Characterization of long-range electron transfer in mixed-metal [zinc,iron] hybrid hemoglobins

Abstract

Measurements characterizing electron transfer from a photoexcited zinc protoporphyrin triplet (3ZnP) to a ferriheme electron acceptor within the [alpha 1,beta 2] electron-transfer complex of [FeIII,Zn] hybrid hemoglobins are reported. Analytical results demonstrate that the hybrids studied are pure, homogeneous proteins with 1:1 ZnP:FeP content. Within the T quaternary structure adopted by these hybrids, the optical spectrum of a FeIIIP is perturbed by the protein environment. Room temperature kinetic studies of the rate of 3ZnP decay as a function of the heme oxidation and ligation state demonstrate that quenching of 3ZnP by FeIII(H2O)P occurs by long-range intramolecular electron transfer with rate constant kt = 100 (+/- 10) s-1 and is not complicated by spin-quenching or energy-transfer processes; results are the same for alpha(Zn) and beta(Zn) hybrids. Replacement of H2O as a ligand to the ferriheme changes the 3ZnP----FeIIIP electron-transfer rate constant, kt, which demonstrates that electron transfer, not conformational conversion, is rate limiting. However, the trend is not readily explained by simple considerations of spin-state and bonding geometry: kt decreases in the order imidazole greater than H2O greater than F- approximately CN- approximately N3-. The reverse electron-transfer process FeIIP----ZnP+ has not been observed directly but has been shown to be much more rapid, with rate constant kb greater than 10(3) s-1, consistent with the possible importance of "hole" superexchange in electron tunneling within protein complexes.

Kinetic-Dynamic Model for Conformational Control of an Electron Transfer Photocycle: Mixed-Metal Hemoglobin Hybrids

“…After removal of bound mercurial with mercaptoethanol, the heme was extracted from the α-chains and the apo-α-protein was reconstituted with ZnP and recombined with the native β-chains to give the [α(ZnP), β(FeCO)] hybrid as described earlier 44-46. The [α(ZnP), β(FeCO)] hybrid was oxidized at 4°C with a 5-fold excess of K 3 Fe(CN) 6 at pH 6 with a N 2 stream over the solution.…”

A Structural Basis of Light Energy and Electron Transfer in Biology (Nobel Lecture)

“…Gray, 1986;Mayo et al, 1986;McGoutry et al, 1987). Gray, 1986;Mayo et al, 1986;McGoutry et al, 1987).…”

Perspectives for Photobiology in Molecular Solar Fuels

“…[67,68] Photoactive porphyrins have been studied in b-type cytochromes when haem is first removed [69] and then replaced with zinc and iron porphyrins. [70,71] In apo-cytochrome b 562 , binding of zinc-chlorin e 6 (ZnCe 6 ) has been studied as ZnCe 6 has a long-lived excited state. [72] Flavin derivatives have also been covalently bound to surface cysteine residues of cytochrome c. [73] Modifications to existing photosynthetic proteins have also been investigated to examine the 'evolutionary argument'.…”