EPR spectroscopic studies of the Fe-S clusters in the O2-tolerant [NiFe]-hydrogenase Hyd-1 from Escherichia coli and characterization of the unique [4Fe-3S] cluster by HYSCORE.
The unusual [4Fe-3S] cluster proximal to the active site plays a crucial role in allowing a class of [NiFe]-hydrogenases to function in the presence of O(2) through its unique ability to undergo two rapid, consecutive one-electron transfers. This property helps to neutralize reactive oxygen species. Mechanistic details and the role of the medial and distal clusters remain unresolved. To probe the Fe-S relay, continuous wave and pulse electron paramagnetic resonance (EPR) studies were conducted on the O(2)-tolerant hydrogenase from Escherichia coli (Hyd-1) and three variants with point mutations at the proximal and/or medial clusters. Reduction potentials of the proximal ([4Fe-3S](5+/4+/3+)) and medial ([3Fe-4S](+/0)) clusters were determined by potentiometry. The medial [3Fe-4S](+/0) reduction potential is exceptionally high, implicating a mechanistic role in O(2)-tolerance. Numerous experiments establish that the distal cluster has a ground state S > 1/2 in all three variants and indicate that this is also the case for native Hyd-1. Concurrent with the Hyd-1 crystal structure, EPR data for the 'superoxidized' P242C variant, in which the medial cluster is 'magnetically silenced', reveal two conformations of the proximal [4Fe-3S](5+) cluster, and X-band HYSCORE spectroscopy shows two (14)N hyperfine couplings attributed to one conformer. The largest, A((14)N) = [11.5,11.5,16.0] ± 1.5 MHz, characterizes the unusual bond between one Fe (Fe(4)) and the backbone amide-N of cysteine-20. The second, A((14)N) = [2.8,4.6,3.5] ± 0.3 MHz, is assigned to N(C19). The (14)N hyperfine couplings are conclusive evidence that Fe(4) is a valence-localized Fe(3+) in the superoxidized state, whose formation permits an additional electron to be transferred rapidly back to the active site during O(2) attack.
Department of Chemistry, Oxford University, South Parks Road, OX1 3QR Oxford, United Kingdom.
Maxie M
MM
Roessler
Rhiannon M
RM
Evans
Rosalind A
RA
Davies
Jeffrey
J
Harmer
Fraser A
FA
Armstrong
D000222:Adaptation, Physiological; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology*; D006864:Hydrogenase / Q000378:metabolism*; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D010100:Oxygen / Q000378:metabolism
D007506:Iron-Sulfur Proteins; C043296:nickel-iron hydrogenase; D006864:Hydrogenase; D010100:Oxygen