National Institutes of Health/National Institute of General Medical Sciences (NIH/NIGMS)
GM100008
United States
National Institutes of Health/National Institute of General Medical Sciences (NIH/NIGMS)
GM124847
United States
Citation
Journal: Proc Natl Acad Sci U S A / Year: 2023 Title: Conformational switching and flexibility in cobalamin-dependent methionine synthase studied by small-angle X-ray scattering and cryoelectron microscopy. Authors: Maxwell B Watkins / Haoyue Wang / Audrey Burnim / Nozomi Ando / Abstract: Cobalamin-dependent methionine synthase (MetH) catalyzes the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate (CH-Hfolate) using the unique chemistry of its cofactor. In doing ...Cobalamin-dependent methionine synthase (MetH) catalyzes the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate (CH-Hfolate) using the unique chemistry of its cofactor. In doing so, MetH links the cycling of -adenosylmethionine with the folate cycle in one-carbon metabolism. Extensive biochemical and structural studies on 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 MetH and a thermophilic homolog from 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-Hfolate and flavodoxin in initiating turnover and reactivation. By combining SAXS with a 3.6-Å cryo-EM structure of the 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.
Details: 50 mM HEPES, 150 mM NaCl, 2.5 mM DTT pH 7.6
Grid
Model: Quantifoil R1.2/1.3 / Material: COPPER / Mesh: 300 / Support film - Material: CARBON / Support film - topology: CONTINUOUS / Pretreatment - Type: GLOW DISCHARGE
Vitrification
Cryogen name: ETHANE-PROPANE / Chamber humidity: 100 % / Chamber temperature: 277 K / Instrument: FEI VITROBOT MARK IV
Details
Sample was mixed with equimolar commercial horse spleen apoferritin for freezing.
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Electron microscopy
Microscope
FEI TITAN KRIOS
Specialist optics
Energy filter - Name: GIF Bioquantum / Energy filter - Slit width: 30 eV
Image recording
Film or detector model: GATAN K3 (6k x 4k) / Digitization - Dimensions - Width: 5760 pixel / Digitization - Dimensions - Height: 5092 pixel / Number grids imaged: 2 / Number real images: 12768 / Average exposure time: 2.5 sec. / Average electron dose: 65.0 e/Å2 Details: Images were collected in movie mode with a total of 50 frames over 2.5 seconds.
Electron beam
Acceleration voltage: 300 kV / Electron source: FIELD EMISSION GUN
Type of model: INSILICO MODEL In silico model: cryoSPARC ab-initio model generation (with 3 models, followed by 3D classification)
Final reconstruction
Number classes used: 1 / Resolution.type: BY AUTHOR / Resolution: 3.6 Å / Resolution method: FSC 0.143 CUT-OFF / Software - Name: cryoSPARC (ver. 3.2) Software - details: Non-uniform refinement in cryosparc (without per-particle defocus or CTF refinement) Number images used: 257706
Initial angle assignment
Type: MAXIMUM LIKELIHOOD / Software - Name: cryoSPARC (ver. 3.2)
Final angle assignment
Type: MAXIMUM LIKELIHOOD / Software - Name: cryoSPARC (ver. 3.2)
Final 3D classification
Number classes: 2 / Avg.num./class: 200000 / Software - Name: cryoSPARC (ver. 3.2) Details: Initial 3D classification of particles into class of interest and two junk classes, final classification of particles between 2-domain (incomplete) and 3-domain (complete) classes
Chain - Chain ID: A / Chain - Residue range: 2-896 / Chain - Source name: AlphaFold / Chain - Initial model type: in silico model
Details
Initial model used was the 3 N-terminal domains of AlphaFold database model A0A1Q9SZ17. Each domain was fit individually by rigid-body fitting in Chimera. The cobalamin ligand was initially fit by alignment of the 1BMT crystal structure to the appropriate binding region.
Refinement
Space: REAL / Protocol: RIGID BODY FIT
Output model
PDB-8g3h: Structure of cobalamin-dependent methionine synthase (MetH) in a resting state
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