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 (PubMed:23981774, PubMed:20506325, PubMed:23974653). Also involved in the production of hydrogen sulfide, a gasotransmitter with signaling and cytoprotective effects on neurons (By similarity). L-homocysteine + L-serine = H2O + L,L-cystathionine Allosterically activated by S-adenosyl-methionine/AdoMet. Activated by S-adenosylhomocysteine/AdoHcy (PubMed:20506325). Binds non-covalently to a heme group that may control the redox sensitivity of the enzyme (PubMed:11483494, PubMed:12173932, PubMed:22738154). Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-homocysteine and L-serine: step 1/2. Homotetramer. In the adult strongly expressed in liver and pancreas, some expression in heart and brain, weak expression in lung and kidney. In the fetus, expressed in brain, liver and kidney. The disease is caused by variants affecting the gene represented in this entry. Belongs to the cysteine synthase/cystathionine beta-synthase family.