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:20506325, PubMed:23974653, PubMed:23981774). Also involved in the production of hydrogen sulfide, a gasotransmitter with signaling and cytoprotective effects on neurons (By similarity). {ECO:0000250|UniProtKB:P32232, ECO:0000269|PubMed:20506325, ECO:0000269|PubMed:23974653, ECO:0000269|PubMed:23981774}. This is the function of ENSG00000160200 (CBS, cystathionine beta-synthase).