P-376 - Crosstalk between gasotransmitters at the H2S-synthesizing human cystathionine β-synthase: heme-based regulation by CO and NO

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Cystathionine β-synthase (CBS) plays a central role in hydrogen sulfide (H2S) metabolism in human (patho) physiology. Each CBS monomer is composed of a central catalytic domain containing a pyridoxal 5’-phosphate active site flanked by two regulatory domains: an N-terminal domain harboring a b-type heme, and a C-terminal S-adenosyl-L-methionine (AdoMet)-binding domain. CBS is intricately regulated at different levels, its function depending on the redox state and availability of exogenous ligands, such as AdoMet and the gasotransmitters nitric oxide (NO) and carbon monoxide (CO). Herein, we employed static and stopped-flow absorption spectroscopy to investigate the kinetics of CO and NO binding to the CBS heme. NO binds faster and with higher affinity than CO, each gasotransmitter initially binding to opposite sides of the heme plane. AdoMet surprisingly enhances CO inhibition of H2S production, as well as CO and NO binding. In the clinically relevant p.P49L CBS variant, we observed much higher CO affinity with respect to WT CBS, likely resulting from increased flexibility of the heme surroundings. Overall, the binding kinetics attest the in vivo relevance of CO and NO inhibition of CBS-catalyzed H2S production, an underlying mechanism in the crosstalk between gasotransmitters.