Abstract
Transition metals are essential elements for living organisms, as they provide the enzymes that use them with novel catalytic properties that are difficult or impossible to access through organic chemistry. This is why they are found at the heart of key reactions in living organisms, notably in energy metabolism. Similarly, transition metals are essential to the metabolism of gases (H2,CO2, N2) by microorganisms[1] and the inorganic cofactors that serve as active sites for these enzymes (hydrogenase, carbon monoxide dehydrogenase or nitrogenase) have suggested, through their resemblance to the structure of a number of minerals, a mineral origin of living organisms through surface catalysis that would have been progressively integrated into peptides[2]. Today, these metal centers are integrated into enzymes developed through the combined intervention of proteins grouped within specific assembly machineries. Thus, in the case of FeFe hydrogenase, at least three proteins are required for the production and insertion of a binuclear iron center called [2Fe]H containing carbon monoxide molecules, cyanide ions and an azadithiolate molecule[3]. In the case of nitrogenase, the active site corresponds to a [Fe7S9CMo] center linked to an R-homocitrate molecule. No fewer than 12 proteins of the Nif machinery are required to produce this complex metal center and insert it into the enzyme[4].
Among the proteins of these machineries are enzymes commonly referred to as " SAM radical proteins " because they use S-adenosyl-L-methionine(SAM) to catalyze radical reactions that are often complex and unparalleled in two-electron polar chemistry. These enzymes use the reduction of a [Fe4S4] center to cleave SAM and transiently generate a highly reactive 5'-deoxyadenosyl radical species which, in turn, triggers the radical reaction[5].
In the case of FeFe hydrogenase active site assembly, the HydG and HydE proteins are members of this protein superfamily. HydG uses L-tyrosine as a substrate to produce, via a complex mechanism, the CO and CN ligands in the form of a precursor called " complex-B ", corresponding to a L-cysteine-FeII(CO)2CNspecies[6]. The latter then serves as a substrate for the HydE protein, also a member of the SAM radical protein family, to continue its transformation to the [2Fe]H compound [7, 8]. In the case of nitrogenase, the NifB protein, also a member of the SAM radical protein family, is the key enzyme in the process, catalyzing the free-radical fusion of two [Fe4S4] centers, combined with the insertion of a carbide ion and a sulfide ion to produce a [Fe8S9C] center known as " NifB-co ", the precursor of the nitrogenase active site. Recent structural results have led to a better understanding of the various steps involved in this reaction [9, 10]. The seminar presented this work, as well as our latest results on the structure-function relationships of these enzymes.
References
[1] Fontecilla-Camps J.C., Amara P., Cavazza C., Nicolet Y. and Volbeda A., " Structure-function relationships of anaerobic gas-processing metalloenzymes ", Nature, vol. 460, 2009, pp. 814-822, https://doi.org/10.1038/nature08299.
[2] Wachtershauser G., " Before enzymes and templates: Theory of surface metabolism ", Microbiology and Molecular Biology Reviews, vol. 52, 1988, pp. 452-484, https://doi.org/10.1128/mr.52.4.452-484.1988.
[3] Britt R.D., Rao G. and Tao L., "Biosynthesis of the catalytic H-cluster of [FeFe] hydrogenase: The roles of the Fe-S maturase proteins HydE, HydF, and HydG", Chemical Science, vol. 11,no. 38, 2020, pp. 10313-10323, https://doi.org/10.1039/D0SC04216A.
[4] Buren S., Jimenez-Vicente E., Echavarri-Erasun C. and Rubio L.M., " Biosynthesis of nitrogenase cofactors ", Chemical Reviews, vol. 120, 2020, pp. 4921-4968, https://doi.org/10.1021/acs.chemrev.9b00489.
[5] Broderick J.B., Duffus B.R., Duschene K.S. and Shepard E.M., " Radical S-adenosylmethionine enzymes ", Chemical Reviews, vol. 114, 2014, pp. 4229-4317, https://doi.org/10.1021/cr4004709.
[6] Rao G., Tao L., Suess D.L.M. and Britt R.D.A., " [4Fe-4S]-Fe(CO)(CN)-L-cysteine intermediate is the first organometallic precursor in [FeFe] hydrogenase H-cluster bioassembly ", Nature Chemistry, vol. 10, 2018, pp. 555-560, https://doi.org/10.1038/s41557-018-0026-7.
[7] Tao L., Pattenaude S.A., Joshi S., Begley T.P., Rauchfuss T.B. and Britt R.D., " Radical SAM enzyme HydE generates adenosylated Fe(I) intermediates en route to the [FeFe]-hydrogenase catalytic H-cluster ", Journal of the American Chemical Society, vol. 142,no. 24, 2020, pp. 10841-10848, https://doi.org/10.1021/jacs.0c03802.
[8] Rohac R., Martin L., Liu L., Basu D., Tao L, Britt R.D., Rauchfuss T.B. and Nicolet Y., " Crystal structure of the [FeFe]-hydrogenase maturase HydE bound to Complex-B ", Journal of the American Chemical Society, vol. 143,no. 22, 2021, pp. 8499-8508, https://doi.org/10.1021/jacs.1c03367.
[9] Fajardo A.S., Legrand P., Payá-Tormo L., Martin L., Pellicer Martinez M.T., Echavarri-Erasun C., Vernède X., Rubio L.M. and Nicolet Y., " Structural insights into the mechanism of the Radical SAM carbide synthase NifB, a key nitrogenase cofactor maturating enzyme ", Journal of the American Chemical Society, vol. 142,no. 25, 2020, pp. 11006-11012, https://doi.org/10.1021/jacs.0c02243.
[10] Jenner L.P., Cherrier M.V., Amara P., Rubio L.M. and Nicolet Y., "An unexpected P-cluster like intermediate en route to the nitrogenase FeMo-co", Chemical Science, vol. 12, 2021, pp. 5269-5274, https://doi.org/10.1039/D1SC00289A.
