A multifunctional enzyme: catalyzes b-replacement reaction between L-serine, L-cysteine, cysteine thioethers or some other beta- substituted alpha-L-amino acids and a variety of mercaptans.

Evolutionary related to cysteine synthase(EC 4.2.99.8), a pyridoxal-phosphate dependent enzyme responsible for the formation of cysteine from O-acetyl-serine and hydrogen sulfide with the concomitant release of acetic acid. In bacteria such as Escherichia coli, two forms of the enzyme are known. In plants there are also two forms, one located in the cytoplasm and the other in chloroplasts. The pyridoxal-phosphate group of CSases has been shown to be attached to a lysine residue which is located in the N-terminal section of these enzymes; the sequence around this residue is highly conserved and can be used as a signature pattern to detect this class of enzymes.
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