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. 2016 Jun;89(6):645-51.
doi: 10.1124/mol.116.103382. Epub 2016 Apr 1.

Using Cryo-EM to Map Small Ligands on Dynamic Metabolic Enzymes: Studies with Glutamate Dehydrogenase

Mario J Borgnia  1 , Soojay Banerjee  1 , Alan Merk  1 , Doreen Matthies  1 , Alberto Bartesaghi  1 , Prashant Rao  1 , Jason Pierson  1 , Lesley A Earl  1 , Veronica Falconieri  1 , Sriram Subramaniam  2 , Jacqueline L S Milne  2
Affiliations

Using Cryo-EM to Map Small Ligands on Dynamic Metabolic Enzymes: Studies with Glutamate Dehydrogenase

Mario J Borgnia et al. Mol Pharmacol. 2016 Jun.

Abstract

Cryo-electron microscopy (cryo-EM) methods are now being used to determine structures at near-atomic resolution and have great promise in molecular pharmacology, especially in the context of mapping the binding of small-molecule ligands to protein complexes that display conformational flexibility. We illustrate this here using glutamate dehydrogenase (GDH), a 336-kDa metabolic enzyme that catalyzes the oxidative deamination of glutamate. Dysregulation of GDH leads to a variety of metabolic and neurologic disorders. Here, we report near-atomic resolution cryo-EM structures, at resolutions ranging from 3.2 Å to 3.6 Å for GDH complexes, including complexes for which crystal structures are not available. We show that the binding of the coenzyme NADH alone or in concert with GTP results in a binary mixture in which the enzyme is in either an "open" or "closed" state. Whereas the structure of NADH in the active site is similar between the open and closed states, it is unexpectedly different at the regulatory site. Our studies thus demonstrate that even in instances when there is considerable structural information available from X-ray crystallography, cryo-EM methods can provide useful complementary insights into regulatory mechanisms for dynamic protein complexes.

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Figures

Fig. 1.
Fig. 1.
Structure and quaternary conformational changes in GDH. (A) Views of open (PDB ID 1NR7) and closed (PDB 3MW9) states of the GDH hexamer, shown in ribbon representation perpendicular to the 2-fold symmetry axis (side view, top) and 3-fold symmetry axis (top view, bottom). Only three protomers are shown in the top view for purposes of visual clarity. The dashed lines and arrows, respectively, highlight the slight extension in length, and twist in shape that occurs with transition from open to the closed state. The open state shown is for unliganded GDH, whereas the closed state has NADH, GTP, and glutamate bound. (B) Superposition of structures for closed and open conformations, along with a series of possible intermediate conformations along the trajectory that serve to illustrate the extent of change in structure across different regions of the protein.
Fig. 2.
Fig. 2.
Cryo-EM structures of GDH in unliganded and NADH-bound states. (A) Refined cryo-EM map of unliganded GDH at ∼3 Å resolution. (B, C) Illustration of density map in the regions that contain the Rossmann nucleotide binding fold (B), pivot and antenna helices (C) in the unliganded GDH map. (D) Cryo-EM-derived density maps for two coexisting conformations that are present when GDH is bound to the cofactor NADH. Each protomer is shown in a different color and densities for NADH bound in both regulatory (red) and catalytic (purple) sites on one protomer are indicated. The overall quaternary structures of the two conformations are essentially the same as that of the open and closed states observed by X-ray crystallography.
Fig. 3.
Fig. 3.
Detailed view of NADH conformation in catalytic and regulatory sites. (A, B) NADH density (purple) and interactions in the catalytic sites of closed (A) and open (B) states. (C, D) NADH density (red) and interactions in the regulatory sites of closed (C) and open (D) states.
Fig. 4.
Fig. 4.
Cryo-EM structure of GDH bound to both NADH and GTP. (A, B) Observation of co-existing open (A) and closed (B) conformations in the GDH-NADH-GTP ternary complex. Densities for GTP (yellow) as well as NADH bound to both catalytic (purple) and regulatory (red) sites in each protomer are shown. (C, D) Detailed inspection of the interactions near the regulatory site show that the orientation of His209 switches between the two states, which may allow interactions with bound GTP in the closed (D), but not open (C) conformation.
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