Electron tunneling in protein crystals

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109

Although bonding networks determine electron-transfer (ET) rates within proteins, the mechanism by which structure and dynamics influence ET across protein interfaces is not well understood. Measurements of photochemically induced ET and subsequent charge recombination between Zn-porphyrin-substituted cytochrome c peroxidase and cytochrome c in single crystals correlate reactivity with defined structures for different association modes of the redox partners. Structures and ET rates in crystals are consistent with tryptophan oxidation mediating charge recombination reactions. Conservative mutations at the interface can drastically affect how the proteins orient and dispose redox centers. Whereas some configurations are ET inactive, the wild-type complex exhibits the fastest recombination rate. Other association modes generate ET rates that do not correlate with predictions based on cofactor separations or simple bonding pathways. Inhibition of photoinduced ET at <273 K indicates gating by smallamplitude dynamics, even within the crystal. Thus, different associations achieve states of similar reactivity, and within those states conformational fluctuations enable interprotein ET.cytochrome ͉ protein dynamics ͉ protein-protein interaction ͉ electron tunneling M any long-range electron-transfer (ET) reactions in biology occur across transient protein-protein interfaces. Reaction rates depend on factors that control both electron tunneling and conformational dynamics coupled to protein association processes (1-3). As such, interprotein ET is sensitive to structure and dynamics at the interface (1, 4-11). Residue substitution (achieved either by use of protein homologs, site-directed mutants, or computations) has been a popular and powerful approach for probing how interface composition influences interprotein ET (7,(10)(11)(12)(13)(14). Nevertheless, effects of residue variation on interface structure are not often known.The natural redox partners yeast cytochrome c peroxidase (CcP) and yeast cytochrome c (yCc), whose structure as a complex was first determined in 1992 (15) and later as a covalent complex (16), have served as a paradigm for studying interprotein ET reactions (8,17). Hoffman and colleagues (8) have exploited ZnCcP substitution to photoactivate ET reactions and examine the effects of many structural and chemical perturbations on interprotein ET. In this system, the ZnCcP triplet state ( 3 ZnCcP) reduces Fe(III)Cc, and then back ET recombines the charge separation (Fig. 1). Recently, it has been demonstrated that a Trp-191 3 Phe CcP variant has much slower ET back-rates (k eb ) than wild-type (WT) CcP in the 1:1 complex with yCc (18). Thus, electron hopping through Trp-191 may accelerate the recombination reaction (Fig. 1), in analogy to the natural reaction between CcP compound I and Fe(II)Cc (19). Nevertheless, much slower ET back-rates in the complex between CcP and hCc compared with yCc, both in solution (20) and in crystals (21), indicate that ET across the protein-protein interface limits the overa...

Section: Structures and Redox Properties Of Znccp In Complex With Ycccontrasting

confidence: 47%

Section: Structures and Redox Properties Of Znccp In Complex With Yccmentioning

confidence: 93%

Effects of interface mutations on association modes and electron-transfer rates between proteins

Kang

Crane

2005

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109

“…Our finding that ET rates in Zn-doped tuna cyt c crystals fall well within the protein range in the Ru-protein tunneling timetable (Fig. 3) (7,62) demonstrates that small interaction zones of low density are quite effective in mediating interprotein redox reactions.…”

Section: Protein-protein Reactionsmentioning

confidence: 86%

“…In crystal lattices of tuna cyt c, chains of protein molecules form helices with a 24.1-Å separation between neighboring metal centers (62). By doping Zn-cyt c into this lattice, interprotein ET between triplet-excited Znporphyrin and a neighboring Fe(III)-cyt c could be investigated; the rate constant was found to be 4(1) ϫ 10 2 s Ϫ1 , and charge recombination was about four times faster [2.0(5) ϫ 10 3 s Ϫ1 ] (62).…”

Section: Protein-protein Reactionsmentioning

confidence: 99%

Long-range electron transfer

Gray

Winkler

2005

Self Cite

756

841

Recent investigations have shed much light on the nuclear and electronic factors that control the rates of long-range electron tunneling through molecules in aqueous and organic glasses as well as through bonds in donor-bridge-acceptor complexes. Couplings through covalent and hydrogen bonds are much stronger than those across van der Waals gaps, and these differences in coupling between bonded and nonbonded atoms account for the dependence of tunneling rates on the structure of the media between redox sites in Ru-modified proteins and protein-protein complexes.electron tunneling ͉ hopping ͉ glass ͉ protein

“…Because the electron transfer rate decreases exponentially with distance, the electron transfer rate between the Cyt and RC in the TS is expected to be much slower than in the bound state. For a 10-Å-longer tunneling distance, when using tunneling decay factor ␤ ϭ 1.61 Ϫ 1.75 Å Ϫ1 for transfer through an aqueous interface (44), electron transfer times in the TS of 10-40 s are expected. These long times preclude a mechanism in which electron transfer occurs at the TS.…”

Section: Discussionmentioning

confidence: 99%

Transition state and encounter complex for fast association of cytochrome c ₂ with bacterial reaction center

Miyashita

Onuchic

Okamura

2004

Electrostatic interactions strongly enhance the electron transfer reaction between cytochrome (Cyt) c 2 and reaction center (RC) from photosynthetic bacteria, yielding a second-order rate constant, k 2 Ϸ 10 9 s ؊1 ⅐M ؊1 , close to the diffusion limit. The proposed mechanism involves an encounter complex (EC) stabilized by electrostatic interactions, followed by a transition state (TS), leading to the bound complex active in electron transfer. The effect of electrostatic interactions was previously studied by Tetreault et al. electron transfer ͉ photosynthesis ͉ electrostatics ͉ site-directed mutagenesis ͉ correlation I ntermolecular electron-transfer reactions play important roles in many biological processes such as photosynthesis and respiration (1). In these reactions, two electron-transfer proteins associate to form an electron donor-acceptor complex in which electron transfer occurs (2). Electrostatic interactions can greatly enhance the rate of protein association by providing long-range guidance in the association process (3). For fast processes assisted by electrostatic interactions, a two-step association mechanism has been proposed (4, 5). The first step is the formation of a loosely bound encounter complex (EC), followed by a rate-limiting process through a transition state (TS), leading to the bound active state. In this paper we examine the association process for the reaction center (RC) and cytochrome (Cyt) c 2 that are involved in the electron-transfer chain in bacterial photosynthesis by using electrostatic calculations and experimental data to obtain a molecular description of the TS and the EC. The results of these calculations describe the ensemble of states that represent a roadmap for the association process and the interactions likely to be important in the dynamics. This approach is complementary to Brownian dynamics simulations (5-7) that yield the diffusional rate constant.The RC is the membrane protein involved in the primary light-induced electron-transfer step in bacterial photosynthesis (8,9). Light absorbed by RC pigments activates photooxidation of the primary donor, D, a bacteriochlorophyll dimer, leading to the reduction of a quinone molecule Q, DQ 3 D ϩ Q Ϫ . The electron from Q Ϫ cycles back to D ϩ through an electron-transfer chain involving the membrane-bound Cyt bc1 complex. The last step in the cycle involves a soluble Cyt c 2 molecule that shuttles the electron back to the RC to complete the cyclic electron-transfer process. This cyclic electron transfer is coupled to proton pumping across the membrane that drives ATP synthesis.The rates of electron transfer between isolated RCs and Cyt c 2 have been measured by using laser-flash techniques and shown to be well optimized for cyclic electron transfer by using the scheme below (refs. 10-13; reviewed in ref. 14).[1]Before the laser flash, the Cyt and RC (denoted DQ) are in equilibrium between a bound state and a free state with the dissociation constant K d Ϸ 10 Ϫ6 M at low ionic strength (Ϸ5 mM) (13). After a laser flash, t...