Residue mutations in [Fe-Fe]-hydrogenase impedes O_2 binding: A QM/MM investigation

摘要

[Fe-Fe]-hydrogenases are enzymes that reversibly catalyze the reaction of protons and electrons to molecular hydrogen, which occurs in anaerobic media. In living systems, [Fe-Fe]-hydrogenases are mostly used for H_2 production. The [Fe-Fe]-hydrogenase H-cluster is the active site, which contains two iron atoms. The latest theoretical investigations (Tye et al., Inorg Chem, 2008, 47, 2380; Liu and Hu, J Am Chem Soc, 2002, 124, 5175) advocate that the structure of di-iron air inhibited species are either Fe_p~(II)-Fe_d~(II)-O-H~-, or Fe _p~(II)-Fe_d~(II)-O-O-H, thus O_2 has to be prevented from binding to Fed in all di-iron subcluster oxidation states in order to retain a catalytically active enzyme. By performing residue mutations1 on [Fe-Fe]- hydrogenases, we were able to weaken O_2 binding to distal iron (Fe_d) of Desulfovibrio desulfuricans hydrogenase (DdH). Individual residue deletions were carried out in the 8 ? apoenzyme layer radially outward from Fed to determine what residue substitutions should be made to weaken O_2 binding. Residue deletions and substitutions were performed for three di-iron subcluster oxidation states, Fe_p~(II)-Fe_d~(II), Fe_p~(II)-Fe_d~I, and Fe_p~I-Fe _d~I of [Fe-Fe]-hydrogenase. Two deletions (ΔThr ~(152) and ΔSer~(202)) were found most effective in weakening O_2 binding to Fed in Fe_p~(II)-Fe _d~I hydrogenase (ΔG_(QM/MM) = +5.4 kcal/mol). An increase in Gibbs energy (+2.2 and +4.4 kcal/mol) has also been found for Fe_p~(II)-Fe_d~(II) and Fe_p~I-Fe_d~I hydrogenase, respectively. π-backdonation.