How do enzymes reduce metals? The mechanism of the reduction of Cr(VI) in chromate by cytochrome c(7) proteins proposed from DFT calculations

Various bacteria are effective in metal reduction, and there is an increasing use of such micro-organisms for decontaminating polluted environments. Iron-containing electron transfer proteins, particularly those of the cytochrome c(7) family, can bind a number of toxic metals in their high oxidation states, and can reduce them via electron transfer mechanisms. We report a computational investigation of the binding of CrO42- to the cytochrome c(7) of Desulfuromonas acetoxidans and explore possible mechanisms for the subsequent reduction of Cr(VI) to Cr(III). Our modelling strategy is to identify the binding site of D. acetoxidans for the chromate di-anion, and to use this structure as a starting point to generate realistic models for DFT calculations of the structures and energetics of species along the pathway for reduction. We address the following aspects of the mechanism: (i) How do the neighbouring residues, particularly the nearby lysines, modulate the reduction process? (ii) What is the speciation of chromium as the oxidation state is reduced from VI? (iii) How is the electron transfer made energetically feasible, considering the initial species (chromate) has a high negative charge? We suggest that both electron transfer from the heme and proton transfer from the lysines occur, followed by a disproportionation mechanism involving Cr(V). This mechanism is compared with our proposed mechanism for the reduction of actinyl species.