Catalyzes the conversion of O-acetyl-L-homoserine (OAH) into homocysteine in the methionine biosynthesis pathway. Has weak activity with O-acetyl-L-serine, O-phospho-L-serine, L-serine, O-succinyl-L-homoserine and L-homoserine. Shows a very low CTT gamma-synthase activity. hydrogen sulfide + O-acetyl-L-homoserine = acetate + L-homocysteine Inhibited by the carbonyl reagents hydroxylamine and phenylhydrazine. Also inhibited by methionine and propargylglycine. Optimum pH is 7.5. Optimum temperature is 70 degrees Celsius. Stable at high temperature. 80% of the activity is retained after treatment at 90 degrees Celsius for 60 min. Homotetramer. Belongs to the trans-sulfuration enzymes family.