Toward the mechanistic understanding of enzymatic CO2 reduction, A.R. Oliveira (a), C. Mota (b), C. Mourato (a), R.M. Domingos (a), M.F. Santos (b), D. Gesto (b), B. Guigliarelli (c), T. Santos-Silva (b), M.J. Romão (b) and
I.A. Pereira (a), ACS Catal. 10, 3844- 3856 (2020); https://doi.org/10.1021/ acscatal.0c00086. (a) Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras (Portugal)
(b) UCIBIO, Applied Molecular Biosciences Unit, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica (Portugal) (c) Aix Marseille Université, CNRS, Marseille (France)
 M.J. Romão, Dalt. Trans. 21, 4053-4068 (2009).  I.A. Pereira, Science 342, 6164, 1329-1330 (2013).  S.M. da Silva et al., Microbiol. 159, 8, 1760-1769 (2013).  S.M. da Silva et al., J. Bacteriol. 193, 12, 2909-2916 (2011).
PRINCIPAL PUBLICATION AND AUTHORS
of an adjacent α-helix (Ile191-Pro198) suffer a distortion; the Sec192 Cα atom shifts up to 1.00 Å from the position occupied in the oxidised form. However, Sec192 remains coordinated to the metal (Figure 39b), supporting a mechanism of stable metal coordination during catalysis. Remarkably, the catalytically relevant His193 moves away from the active site, adopting a different rotamer, and a water molecule occupies the previous position of the imidazole ring, 3.18 Å away from Se (Figure 39b).
Conformational changes are also observed for the two MGD cofactors and surrounding environment. In the oxidised form, Gln890 is in close contact to MGD1, via a hydrogen bond to N5 of the piperazine central ring (Figure 39a). Additionally, Gln890 -NH2 is a H-bond donor to one of the dithiolene sulfur atoms, and to the carboxylate of Glu443. In the reduced form, Gln890 adopts a new rotamer, and the interaction with the dithiolene sulfur atom is maintained, but the carboxyl group is now stabilised by an interaction with a water
molecule present only in the reduced structure (Figure 39b). This water molecule hydrogen- bonds the dinucleotide moiety from MGD2, promoting a major distortion of the ribose. The reorientation of Gln890 suggests a putative change in the redox state of the proximal pterin (tetrahydro into dihydro oxidation states). The structural changes observed in FdhAB, together with mutagenesis data on other DMSOR family enzymes, show the importance of the H-bond interactions bridging both pyranopterins in controlling the active site redox chemistry and catalysis, highlighting that computational predictions of enzymatic mechanisms should take into account the putative redox role of the MGD cofactors.
The FdhAB from D. vulgaris is an excellent model for studying catalytic CO2 reduction and probing the mechanism of this conversion. Additional studies with future variants and new crystal structures will enable the production of improved engineered forms of this important enzyme.
Fig. 39: Active site of D. vulgaris FdhAB. a) Close-up view of the FdhAB active site in the oxidised form. b) Close-up view of the FdhAB active site in the formate-reduced form.
In both cases, 2Fo-Fc electron density map is contoured at the 1σ level.