The cyanoalanine synthesis reaction is more efficient than the cysteine synthase activity. Probably involved in the detoxification of cyanide that arises from ethylene biosynthesis. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine hydrogen cyanide + L-cysteine = 3-cyano-L-alanine + H(+) + hydrogen sulfide Optimum pH is 10.0. Expressed in green and etiolated seedlings. Belongs to the cysteine synthase/cystathionine beta-synthase family.