Electron flow through metalloproteins.
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ABSTRACT:
SUBMITTER: Winkler JR
PROVIDER: S-EPMC3981952 | biostudies-literature | 2014 Apr
REPOSITORIES: biostudies-literature
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Project description:We have designed ruthenium-modified Pseudomonas aeruginosa azurins that incorporate 3-nitrotyrosine (NO2YOH) between Ru(2,2'-bipyridine)2(imidazole)(histidine) and Cu redox centers in electron transfer (ET) pathways. We investigated the structures and reactivities of three different systems: RuH107NO2YOH109, RuH124NO2YOH122, and RuH126NO2YOH122. RuH107NO2YOH109, unlabeled H124NO2YOH122, and unlabeled H126NO2YOH122 were structurally characterized. The pKa's of NO2YOH at positions 122 and 109 are 7.2 and 6.0, respectively. Reduction potentials of 3-nitrotyrosinate (NO2YO(-))-modified azurins were estimated from cyclic and differential pulse voltammetry data: oxidation of NO2YO(-)122 occurs near 1.1 versus NHE; oxidation of NO2YO(-)109 is near 1.2 V. Our analysis of transient optical spectroscopic experiments indicates that hopping via NO2YO(-) enhances Cu(I) oxidation rates over single-step ET by factors of 32 (RuH107NO2YO(-)109), 46 (RuH126NO2YO(-)122), and 13 (RuH124NO2YO(-)122).
| S-EPMC3839300 | biostudies-literature
Project description:One of the hallmark advances in our understanding of metalloprotein function is showcased in our ability to design new, non-native, catalytically active protein scaffolds. This review highlights progress and milestone achievements in the field of de novo metalloprotein design focused on reports from the past decade with special emphasis on de novo designs couched within common subfields of bioinorganic study: heme binding proteins, monometal- and dimetal-containing catalytic sites, and metal-containing electron transfer sites. Within each subfield, we highlight several of what we have identified as significant and important contributions to either our understanding of that subfield or de novo metalloprotein design as a discipline. These reports are placed in context both historically and scientifically. General suggestions for future directions that we feel will be important to advance our understanding or accelerate discovery are discussed.
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Project description:The [Re(I)(CO)(3)(4,7-dimethyl-1,10-phenanthroline)(histidine-124)(tryptophan-122)] complex, denoted [Re(I)(dmp)(W122)], of Pseudomonas aeruginosa azurin behaves as a single photoactive unit that triggers very fast electron transfer (ET) from a distant (2 nm) Cu(I) center in the protein. Analysis of time-resolved (ps-?s) IR spectroscopic and kinetics data collected on [Re(I)(dmp)(W122)AzM] (in which M=Zn(II), Cu(II), Cu(I); Az=azurin) and position-122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants, together with excited-state DFT/time-dependent (TD)DFT calculations and X-ray structural characterization, reveal the character, energetics, and dynamics of the relevant electronic states of the [Re(I)(dmp)(W122)] unit and a cascade of photoinduced ET and relaxation steps in the corresponding Re-azurins. Optical population of [Re(I)(imidazole-H124)(CO)(3)]?dmp (1)CT states (CT=charge transfer) is followed by around 110 fs intersystem crossing and about 600 ps structural relaxation to a (3)CT state. The IR spectrum indicates a mixed Re(I)(CO)(3),A?dmp/???(*)(dmp) character for aromatic amino acids A122 (A=W, Y, F) and Re(I)(CO)(3)?dmp metal-ligand charge transfer (MLCT) for [Re(I)(dmp)(K122)AzCu(II)]. In a few ns, the (3)CT state of [Re(I)(dmp)(W122)AzM] establishes an equilibrium with the [Re(I)(dmp(.-))(W122(.+))AzM] charge-separated state, (3)CS, whereas the (3)CT state of the other Y, F, and K122 proteins decays to the ground state. In addition to this main pathway, (3)CS is populated by fs- and ps-W(indole)?Re(II) ET from (1)CT and the initially "hot" (3)CT states, respectively. The (3)CS state undergoes a tens-of-ns dmp(.-)?W122(.+) ET recombination leading to the ground state or, in the case of the Cu(I) azurin, a competitively fast (?30 ns over 1.12?nm) Cu(I)?W(.+) ET, to give [Re(I)(dmp(.-))(W122)AzCu(II)]. The overall photoinduced Cu(I)?Re(dmp) ET through [Re(I)(dmp)(W122)AzCu(I)] occurs over a 2 nm distance in <50 ns after excitation, with the intervening fast (3)CT-(3)CS equilibrium being the principal accelerating factor. No reaction was observed for the three Y, F, and K122 analogues. Although the presence of [Re(dmp)(W122)AzCu(II)] oligomers in solution was documented by mass spectrometry and phosphorescence anisotropy, the kinetics data do not indicate any significant interference from the intermolecular ET steps. The ground-state dmp-indole ?-? interaction together with well-matched W/W(.+) and excited-state [Re(II)(CO)(3)(dmp(.-))]/[Re(I)(CO)(3)(dmp(.-))] potentials that result in very rapid electron interchange and (3)CT-(3)CS energetic proximity, are the main factors responsible for the unique ET behavior of [Re(I)(dmp)(W122)]-containing azurins.
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Project description:The unique photochemical properties of Ru(II)-diimine complexes have helped initiate a series of seminal electron transfer studies in metalloenzymes. It has thus been possible to experimentally determine rate constants for long-range electron transfers. These studies have laid the foundation for the investigation of reactive intermediates in heme proteins and for the design of light-activated biocatalysts. Various metalloenzymes such as hydrogenase, carbon monoxide dehydrogenase, nitrogenase, laccase and cytochrome P450 BM3 have been functionalized with Ru(II)-diimine complexes. Upon visible light-excitation, these photosensitized metalloproteins are capable of sustaining photocatalytic activity to reduce small molecules such as protons, acetylene, hydrogen cyanide and carbon monoxide or activate molecular dioxygen to produce hydroxylated products. The Ru(II)-diimine photosensitizers are hence able to deliver multiple electrons to metalloenzymes buried active sites, circumventing the need for the natural redox partners. In this review, we will highlight the key achievements of the light-driven biocatalysts, which stem from the extensive electron transfer investigations. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.
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Project description:Redox potentials are a major contributor in controlling the electron transfer (ET) rates and thus regulating the ET processes in the bioenergetics. To maximize the efficiency of the ET process, one needs to master the art of tuning the redox potential, especially in metalloproteins, as they represent major classes of ET proteins. In this review, we first describe the importance of tuning the redox potential of ET centers and its role in regulating the ET in bioenergetic processes including photosynthesis and respiration. The main focus of this review is to summarize recent work in designing the ET centers, namely cupredoxins, cytochromes, and iron-sulfur proteins, and examples in design of protein networks involved these ET centers. We then discuss the factors that affect redox potentials of these ET centers including metal ion, the ligands to metal center and interactions beyond the primary ligand, especially non-covalent secondary coordination sphere interactions. We provide examples of strategies to fine-tune the redox potential using both natural and unnatural amino acids and native and nonnative cofactors. Several case studies are used to illustrate recent successes in this area. Outlooks for future endeavors are also provided. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.
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Project description:We have constructed and structurally characterized a Pseudomonas aeruginosa azurin mutant Re126WWCuI , where two adjacent tryptophan residues (W124 and W122, indole separation 3.6-4.1 Å) are inserted between the CuI center and a Re photosensitizer coordinated to the imidazole of H126 (ReI(H126)(CO)3(4,7-dimethyl-1,10-phenanthroline)+). CuI oxidation by the photoexcited Re label (*Re) 22.9 Å away proceeds with a ∼70 ns time constant, similar to that of a single-tryptophan mutant (∼40 ns) with a 19.4 Å Re-Cu distance. Time-resolved spectroscopy (luminescence, visible and IR absorption) revealed two rapid reversible electron transfer steps, W124 → *Re (400-475 ps, K 1 ≅ 3.5-4) and W122 → W124•+ (7-9 ns, K 2 ≅ 0.55-0.75), followed by a rate-determining (70-90 ns) CuI oxidation by W122•+ ca. 11 Å away. The photocycle is completed by 120 μs recombination. No photochemical CuI oxidation was observed in Re126FWCuI , whereas in Re126WFCuI , the photocycle is restricted to the ReH126W124 unit and CuI remains isolated. QM/MM/MD simulations of Re126WWCuI indicate that indole solvation changes through the hopping process and W124 → *Re electron transfer is accompanied by water fluctuations that tighten W124 solvation. Our finding that multistep tunneling (hopping) confers a ∼9000-fold advantage over single-step tunneling in the double-tryptophan protein supports the proposal that hole-hopping through tryptophan/tyrosine chains protects enzymes from oxidative damage.
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Project description:Elucidating the factors that control charge transfer rates in relatively flexible conjugates is of importance for understanding energy flows in biology as well as assisting the design and construction of electronic devices. Here, we report ultrafast electron transfer (ET) and hole transfer (HT) between a corrole (Cor) donor linked to a perylene-diimide (PDI) acceptor by a tetrameric alanine (Ala)4 Selective photoexcitation of the donor and acceptor triggers subpicosecond and picosecond ET and HT. Replacement of the (Ala)4 linker with either a single alanine or phenylalanine does not substantially affect the ET and HT kinetics. We infer that electronic coupling in these reactions is not mediated by tetrapeptide backbone nor by direct donor-acceptor interactions. Employing a combination of NMR, circular dichroism, and computational studies, we show that intramolecular hydrogen bonding brings the donor and the acceptor into proximity in a "scorpion-shaped" molecular architecture, thereby accounting for the unusually high ET and HT rates. Photoinduced charge transfer relies on a (Cor)NH…O=C-NH…O=C(PDI) electronic-coupling pathway involving two pivotal hydrogen bonds and a central amide group as a mediator. Our work provides guidelines for construction of effective donor-acceptor assemblies linked by long flexible bridges as well as insights into structural motifs for mediating ET and HT in proteins.
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