Two cysteine synthase enzymes are found. Both catalyze the same reaction. Cysteine synthase B can also use thiosulfate in place of sulfide to give cysteine thiosulfonate as a product. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer. Belongs to the cysteine synthase/cystathionine beta-synthase family.