Regulation of the cyanide-resistant alternative oxidase of plant mitochondria -: Identification of the cysteine residue involved in α-keto acid stimulation and intersubunit disulfide bond formation

The cyanide-resistant alternative oxidase of plant mitochondria is a homodimeric protein whose activity can be regulated by a redox-sensitive intersubunit sulfhydryl/disulfide system and by alpha-keto acids. After determining that the Arabidopsis alternative oxidase possesses the redox-sensitive sulfhydryl/disulfide system, site-directed mutagenesis of an Arabidopsis cDNA clone was used to individually change the two conserved Cys residues, Cys-128 and Cys-78, to Ala. Using diamide oxidation and chemical cross-linking of the protein expressed in Escherichia coli, Cys-78 was shown to be: 1) the Cys residue involved in the sulfhydryl/disulfide system; and 2) not required for subunit dimerization. The C128A mutant was stimulated by pyruvate, while the C78A mutant protein had little activity and displayed no stimulation by pyruvate. Mutating Cys-78 to Glu produced an active enzyme which was insensitive to pyruvate, consistent with alpha-keto acid activation occurring through a thiohemiacetal, These results indicate that Cys-78 serves as both the regulatory sulfhydryl/disulfide and the site of activation by alpha-keto acids. In light of these results, the previously observed effects of sulfhydryl reagents on the alternative oxidase of isolated soybean mitochondria were re-examined and were found to be in agreement with a single sulfhydryl residue being the site both of alpha-keto acid activation and of the regulatory sulfhydryl/disulfide system.