Electrostatic role of the non-heme iron complex in bacterial photosynthetic reaction center

Electron Transfer Route between Quinones in Type-II Reaction Centers

In photosynthetic reaction centers from purple bacteria (PbRCs) and photosystem II (PSII), the photoinduced charge separation is terminated by an electron transfer between the primary (QA) and secondary (QB) quinones. Here, we investigate the electron transfer route, calculating the superexchange coupling (HQA-QB) for electron transfer from QA to QB in the protein environment. HQA-QB is significantly larger in PbRC than in PSII. In superexchange electron tunneling, the electron transfer via unoccupied molecular orbitals of the nonheme Fe complex (QA → Fe → QB) is pronounced in PbRC, whereas the electron transfer via occupied molecular orbitals (Fe → QB followed by QA → Fe) is pronounced in PSII. The significantly large HQA-QB is caused by a water molecule that donates the H-bond to the ligand Glu-M234 in PbRC. The corresponding water molecule is absent in PSII due to the existence of D1-Tyr246. HQA-QB increases in response to the Ser-L223···QB H-bond formation caused by an extension of the H-bond network, which facilitates charge delocalization over the QB site. This explains the observed discrepancy in the QA-to-QB electron transfer between PbRC and PSII, despite their structural similarity.

Long-Range Electron Tunneling from the Primary to Secondary Quinones in Photosystem II Enhanced by Hydrogen Bonds with a Nonheme Fe Complex

The mechanisms governing the long-range electron tunneling from the primary (Q_A) to secondary (Q_B) quinones in photosystem II are clarified by analyzing superexchange pathways through a nonheme Fe complex, using a quantum mechanics/molecular mechanics/polarizable continuum model approach. The electron tunneling rate is evaluated using the Marcus-Levich-Jortner theory considering electronic coupling, energy difference, and Franck-Condon factor. The superexchange Q_A → Q_B electron tunneling is enhanced by hybridized σ/σ* orbitals of histidines (D2-His214 and D1-His215) via penetration of the wave function into hydrogen bonds with both Q_A and Q_B. Despite a large energy gap to the intermediate states, the contributions of the histidine σ/σ* orbitals to the superexchange coupling are larger than those of π/π* orbitals. Fe²⁺ is not an essential component for the Q_A → Q_B electron tunneling because hybridized histidine molecular orbitals can be coupled with both Q_A and Q_B simultaneously in the absence of Fe d orbitals.

Charge separation in photosystem II: a comparative and evolutionary overview

Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.

Electron transfer as a potential cause of diacetyl toxicity in popcorn lung disease

Diacetyl, a butter-flavoring component, has recently attracted scientific and media attention because it has been implicated as an agent that induces popcorn lung disease in exposed plant workers. This disease, officially referred to as bronchiolitis obliterans, entails exposure-induced compromise to the lung's epithelial barrier function. In this review, we present a novel molecular mechanism (electron transfer, ET) designed to explain how diacetyl and its imine derivatives might interact to produce lung damage. We relate the fact that diacetyl and related compounds possess reduction potentials amenable to electron transfer (ET) in vivo. The electrochemical nature of these toxicants can potentially disrupt normal ET processes, generate reactive oxygen species (ROS), and participate in cell signaling events. Condensation of diacetyl with protein may also play a role in the toxicity caused by this compound. ET is a common feature of toxic substances, usually involving their metabolites which can operate per se or through reactions that generate ROS and oxidative stress (OS). Examples of agents capable of ET are quinone and metal compounds, aromatic nitro compounds, and iminium salts. Among compounds that generate ET, the alpha-dicarbonyl ET class, of which diacetyl is a member, is much less studied. This review emphasizes diacetyl as an agent that acts through oxidative processes to cause its effects. However, we also treat related substances that appear to act by a similar mechanism. This mechanism forms a theoretical framework capable of describing the mechanism by which diacetyl may induce its effects and is in accord with various physiological activities displayed by other alpha-dicarbonyl substances. Examples of substances that may act by mechanisms similar to that displayed by diacetyl include cyclohexane-1,2-dione, marinopyrroles, reactive carbonyl species, the bacterial signaling agent DPD, and advanced glycation end products.

Unifying electrostatic mechanism for metal cations in receptors and cell signaling

Previously, an electrostatic mechanism was proposed for receptor-ligand action and for cell signaling by phosphate and sulfate. The hypothesis is further elaborated by application to metal ions, mainly calcium, magnesium, zinc, iron, and copper, in receptors and cell signaling. Evidence is provided for involvement of electrostatics in various reaction modes in biosystems. Calcium plays an important role electrochemically in neurotransmission. In some cases, electron transfer and redox processes are also involved. Electrostatics are known to participate in plant biochemistry. Mechanistically, the electrostatic field may act as a conduit for electrons and radicals and in involvement with energetics.

Unifying electrostatic mechanism for receptor-ligand activity

A prior article in skeletal form proposed an electrostatic mechanism for receptor-ligand activity. The present review provides an elaboration, including supporting evidence. The fundamental aspect entails the presence of molecular electrostatic potential associated with ions and dipoles in the ligand. The ligand can be regarded as an electrical link that joins prevalent electrostatic fields present in the surrounding protein matrix. The exact role of these fields is speculative. One possibility is to function as conduits for electrons and radicals in cell signaling. There is increasing support for important participation of these species in signal transduction. There might also be a favorable influence on energetics involving the electron transfer process. A summary of receptor biology is also provided, including receptors for acetylcholine (nicotinic and muscarinic), GABA, adrenergic, and glutamate.

Unifying electrostatic mechanism for phosphates and sulfates in cell signaling

Prior proposals suggested the importance of electrochemistry in signal transduction and receptor-ligand activity. Electrostatic fields associated with ions and dipoles were assigned important roles. Little is known concerning the precise mode of action in cell signaling by widespread phosphorylation. According to the hypothetical framework, molecular electrostatic potential associated with phosphate anion is a key element as a link in the communication grid, possibly inducing favorable energetics in the electron transfer process. Similar involvement appears plausible for the sulfate anion. Supporting evidence for the electrostatic mechanism is presented. Representative literature on phosphorylation in the biological domain is reviewed with emphasis on cell signaling. The treatment includes phosphates from protein, lipids, and other molecules, plus the role of reactive oxygen species. Protein sulfation is also discussed.

The Unusually Strong Hydrogen Bond between the Carbonyl of QAand His M219 in theRhodobacter sphaeroidesReaction Center Is Not Essential for Efficient Electron Transfer from QA-to QB

In native reaction centers (RCs) from photosynthetic purple bacteria the primary quinone (QA) and the secondary quinone (QB) are interconnected via a specific His-Fe-His bridge. In Rhodobacter sphaeroides RCs the C4=O carbonyl of QA forms a very strong hydrogen bond with the protonated Npi of His M219, and the Ntau of this residue is in turn coordinated to the non-heme iron atom. The second carbonyl of QA is engaged in a much weaker hydrogen bond with the backbone N-H of Ala M260. In previous work, a Trp side chain was introduced by site-directed mutagenesis at the M260 position in the RC of Rb. sphaeroides, resulting in a complex that is completely devoid of QA and therefore nonfunctional. A photochemically competent derivative of the AM260W mutant was isolated that contains a Cys side chain at the M260 position (denoted AM260(W-->C)). In the present work, the interactions between the carbonyl groups of QA and the protein in the AM260(W-->C) suppressor mutant have been characterized by light-induced FTIR difference spectroscopy of the photoreduction of QA. The QA-/QA difference spectrum demonstrates that the strong interaction between the C4=O carbonyl of QA and His M219 is lost in the mutant, and the coupled CO and CC modes of the QA- semiquinone are also strongly perturbed. In parallel, a band assigned to the perturbation of the C5-Ntau mode of His M219 upon QA- formation in the native RC is lacking in the spectrum of the mutant. Furthermore, a positive band between 2900 and 2400 cm-1 that is related to protons fluctuating within a network of highly polarizable hydrogen bonds in the native RC is reduced in amplitude in the mutant. On the other hand, the QB-/QB FTIR difference spectrum is essentially the same as for the native RC. The kinetics of electron transfer from QA- to QB were measured by the flash-induced absorption changes at 780 nm. Compared to native RCs the absorption transients are slowed by a factor of about 2 for both the slow phase (in the hundreds of microseconds range) and fast phase (microseconds to tens of microseconds range) in AM260(W-->C) RCs. We conclude that the unusually strong hydrogen bond between the carbonyl of QA and His M219 in the Rb. sphaeroides RC is not obligatory for efficient electron transfer from QA- to QB.