Purification and Characterization of 3-Methylcrotonyl-Coenzyme A Carboxylase from Higher Plant Mitochondria

3-Methylcrotonyl-coenzyme A (CoA) carboxylase was purified to homogeneity from pea (Pisum sativum L.) leaf and potato (Solanum tuberosum L.) tuber mitochondria. The native enzyme has an apparent molecular weight of 530,000 in pea leaf and 500,000 in potato tuber as measured by gel filtration. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate disclosed two nonidentical subunits. The larger subunit (B subunit) is biotinylated and has an apparent molecular weight of 76,000 in pea leaf and 74,000 in potato tuber. The smaller subunit (A subunit) is biotin free and has an apparent molecular weight of 54,000 in pea leaf and 53,000 in potato tuber. The biotin content of the enzyme is 1 mol/133,000 g of protein and 1 mol/128,000 g of protein in pea leaf and potato tuber, respectively. These values are consistent with an A4B4 tetrameric structure for the native enzyme. Maximal 3-methylcrotonyl-CoA carboxylase activity was found at pH 8 to 8.3 and at 35 to 38deg]C in the presence of Mg2+. Kinetic constants (apparent Km values) for the enzyme substrates 3-methylcrotonyl-CoA, ATP, and HCO3- were: 0.1 mM, 0.1 mM, and 0.9 mM, respectively, for pea leaf 3-methylcrotonyl-CoA carboxylase and 0.1 mM, 0.07 mM, and 0.34 mM, respectively, for potato tuber 3-methylcrotonyl-CoA carboxylase. A steady-state kinetic analysis of the carboxylase-catalyzed carboxylation of 3-methylcrotonyl-CoA gave rise to parallel line patterns in double reciprocal plots of initial velocity with the substrate pairs 3-methylcrotonyl-CoA plus ATP and 3-methylcrotonyl-CoA plus HCO3- and an intersecting line pattern with the substrate pair HCO3- plus ATP. It was concluded that the kinetic mechanism involves a double displacement. Purified 3-methylcrotonyl-CoA carboxylase was inhibited by end products of the reaction catalyzed, namely ADP and orthophosphate, and by 3-hydroxy-3-methylglutaryl-CoA. Finally, as for the 3-methylcrotonyl-CoA carboxylases from mammalian and bacterial sources, plant 3-methylcrotonyl-CoA carboxylase was sensitive to sulfhydryl and arginyl reagents.