The redox properties of the unique heme in cystathionine β-synthase

Cystathionine β-synthase (CBS) catalyzes the condensation of homocysteine with serine or cysteine to form cystathionine and H 2 O or H 2 S. Human CBS has a non-catalytic heme bound to cysteine and histidine. The heme environment is conserved among several eukaryotic species but does not resemble any other protein. Fe(III)-CBS is relatively unreactive; however, reduction to Fe(II)-CBS can be achieved by strong chemical and biochemical reductants. Once reduced, Fe(II)-CBS can react with CO, which leads to a carbonylated and inactive species. Fe(II)-CBS can also be reoxidized by oxygen back to Fe(III)-CBS, thus forming superoxide radical. Kinetic considerations favor reoxidation unless this occurs under very low oxygen concentrations.