Comparative study of enzyme activity and heme reactivity in Drosophila melanogaster and Homo sapiens cystathionine beta-synthases

Cystathionine β-synthase (CBS) is the first and rate-limiting enzyme in the transsulfuration pathway, which is critical for eukaryotes to synthesize cysteine from methionine. CBS uses a coenzyme pyridoxal-5′-phosphate (PLP) for catalysis and S-adenosylmethionine regulates the activity of human CBS, but not yeast CBS. Human and fruit fly CBS contain heme; however, the role for heme is not clear. This paper reports biochemical and spectroscopic characterization of CBS from fruit fly Drosophila melanogaster (DmCBS) and the CO/NO gas binding reactions of DmCBS and human CBS. Like CBS enzymes from lower organisms (e.g. yeast), DmCBS is intrinsically highly active and is not regulated by AdoMet. The DmCBS heme coordination environment, reactivity and the accompanying effects on enzyme activity are similar to those of human CBS. The DmCBS heme bears histidine and cysteine axial ligands, and the enzyme becomes inactive when the cysteine ligand is replaced. The Fe(II)heme in DmCBS is less stable than that in human CBS, undergoing more facile reoxidation and ligand exchange. In both CBS proteins, the overall stability of the protein is correlated with the heme oxidation state. Human and DmCBS Fe(II) hemes react relatively slowly with CO and NO, and the rate of the CO-binding reaction is faster at low pH than at high pH. Together the results suggest that heme incorporation and AdoMet regulation in CBS are not correlated, possibly providing two independent means to regulate the enzyme.