Cobalamin-dependent methionine synthase (MetH) catalyzes the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate (CH 3 -H 4 folate) using the unique chemistry of its cofactor. In doing so, MetH links the cycling of S -adenosylmethionine with the folate cycle in one-carbon metabolism. Extensive biochemical and structural studies on Escherichia coli  MetH have shown that this flexible, multidomain enzyme adopts two major conformations to prevent a futile cycle of methionine production and consumption. However, as MetH is highly dynamic as well as both a photosensitive and oxygen-sensitive metalloenzyme, it poses special challenges for structural studies, and existing structures have necessarily come from a “divide and conquer” approach. In this study, we investigate E. coli MetH and a thermophilic homolog from Thermus filiformis using small-angle X-ray scattering (SAXS), single-particle cryoelectron microscopy (cryo-EM), and extensive analysis of the AlphaFold2 database to present a structural description of the full-length MetH in its entirety. Using SAXS, we describe a common resting-state conformation shared by both active and inactive oxidation states of MetH and the roles of CH 3 -H 4 folate and flavodoxin in initiating turnover and reactivation. By combining SAXS with a 3.6-Å cryo-EM structure of the T. filiformis MetH, we show that the resting-state conformation consists of a stable arrangement of the catalytic domains that is linked to a highly mobile reactivation domain. Finally, by combining AlphaFold2-guided sequence analysis and our experimental findings, we propose a general model for functional switching in MetH.

Cobalamin-dependent methionine synthase (MetH) catalyzes the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate (CH3-H4folate) using the unique chemistry of its cofactor. In doing so, MetH links the cycling ofS-adenosylmethionine with the folate cycle in one-carbon metabolism. Extensive biochemical and structural studies onEscherichia coliMetH have shown that this flexible, multi-domain enzyme adopts two major conformations to prevent a futile cycle of methionine production and consumption. However, as MetH is highly dynamic as well as both a photosensitive and oxygen-sensitive metalloenzyme, it poses special challenges for structural studies, and existing structures have necessarily come from a "divide and conquer" approach. In this study, we investigateE. coliMetH and a thermophilic homolog fromThermus filiformisusing small-angle X-ray scattering (SAXS), single-particle cryo-electron microscopy (cryo-EM), and extensive analysis of the AlphaFold2 database to present the first structural description of MetH in its entirety. Using SAXS, we describe a common resting-state conformation shared by both active and inactive oxidation states of MetH and the roles of CH3-H4folate and flavodoxin in initiating turnover and reactivation. By combining SAXS with a 3.6-Å cryo-EM structure of theT. filiformisMetH, we show that the resting-state conformation consists of a stable arrangement of the catalytic domains that is linked to a highly mobile reactivation domain. Finally, by combining AlphaFold2-guided sequence analysis and our experimental findings, we propose a general model for functional switching in MetH.