Affinity and activity of non-native quinones at the Q(B) site of bacterial photosynthetic reaction centers

Efficiency Limits of Energy Conversion by Light-Driven Redox Chains

The conversion of absorbed sunlight to spatially separated electron-hole pairs is a crucial outcome of natural photosynthesis. Many organisms achieve near-unit quantum yields of charge separation (one electron-hole pair per incident photon) by dissipating as heat more than half of the light energy that is deposited in the primary donor. Might alternative choices have been made by Nature that would sacrifice quantum yield in favor of producing higher energy electron/hole pairs? Here, we use a multisite electron hopping model to address the kinetic and thermodynamic compromises that can be made in electron transfer chains, with the aim of understanding Nature's choices and opportunities in bioinspired energy-converting systems. We find that if the electron-transfer coordinates are even weakly coupled to a high-frequency vibrational mode, substantial energy dissipation is necessary to achieve the maximum possible energy storage in an electron-transfer chain. Since high-frequency vibronic coupling is common in physiological redox cofactors, we posit that biological reaction centers have recruited a strategy to convert light energy into redox potential with the near-optimum energy efficiency that is possible in an electron-transfer chain. Our simulations also find that charge separation in electron-transfer chains is subject to a minimum intercofactor separation distance, beneath which energy-dissipating charge recombination is unavoidable. We find that high quantum yield and low energy dissipation can thus be realized simultaneously for multistep electron transfer if recombination pathways are uncoupled from high-frequency vibrations and if the cofactors are held at small-to-intermediate distances apart (ca. 3 to 8 Å edge-to-edge). Our analysis informs the design of bioinspired light-harvesting structures that may exceed 60% energy efficiency, as opposed to the ∼30% efficiency achieved in natural photosynthesis.

The aprotic electrochemistry of quinones

Quinones play important roles in biological electron transfer reactions in almost all organisms, with specific roles in many physiological processes and chemotherapy. Quinones participate in two-electron, two-proton reactions in aqueous solution at equilibrium near neutral pH, but protons often lag behind the electron transfers. The relevant reactions in proteins are often sequential one electron redox processes without involving protons. Here we report the aprotic electrochemistry of the two half-couples, Q/Q^.- and Q^.-/Q⁼, of 11 parent quinones and 118 substituted 1,4-benzoquinones, 91 1,4-naphthoquinones, and 107 9,10-anthraquinones. The measured redox potentials are fit quite well with the Hammett para sigma (σ_para) parameter. Occasional exceptions can involve important groups, such as methoxy substituents in ubiquinone and hydroxy substituents in therapeutics. These can generally be explained by reasonable conjectures involving steric clashes and internal hydrogen bonds. We also provide data for 25 other quinones, 2 double quinones and 15 non-quinones, all measured under similar conditions.

Introduction of the Menaquinone Biosynthetic Pathway into Rhodobacter sphaeroides and de Novo Synthesis of Menaquinone for Incorporation into Heterologously Expressed Integral Membrane Proteins

Quinones are redox-active molecules that transport electrons and protons in organelles and cell membranes during respiration and photosynthesis. In addition to the fundamental importance of these processes in supporting life, there has been considerable interest in exploiting their mechanisms for diverse applications ranging from medical advances to innovative biotechnologies. Such applications include novel treatments to target pathogenic bacterial infections and fabricating biohybrid solar cells as an alternative renewable energy source. Ubiquinone (UQ) is the predominant charge-transfer mediator in both respiration and photosynthesis. Other quinones, such as menaquinone (MK), are additional or alternative redox mediators, for example in bacterial photosynthesis of species such as Thermochromatium tepidum and Chloroflexus aurantiacus. Rhodobacter sphaeroides has been used extensively to study electron transfer processes, and recently as a platform to produce integral membrane proteins from other species. To expand the diversity of redox mediators in R. sphaeroides, nine Escherichia coli genes encoding the synthesis of MK from chorismate and polyprenyl diphosphate were assembled into a synthetic operon in a newly designed expression plasmid. We show that the menFDHBCE, menI, menA, and ubiE genes are sufficient for MK synthesis when expressed in R. sphaeroides cells, on the basis of high performance liquid chromatography and mass spectrometry. The T. tepidum and C. aurantiacus photosynthetic reaction centers produced in R. sphaeroides were found to contain MK. We also measured in vitro charge recombination kinetics of the T. tepidum reaction center to demonstrate that the MK is redox-active and incorporated into the Q_A pocket of this heterologously expressed reaction center.

Redox potential tuning through differential quinone binding in the photosynthetic reaction center of Rhodobacter sphaeroides

Ubiquinone forms an integral part of the electron transport chain in cellular respiration and photosynthesis across a vast number of organisms. Prior experimental results have shown that the photosynthetic reaction center (RC) from Rhodobacter sphaeroides is only fully functional with a limited set of methoxy-bearing quinones, suggesting that specific interactions with this substituent are required to drive electron transport and the formation of quinol. The nature of these interactions has yet to be determined. Through parameterization of a CHARMM-compatible quinone force field and subsequent molecular dynamics simulations of the quinone-bound RC, we have investigated and characterized the interactions of the protein with the quinones in the Q(A) and Q(B) sites using both equilibrium simulation and thermodynamic integration. In particular, we identify a specific interaction between the 2-methoxy group of ubiquinone in the Q(B) site and the amide nitrogen of GlyL225 that we implicate in locking the orientation of the 2-methoxy group, thereby tuning the redox potential difference between the quinones occupying the Q(A) and Q(B) sites. Disruption of this interaction leads to weaker binding in a ubiquinone analogue that lacks a 2-methoxy group, a finding supported by reverse electron transfer electron paramagnetic resonance experiments of the Q(A)⁻Q(B)⁻ biradical and competitive binding assays.

The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers

Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (Q_A) and secondary (Q_B) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. ¹³C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQ_A and SQ_B, were compared with DFT calculations of the ¹³C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔE_m) between Q_A and Q_B of 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as Q_B in mutant reaction centers with a ΔE_m of ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from Q_A to Q_B in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

Molecular mechanisms for generating transmembrane proton gradients

Membrane proteins use the energy of light or high energy substrates to build a transmembrane proton gradient through a series of reactions leading to proton release into the lower pH compartment (P-side) and proton uptake from the higher pH compartment (N-side). This review considers how the proton affinity of the substrates, cofactors and amino acids are modified in four proteins to drive proton transfers. Bacterial reaction centers (RCs) and photosystem II (PSII) carry out redox chemistry with the species to be oxidized on the P-side while reduction occurs on the N-side of the membrane. Terminal redox cofactors are used which have pKas that are strongly dependent on their redox state, so that protons are lost on oxidation and gained on reduction. Bacteriorhodopsin is a true proton pump. Light activation triggers trans to cis isomerization of a bound retinal. Strong electrostatic interactions within clusters of amino acids are modified by the conformational changes initiated by retinal motion leading to changes in proton affinity, driving transmembrane proton transfer. Cytochrome c oxidase (CcO) catalyzes the reduction of O2 to water. The protons needed for chemistry are bound from the N-side. The reduction chemistry also drives proton pumping from N- to P-side. Overall, in CcO the uptake of 4 electrons to reduce O2 transports 8 charges across the membrane, with each reduction fully coupled to removal of two protons from the N-side, the delivery of one for chemistry and transport of the other to the P-side.