Redox activity of some non-innocent amino acids

Constraints on the Radical Cation Center of Cytochrome c Peroxidase for Electron Transfer from Cytochrome c

“…This difference in potential is consistent with measurements of the isolated moieties: E°(ZnP°/ZnP + ) = 1.2 V, 15 whereas E°(W°/ 45,81,90,91 It is worth noting that the actual reduction potentials of W191 •+ and ZnP + in CcP are likely less than these values. W •+ potentials usually exceed 1 V, 45,81,90,91 and most peroxidase Cpd II [Fe(IV)O] potentials are >0.9 V; 92,93 however, in WT CcP:Cc, the two-electron couple 94,95 Thus, the protein environment may substantially lower the reduction potentials of the 191 side chain and the porphyrin moiety. 96,97 Importantly, a lowered potential for W •+ is still consistent with a very small population of the charge-separated intermediate in the WT ZnCcP:Cc system.…”

“…This difference in potential is consistent with measurements of the isolated moieties: E °(ZnP°/ZnP + ) = 1.2 V, whereas E °(W°/W •+ ) = 1.1–1.4 V. ,,, It is worth noting that the actual reduction potentials of W191 •+ and ZnP + in CcP are likely less than these values. W •+ potentials usually exceed 1 V, ,,, and most peroxidase Cpd II [Fe­(IV)O] potentials are >0.9 V; , however, in WT CcP:Cc, the two-electron couple E °[W •+ Fe­(IV)/W°Fe­(III)] = 1 / 2 { E °(W •+ /W°) + E °[Fe­(IV)/Fe­(III)]} = 0.740 V. , Thus, the protein environment may substantially lower the reduction potentials of the 191 side chain and the porphyrin moiety. , Importantly, a lowered potential for W •+ is still consistent with a very small population of the charge-separated intermediate in the WT ZnCcP:Cc system. Provided that the reduction potential of the hopping site remains more than ∼200 mV higher than that of the donor Cc­(II) [ E °(Cc) = 290 mV; , i.e., Δ G = −200 mV], the standard Marcus equation { k ∼ k eb (W •+ ) exp­[−(λ + Δ G ) 2 /4 λkT ]} predicts that the back ET rate to a 191 radical will remain ∼100 times higher than the forward rate of Cc­(III) reduction [∼10 2 s –1 (Table )].…”

“…For solution and crystalline complexes of ZnCcP with Cc(III), k obs ≥ 4000 s −1 , 12,18,89 and thus, eq 2 implies that 120 mV < ΔE°< 180 mV. This difference in potential is consistent with measurements of the isolated moieties: E°(ZnP°/ZnP + ) = 1.2 V, 15 whereas E°(W°/ 45,81,90,91 It is worth noting that the actual reduction potentials of W191 •+ and ZnP + in CcP are likely less than these values. W •+ potentials usually exceed 1 V, 45,81,90,91 and most peroxidase Cpd II [Fe(IV)O] potentials are >0.9 V; 92,93 however, in WT CcP:Cc, the two-electron couple 94,95 Thus, the protein environment may substantially lower the reduction potentials of the 191 side chain and the porphyrin moiety.…”

“…First, although, E o (Y •+ /Y o ) will likely be comparable to E o (W •+ /W o ); 43, 90, 91, 100–102 even a redox potential increase of 50–100 mV over that of Trp would decrease k eb 1 to the range of k eb 2 . Unlike Trp191, which hydrogen bonds to Asp235, there is no hydrogen-bond acceptor for the Tyr hydroxyl and thus the Tyr redox potential may not be suitably reduced by the protein environment.…”

“…With the crystal structure of the W191Y complex ruling out any substantial structural perturbations caused by the substitution, the inability of Tyr191 to accelerate back ET like Trp may derive from one of the two considerations mentioned previously. First, although E °(Y •+ /Y°) will likely be comparable to E °(W •+ /W°), ,,,− even a redox potential increase of 50–100 mV over that of Trp would decrease k eb 1 to the range of k eb 2 . Unlike Trp191, which hydrogen bonds to Asp235, there is no hydrogen bond acceptor for the Tyr hydroxyl, and thus, the Tyr redox potential may not be suitably reduced by the protein environment.…”