Cystathionine β-synthase (CBS) is a key enzyme in human (patho)physiology with a central role in hydrogen sulfide metabolism. The enzyme is composed of a pyridoxal 5′-phosphate- binding catalytic domain, flanked by the following two domains: a heme-binding N-terminal domain and a regulatory C-terminal domain binding S-adenosyl-L-methionine (AdoMet). CO or NO· binding at the ferrous heme negatively modulates the enzyme activity. Conversely, AdoMet binding stimulates CBS activity. Here, we provide experimental evidence for a functional communication between the two domains. We report that AdoMet binding significantly enhances CBS inhibition by CO. Consistently, we observed increased affinity (∼5-fold) and faster association (∼10-fold) of CO to the ferrous heme at physiological AdoMet concentrations. NO· binding to reduced CBS was also enhanced by AdoMet, although to a lesser extent (∼2-fold higher affinity) as compared with CO. Importantly, CO and NO· binding was unchanged by AdoMet in a truncated form of CBS lacking the C-terminal regulatory domain. These unprecedented observations demonstrate that CBS activation by AdoMet puzzlingly sensitizes the enzyme toward inhibition by exogenous ligands, like CO and NO·. This further supports the notion that CBS regulation is a complex process, involving the concerted action of multiple physiologically relevant effectors.