Catalyzes the formation of L-cystathionine from O-succinyl-L-homoserine (OSHS) and L-cysteine, via a gamma-replacement reaction (By similarity). In the absence of thiol, catalyzes gamma-elimination to form 2-oxobutanoate, succinate and ammonia. L-cysteine + O-succinyl-L-homoserine = H(+) + L,L-cystathionine + succinate Binds 1 pyridoxal phosphate per subunit. Four natural products, alpha-lapachone, 9-hydroxy-alpha-lapachone, Paulownin, and Yangambin, show strong inhibitory activities against CGS. All these four inhibitors prevent the binding of OSHS to CGS in a non-competitive fashion. These compounds are specific inhibitors against CGS from H.pylori relative to E.coli since they exhibit very low inhibition activities against CGS from E.coli. The KM value was measured when assaying the gamma-elimination reaction. Optimum pH is 7.5. Optimum temperature is 45 degrees Celsius. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-cystathionine from O-succinyl-L-homoserine: step 1/1. Homotetramer. CGS from H.pylori is an attractive target for antibiotics discovery since the four natural inhibitors alpha-lapachone, 9-hydroxy-alpha-lapachone, Paulownin, and Yangambin also display strong antibacterial activity in vitro, highly inhibiting the growth of H.pylori, whereas no antibacterial activity against E.coli in vitro. These inhibitors might thus be used as potential lead compounds for further research in the discovery of anti-H.pylori agents. Belongs to the trans-sulfuration enzymes family.