Structure and reactivity of cysteine residues in mitochondrial serine hydroxymethyltransferase

Six cysteine-containing tryptic peptides were isolated and sequenced from rabbit liver mitochondrial serine hydroxymethyltransferase. 2 of the 6 cysteine residues were located on the surface of the enzyme. These 2 cysteine residues are sufficiently close to each other that they form a disulfide bond when oxidized by periodate. Another of the cysteine residues is exposed upon removal of the active site pyridoxal phosphate. The remaining three cysteines are buried and react with sulfhydryl reagents only when the enzyme is denatured. Blocking of one of the surface sulfhydryls with any of several sulfhydryl reagents results in increased catalytic activity when allothreonine is the substrate but decreased activity when serine and tetrahydrofolate are the substrates. The activation and inhibition effects are on Vmax and not on the affinity of the enzyme for its substrates. Of the six cysteine peptides from the mitochondrial enzyme, three show substantial homology with cysteine-containing peptides from the cytosolic form of the enzyme. For both enzyme forms, one of these homologous pairs is a cysteine residue on the surface of the enzyme. These results suggest that the mitochondrial and cytosolic forms of rabbit liver serine hydroxymethyltransferases are the products of separate genes.