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

Abstract

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...

Direct Visualization Reveals Dynamics of a Transient Intermediate During Protein Assembly

“…Consistent with this notion, the stability of the early intermediate and the rate of stable complex assembly have a strong dependence on ionic strength (32). Critical roles of electrostatic interactions in enhancing protein interaction kinetics have been predicted theoretically (33,34) and demonstrated in multiple cases (35)(36)(37)(38); our results further emphasize the role of such interactions in stabilizing assembly intermediates, which provides an effective way to accelerate the overall assembly process.…”

Structure of Complex III with Bound Cytochrome c in Reduced State and Definition of a Minimal Core Interface for Electron Transfer

“…In the complex of the cyt c peroxidase with cyt c, charged residue pairs are separated by 3.8 -10 Å (with the exception of one pair with 3.2 Å) (6). At the interface of the reaction center⅐cyt c 2 complex oppositely charged residue pairs are separated by very similar distances (4.8 -9 Å) (7), and computational and kinetic analysis indicates that they are involved in encounter complex formation but do not significantly contribute to the binding energy of the bound state (7,36,37).…”

Structural Basis of Efficient Electron Transport between Photosynthetic Membrane Proteins and Plastocyanin in Spinach Revealed Using Nuclear Magnetic Resonance

“…This binding mode explains the rapid dissociation of the complexes (Finazzi et al, 2005;Busch and Hippler, 2011) and the pseudospecificity of the acidic patches in fern and spinach Pcs (Sato et al, 2004). Rapid binding facilitated by transient electrostatic interactions has also been proposed in various systems (Schreiber and Fersht, 1996;Selzer et al, 2000;Miyashita et al, 2004;Alsallaq and Zhou, 2008;Schreiber et al, 2009), and evidence is accumulating for the formation of transient complexes, based on NMR studies using paramagnetic relaxation enhancement Tang et al, 2006;Volkov et al, 2006;Clore et al, 2007;Bashir et al, 2011;Clore, 2011). Although the introduction of paramagnetic probes into specific sites of PSI and cyt b 6 f is a challenging task, paramagnetic relaxation enhancement analyses would provide further information about the encounter complex formation in the Pc-PSI and Pc-cyt b 6 f complexes.…”