Photosystems I and II convert solar energy into the chemical energy that powers life. Chlorophyll a photochemistry, using red light (680 to 700 nm), is near universal and is considered to define the energy "red limit" of oxygenic photosynthesis. We present biophysical studies on the photosystems from a cyanobacterium grown in far-red light (750 nm). The few long-wavelength chlorophylls present are well resolved from each other and from the majority pigment, chlorophyll a. Charge separation in photosystem I and II uses chlorophyll f at 745 nm and chlorophyll f (or d) at 727 nm, respectively. Each photosystem has a few even longer-wavelength chlorophylls f that collect light and pass excitation energy uphill to the photochemically active pigments. These photosystems function beyond the red limit using far-red pigments in only a few key positions.

The midpoint potential (E m ) of Q A =Q −• A , the one-electron acceptor quinone of Photosystem II (PSII), provides the thermodynamic reference for calibrating PSII bioenergetics. Uncertainty exists in the literature, with two values differing by ∼80 mV. Here, we have resolved this discrepancy by using spectroelectrochemistry on plant PSII-enriched membranes. Removal of bicarbonate (HCO 3 − ) shifts the E m from ∼−145 mV to −70 mV. The higher values reported earlier are attributed to the loss of HCO 3 − during the titrations (pH 6.5, stirred under argon gassing). P hotosystem II (PSII), the water/plastoquinone photooxidoreductase, is at the heart of the major energy cycle that powers the biosphere. Chlorophyll-based photochemistry drives charge separation followed by electron transfer reactions that result in the reduction of quinone on one side of the thylakoid membrane ( Fig. 1) and the oxidation of water on the other. The photochemistry is intrinsically a one-photon/one-electron process, but the quinone reduction and water oxidation are two-and fourelectron processes, respectively. As a result, both processes involve the accumulation of intermediates that are stabilized by the protein and by coupling to protonation reactions (1-3).The electron acceptor side of PSII contains a nonheme ferrous iron (Fe 2+ ) that is flanked by the two quinones, Q A and Q B (Fig. 1) (reviewed in refs. 3 and 4). The Fe 2+ is coordinated by four histidine residues, two from D1 and two from D2, and a bicarbonate ion (HCO 3 − ) that provides a bidentate ligand (3-5). The HCO 3 − is thought to play a role in the Q B protonation pathway (reviewed in refs. 3-5). Recent EPR (6) and computation chemistry studies (7) based on the highest-resolution X-ray crystallographic model (8) implicated HCO 3 − in the second of two protonation steps that are associated with Q B reduction. Measurements of the dissociation constant of HCO 3 − (K d = 40-80 μM), compared with estimates of the HCO 3 − concentration in the stroma (reviewed in ref. 9), led to the assumption that HCO 3 − remains bound under physiological conditions (10). Recently, however, it was reported that the HCO 3 − bound to PSII in Chlamydomonas can be displaced by acetate when present in the culture medium (11).The redox potential of Q A has been the subject of research for decades (12). The current detailed thermodynamic picture of PSII redox chemistry is based on estimates of energy differences between the electron transfer components, but these estimates A , differing by ∼150 mV, depending on the nature of the PSII (12-14). The fully active enzyme showed an E m value that was ∼150 mV lower than that in PSII lacking the Mn 4 CaO 5 cluster. This shift was, in fact, due to the binding (or absence thereof) of the Ca 2+ ion involved in water splitting, but because the Mn cluster provides the Ca binding site, the potential is indirectly determined by the presence of Mn cluster (12)(13)(14). Given the high valence state on the Mn cluster even in the lowest redox state of the water splitti...

Factors governing the photoelectrochemical output of photosynthetic microorganisms are poorly understood, and energy loss may occur due to inefficient electron transfer (ET) processes. Here, we systematically compare the photoelectrochemistry of photosystem II (PSII) protein-films to cyanobacteria biofilms to derive: (i) the losses in light-to-charge conversion efficiencies, (ii) gains in photocatalytic longevity, and (iii) insights into the ET mechanism at the biofilm interface. This study was enabled by the use of hierarchically structured electrodes, which could be tailored for high/stable loadings of PSII core complexes and Synechocystis sp. PCC 6803 cells. The mediated photocurrent densities generated by the biofilm were 2 orders of magnitude lower than those of the protein-film. This was partly attributed to a lower photocatalyst loading as the rate of mediated electron extraction from PSII in vitro is only double that of PSII in vivo. On the other hand, the biofilm exhibited much greater longevity (>5 days) than the protein-film (<6 h), with turnover numbers surpassing those of the protein-film after 2 days. The mechanism of biofilm electrogenesis is suggested to involve an intracellular redox mediator, which is released during light irradiation.

This communication reports the first electrochemical study of the human P450 2E1 either absorbed or covalently linked to different electrode surfaces. Glassy-carbon and gold electrodes gave reversible electrochemical signals of an active P450 2E1. Molecular modeling of the enzyme helped to rationalize the results. A monolayer coverage was obtained on gold modified with cystamine/maleimide that covalently linked surface accessible cysteines of P450 2E1. The midpoint potential measured for the oriented P450 2E1 was -177 +/- 5 mV comparable to that of the FeIII/FeII of other P450 enzymes. The observed electron-transfer rate for this electrode was 10 s-1. The turnover of the active enzyme was measured with the P450 2E1 specific substrate p-nitrophenol, resulting in a KM of 130 +/- 3 muM and the formation of 2.2 muM of the p-nitrocatechol product upon application of a -300 mV bias.

No abstract