Coordination chemistry of the heme in cystathionine beta-synthase: Formation of iron(II)-isonitrile complexes

Interaction of rat and human cystathionine-beta -synthase (CBS) with variou s potential ligands has been studied by visible and EPR spectroscopy in ord er to explore the coordination chemistry of this atypical hemeprotein, Ferr ic CBS did not react with any classical hemeprotein ligands, such as variou s imidazole and pyridine derivatives, N-3(-) and isonitriles RNC. Ferrous C BS also failed to bind these nitrogenous ligands or nitrosoalkanes. However , it reacts with various isonitriles RNC, leading to complexes characterize d by a Soret peak at 433 +/- 2 nm. Binding of isonitriles to ferrous CBS is a relatively slow process; its rate markedly depends on the nature of R. I t thus seems that the only exogenous ligands able to bind CBS iron are carb on-centered, very strong heme-Fe(II) ligands such as CNR, CO, and CN-, pres umably after dissociation of the CBS-iron(II)-cysteinate bond. Isonitriles appear as interesting tools for further studies on the topology of CBS acti ve site. (C) 2001 Academic Press.