Architecture and regulation of filamentous human cystathionine beta-synthase

Cystathionine beta-synthase (CBS) is an essential metabolic enzyme across all domains of life involved in the production of glutathione, cysteine, and hydrogen sulphide 1–4 . Human CBS appends to its conserved catalytic domain a regulatory domain that modulates activity by S-adenosyl-L-methionine (SAM) and promotes oligomerization 5–12 , however the molecular basis is unknown. Here we show using cryo-electron microscopy that full-length human CBS in the basal and SAM-bound activated states polymerises as filaments mediated by a conserved regulatory domain loop. In the basal state, CBS regulatory domains sterically block the catalytic domain active site, resulting in a low activity filament with three CBS dimers per turn. This steric block is removed when in the activated state, one molecule of SAM binds to the regulatory domain, forming a high activity filament with two CBS dimers per turn. These large conformational changes result in a central filament of SAM stabilised regulatory domains at the core, decorated with highly flexible catalytic domains. Polymerization stabilises CBS and increases the cooperativity of allosteric activation by SAM. Together our findings elaborate our understanding of CBS enzyme regulation, and open new avenues for investigating the pathogenic mechanism and therapeutic opportunities for CBS-associated disorders 3,13–17 .