyme was inactive as a methylmalonyl-CoA decarboxylase but retained carboxyltransferase activity. The .beta. subunits were specifically protected by Na+ ions from tryptic hydrolysis. Based on these and other observations the following functions may be assigned to the different polypeptide chains of methylmalonyl-CoA decarboxylase: carboxyltransferase (.alpha.), carboxybiotin-carrier-protein decarboxylase (.beta.), biotin carrier protein (.gamma.). The function of the .delta. chain is unknown.