Modification of quinone electrochemistry by the proteins in the biological electron transfer chains: examples from photosynthetic reaction centers

Abstract

Quinones such as ubiquinone are the lipid soluble electron and proton carriers in the membranes of mitochondria, chloroplasts and oxygenic bacteria. Quinones undergo controlled redox reactions bound to specific sites in integral membrane proteins such as the cytochrome bc1 oxidoreductase. The quinone reactions in bacterial photosynthesis are amongst the best characterized, presenting a model to understand how proteins modulate cofactor chemistry. The free energy of ubiquinone redox reactions in aqueous solution and in the QA and QB sites of the bacterial photosynthetic reaction centers (RCs) are compared. In the primary QA site ubiquinone is reduced only to the anionic semiquinone (Q•−) while in the secondary QB site the product is the doubly reduced, doubly protonated quinol (QH2). The ways in which the protein modifies the relative energy of each reduced and protonated intermediate are described. For example, the protein stabilizes Q•− while destabilizing Q= relative to aqueous solution through electrostatic interactions. In addition, kinetic and thermodynamic mechanisms for stabilizing the intermediate semiquinones are compared. Evidence for the protein sequestering anionic compounds by slowing both on and off rates as well as by binding the anion more tightly is reviewed.

Carboxylate Shifts Steer Interquinone Electron Transfer in Photosynthesis

“…from its freezing-out at cryogenic temperatures (15,16,59,60) and from the driving force-independent kinetics (15)(16)(17)61). The gating concept explains why the experimentally accessible (monophasic) reaction kinetics are slower than predicted by ET theory (62).…”

“…on the ET kinetics and accompanying protonation reactions (reviewed in Refs. [15][16][17][18][19], preferentially on BRCs for which high resolution (Յ2 Å) crystal data are available (20 -23). Crystal structures of PSII so far were reported only at a lower resolution of Ն2.9 Å (7, 8) (a structure at higher resolution may become available in the future) (24).…”

“…Crystal structures of PSII so far were reported only at a lower resolution of Ն2.9 Å (7, 8) (a structure at higher resolution may become available in the future) (24). For both PSII and BRCs, in particular the function of the non-heme iron and of its carboxyl ligand in the ET reactions has remained enigmatic (15)(16)(17)(18)(19).…”

Electronic Connection Between the Quinone and CytochromecRedox Pools and Its Role in Regulation of Mitochondrial Electron Transport and Redox Signaling

“…states were detected experimentally. Among these six states, only two are found to be relatively stable in bulk lipid phase (115,281 (217). The difference between the redox midpoint potentials between these two couples defines the stability constant for USQ (K stab in FIGURE 3): the larger the difference, the lower equilibrium concentration of USQ (79,200).…”

The Electrochemical Reaction Mechanism and Applications of Quinones

“…For example, the Q A site and Q B site of bacterial photosynthetic reaction center show very different reactivity and a great deal of endeavors are still continuing to fully understand the kind of interactions that lead to such difference. 4 Similarly, the Q cycle in electron transport chain indicates that the electrochemical property of quinones at Q i and Q o sites should be carefully tuned in order to suppress reactive oxygen species generation. 5 The mechanism involved in quinone toxicity is also of great interest, which mainly consists of reactive oxygen species generation and reaction with thiol nucleophiles.…”