Human 3-hydroxy-3-methylglutaryl-CoA lyase catalyzes formation of acetyl-CoA and acetoacetate in a reaction that requires divalent cation and is stimulated by sulfhydryl protective reagents. The enzyme is a homodimer and inter-subunit adducts form in the absence of reducing agents or upon treatment with cysteine selective crosslinking agents. To address the influence of cysteines on enzyme activity and formation of inter-subunit and intra-subunit adducts, single serine substitutions have been engineered for each enzyme cysteine. Enzyme activity varies for each cysteine→serine mutant protein and different mutations have widely different effects on recovery of activity upon DTT treatment of non-reduced enzyme. These levels of enzyme activity do not strongly correlate with formation of inter-subunit adducts by these HMGCL mutants. C170S, C266S, and C323S proteins do not form inter-subunit disulfide adducts but such an adduct is restored in the C170S/C174S double mutant. Coexpression of HMGCL proteins encoded by C266S and C323S expression plasmids supports formation of a C266S/C323S heterodimer which does form a covalent inter-subunit adduct. These observations are interpreted in the context of competition between cysteines in formation of intra-subunit and inter-subunit heterodisulfide adducts.
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