Hydro-lyase catalyzing the first step of the transsulfuration pathway, where the hydroxyl group of L-serine is displaced by L-homocysteine in a beta-replacement reaction to form L-cystathionine, the precursor of L-cysteine. This catabolic route allows the elimination of L-methionine and the toxic metabolite L-homocysteine (By similarity). Also involved in the production of hydrogen sulfide, a gasotransmitter with signaling and cytoprotective effects on neurons (PubMed:20149843, PubMed:8558235). L-homocysteine + L-serine = H2O + L,L-cystathionine Allosterically activated by S-adenosyl-methionine/AdoMet. Activated by S-adenosylhomocysteine/AdoHcy. Binds non-covalently to a heme group that may control the redox sensitivity of the enzyme. Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-homocysteine and L-serine: step 1/2. Homotetramer. Expressed in liver, kidney and brain. Highly expressed in the hippocamus and cerebellum. Belongs to the cysteine synthase/cystathionine beta-synthase family.