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Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes provided that the conjugated backbone is twisted out of plane. However, this hypothesis has been forwarded only in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a "minimal" one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in a trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission despite the canthaxanthin twist in OCPo.

The light reactions of photosynthesis couple electron and proton transfers across the thylakoid membrane, generating NADPH, and proton motive force (pmf) that powers the endergonic synthesis of ATP by ATP synthase. ATP and NADPH are required for CO2 fixation into carbohydrates by the Calvin-Benson-Bassham cycle. The dominant ΔpH component of the pmf also plays a photoprotective role in regulating photosystem II light harvesting efficiency through nonphotochemical quenching (NPQ) and photosynthetic control via electron transfer from cytochrome b6f (cytb6f) to photosystem I. ΔpH can be adjusted by increasing the proton influx into the thylakoid lumen via upregulation of cyclic electron transfer (CET) or decreasing proton efflux via downregulation of ATP synthase conductivity (gH+). The interplay and relative contributions of these two elements of ΔpH control to photoprotection are not well understood. Here, we showed that an Arabidopsis (Arabidopsis thaliana) ATP synthase mutant hunger for oxygen in photosynthetic transfer reaction 2 (hope2) with 40% higher proton efflux has supercharged CET. Double crosses of hope2 with the CET-deficient proton gradient regulation 5 and ndh-like photosynthetic complex I lines revealed that PROTON GRADIENT REGULATION 5 (PGR5)-dependent CET is the major pathway contributing to higher proton influx. PGR5-dependent CET allowed hope2 to maintain wild-type levels of ΔpH, CO2 fixation and NPQ, however photosynthetic control remained absent and PSI was prone to photoinhibition. Therefore, high CET in the absence of ATP synthase regulation is insufficient for PSI photoprotection., Competing Interests: Conflict of interest statement. None declared., (© American Society of Plant Biologists 2023. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Carotenoids are crucial photosynthetic pigments utilized for light harvesting, energy transfer, and photoprotection. Although most of the enzymes involved in carotenoid biosynthesis in chlorophototrophs are known, some are yet to be identified or fully characterized in certain organisms. A recently characterized enzyme in oxygenic phototrophs is 15- cis -zeta(ζ)-carotene isomerase (Z-ISO), which catalyzes the cis -to- trans isomerization of the central 15-15' cis double bond in 9,15,9'-tri- cis -ζ-carotene to produce 9,9'-di- cis -ζ-carotene during the four-step conversion of phytoene to lycopene. Z-ISO is a heme B-containing enzyme best studied in angiosperms. Homologs of Z-ISO are present in organisms that use the multi-enzyme poly- cis phytoene desaturation pathway, including algae and cyanobacteria, but appear to be absent in green bacteria. Here we confirm the identity of Z-ISO in the model unicellular cyanobacterium Synechocystis sp. PCC 6803 by showing that the protein encoded by the slr1599 open reading frame has ζ-carotene isomerase activity when produced in Escherichia coli . A Synechocystis Δslr1599 mutant synthesizes a normal quota of carotenoids when grown under illumination, where the photolabile 15-15' cis double bond of 9,15,9'-tri- cis -ζ-carotene is isomerized by light, but accumulates this intermediate and fails to produce 'mature' carotenoid species during light-activated heterotrophic growth, demonstrating the requirement of Z-ISO for carotenoid biosynthesis during periods of darkness. In the absence of a structure of Z-ISO, we analyze AlphaFold models of the Synechocystis , Zea mays (maize), and Arabidopsis thaliana enzymes, identifying putative protein ligands for the heme B cofactor and the substrate-binding site.

In oxygenic photosynthesis, the cytochrome b6f (cytb6f) complex links the linear electron transfer (LET) reactions occurring at photosystems I and II and generates a transmembrane proton gradient via the Q-cycle. In addition to this central role in LET, cytb6f also participates in a range of processes including cyclic electron transfer (CET), state transitions and photosynthetic control. Many of the regulatory roles of cytb6f are facilitated by auxiliary proteins that differ depending upon the species, yet because of their weak and transient nature the structural details of these interactions remain unknown. An apparent key player in the regulatory balance between LET and CET in cyanobacteria is PetP, a ∼10 kDa protein that is also found in red algae but not in green algae and plants. Here, we used cryogenic electron microscopy to determine the structure of the Synechocystis sp. PCC 6803 cytb6f complex in the presence and absence of PetP. Our structures show that PetP interacts with the cytoplasmic side of cytb6f, displacing the C-terminus of the PetG subunit and shielding the C-terminus of cytochrome b6, which binds the heme cn cofactor that is suggested to mediate CET. The structures also highlight key differences in the mode of plastoquinone binding between cyanobacterial and plant cytb6f complexes, which we suggest may reflect the unique combination of photosynthetic and respiratory electron transfer in cyanobacterial thylakoid membranes. The structure of cytb6f from a model cyanobacterial species amenable to genetic engineering will enhance future site-directed mutagenesis studies of structure-function relationships in this crucial ET complex., (© 2022 The Author(s).)

(Bacterio)chlorophylls are modified tetrapyrroles that are used by phototrophic organisms to harvest solar energy, powering the metabolic processes that sustain most of the life on Earth. Biosynthesis of these pigments involves enzymatic modification of the side chains and oxidation state of a porphyrin precursor, modifications that differ by species and alter the absorption properties of the pigments. (Bacterio)chlorophylls are coordinated by proteins that form macromolecular assemblies to absorb light and transfer excitation energy to a special pair of redox-active (bacterio)chlorophyll molecules in the photosynthetic reaction centre. Assembly of these pigment-protein complexes is aided by an isoprenoid moiety esterified to the (bacterio)chlorin macrocycle, which anchors and stabilizes the pigments within their protein scaffolds. The reduction of the isoprenoid 'tail' and its addition to the macrocycle are the final stages in (bacterio)chlorophyll biosynthesis and are catalysed by two enzymes, geranylgeranyl reductase and (bacterio)chlorophyll synthase. These enzymes work in conjunction with photosynthetic complex assembly factors and the membrane biogenesis machinery to synchronize delivery of the pigments to the proteins that coordinate them. In this review, we summarize current understanding of the catalytic mechanism, substrate recognition and regulation of these crucial enzymes and their involvement in thylakoid biogenesis and photosystem repair in oxygenic phototrophs., Competing Interests: The authors declare that there are no competing interests associated with the manuscript., (© 2022 The Authors.)

Cytochrome b 6 f (cytb 6 f) lies at the heart of the light-dependent reactions of oxygenic photosynthesis, where it serves as a link between photosystem II (PSII) and photosystem I (PSI) through the oxidation and reduction of the electron carriers plastoquinol (PQH 2 ) and plastocyanin (Pc). A mechanism of electron bifurcation, known as the Q-cycle, couples electron transfer to the generation of a transmembrane proton gradient for ATP synthesis. Cytb 6 f catalyses the rate-limiting step in linear electron transfer (LET), is pivotal for cyclic electron transfer (CET) and plays a key role as a redox-sensing hub involved in the regulation of light-harvesting, electron transfer and photosynthetic gene expression. Together, these characteristics make cytb 6 f a judicious target for genetic manipulation to enhance photosynthetic yield, a strategy which already shows promise. In this review we will outline the structure and function of cytb 6 f with a particular focus on new insights provided by the recent high-resolution map of the complex from Spinach., (Copyright © 2021 Elsevier B.V. All rights reserved.)

TAP38/STN7-dependent (de)phosphorylation of light-harvesting complex II (LHCII) regulates the relative excitation rates of photosystems I and II (PSI, PSII) (state transitions) and the size of the thylakoid grana stacks (dynamic thylakoid stacking). Yet, it remains unclear how changing grana size benefits photosynthesis and whether these two regulatory mechanisms function independently. Here, by comparing Arabidopsis wild-type, stn7 and tap38 plants with the psal mutant, which undergoes dynamic thylakoid stacking but lacks state transitions, we explain their distinct roles. Under low light, smaller grana increase the rate of PSI reduction and photosynthesis by reducing the diffusion distance for plastoquinol; however, this beneficial effect is only apparent when PSI/PSII excitation balance is maintained by state transitions or far-red light. Under high light, the larger grana slow plastoquinol diffusion and lower the equilibrium constant between plastocyanin and PSI, maximizing photosynthesis by avoiding PSI photoinhibition. Loss of state transitions in low light or maintenance of smaller grana in high light also both bring about a decrease in cyclic electron transfer and over-reduction of the PSI acceptor side. These results demonstrate that state transitions and dynamic thylakoid stacking work synergistically to regulate photosynthesis in variable light.

Chlorophyll synthase (ChlG) catalyses a terminal reaction in the chlorophyll biosynthesis pathway, attachment of phytol or geranylgeraniol to the C17 propionate of chlorophyllide. Cyanobacterial ChlG forms a stable complex with high light-inducible protein D (HliD), a small single-helix protein homologous to the third transmembrane helix of plant light-harvesting complexes (LHCs). The ChlG-HliD assembly binds chlorophyll, β-carotene, zeaxanthin and myxoxanthophyll and associates with the YidC insertase, most likely to facilitate incorporation of chlorophyll into translated photosystem apoproteins. HliD independently coordinates chlorophyll and β-carotene but the role of the xanthophylls, which appear to be exclusive to the core ChlG-HliD assembly, is unclear. Here we generated mutants of Synechocystis sp. PCC 6803 lacking specific combinations of carotenoids or HliD in a background with FLAG- or His-tagged ChlG. Immunoprecipitation experiments and analysis of isolated membranes demonstrate that the absence of zeaxanthin and myxoxanthophyll significantly weakens the interaction between HliD and ChlG. ChlG alone does not bind carotenoids and accumulation of the chlorophyllide substrate in the absence of xanthophylls indicates that activity/stability of the 'naked' enzyme is perturbed. In contrast, the interaction of HliD with a second partner, the photosystem II assembly factor Ycf39, is preserved in the absence of xanthophylls. We propose that xanthophylls are required for the stable association of ChlG and HliD, acting as a 'molecular glue' at the lateral transmembrane interface between these proteins; roles for zeaxanthin and myxoxanthophyll in ChlG-HliD complexation are discussed, as well as the possible presence of similar complexes between LHC-like proteins and chlorophyll biosynthesis enzymes in plants., (© 2020 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society.)

In the model cyanobacterium Synechocystis sp. PCC 6803, the terminal enzyme of chlorophyll biosynthesis, chlorophyll synthase (ChlG), forms a complex with high light-inducible proteins, the photosystem II assembly factor Ycf39 and the YidC/Alb3/OxaI membrane insertase, co-ordinating chlorophyll delivery with cotranslational insertion of nascent photosystem polypeptides into the membrane. To gain insight into the ubiquity of this assembly complex in higher photosynthetic organisms, we produced functional foreign chlorophyll synthases in a cyanobacterial host. Synthesis of algal and plant chlorophyll synthases allowed deletion of the otherwise essential native cyanobacterial gene. Analysis of purified protein complexes shows that the interaction with YidC is maintained for both eukaryotic enzymes, indicating that a ChlG-YidC/Alb3 complex may be evolutionarily conserved in algae and plants., (© 2018 The Authors. FEBS Letters published by John Wiley & Sons Ltd on behalf of Federation of European Biochemical Societies.)

Intracytoplasmic vesicles (chromatophores) in the photosynthetic bacterium Rhodobacter sphaeroides represent a minimal structural and functional unit for absorbing photons and utilising their energy for the generation of ATP. The cytochrome bc 1 complex (cytbc 1 ) is one of the four major components of the chromatophore alongside the reaction centre-light harvesting 1-PufX core complex (RC-LH1-PufX), the light-harvesting 2 complex (LH2), and ATP synthase. Although the membrane organisation of these complexes is known, their local lipid environments have not been investigated. Here we utilise poly(styrene-alt-maleic acid) (SMA) co-polymers as a tool to simultaneously determine the local lipid environments of the RC-LH1-PufX, LH2 and cytbc 1 complexes. SMA has previously been reported to effectively solubilise complexes in lipid-rich membrane regions whilst leaving lipid-poor ordered protein arrays intact. Here we show that SMA solubilises cytbc 1 complexes with an efficiency of nearly 70%, whereas solubilisation of RC-LH1-PufX and LH2 was only 10% and 22% respectively. This high susceptibility of cytbc 1 to SMA solubilisation is consistent with this complex residing in a locally lipid-rich region. SMA solubilised cytbc 1 complexes retain their native dimeric structure and co-purify with 56±6 phospholipids from the chromatophore membrane. We extended this approach to the model cyanobacterium Synechocystis sp. PCC 6803, and show that the cytochrome b 6 f complex (cytb 6 f) and Photosystem II (PSII) complexes are susceptible to SMA solubilisation, suggesting they also reside in lipid-rich environments. Thus, lipid-rich membrane regions could be a general requirement for cytbc 1 /cytb 6 f complexes, providing a favourable local solvent to promote rapid quinol/quinone binding and release at the Q 0 and Q i sites., (Copyright © 2018 The Authors. Published by Elsevier B.V. All rights reserved.)

Background: Our recent experimental study showed that peripheral muscle tissue oxygen saturation (StO2), determined noninvasively by near-infrared spectroscopy (NIRS), was more reliable than systemic hemodynamics or invasive oxygenation variables as an index of traumatic shock. The purpose of this study was to establish the normal range of thenar muscle StO2 in humans and the relationship between shock state and StO2 in trauma patients., Methods: This was a prospective, nonrandomized, observational, descriptive study in normal human volunteers (n = 707) and patients admitted to the resuscitation area of our Level I trauma center (n = 150). To establish a normal StO2 range, an NIRS probe was applied to the thenar eminence of volunteers (normals). Subsequently, in a group of trauma patients, an NIRS probe was applied to the thenar eminence and data were collected and stored for offline analysis. StO2 monitoring was performed continuously and noninvasively, and values were recorded at 2-minute intervals. Five moribund trauma patients were excluded. Members of our trauma faculty, blinded to StO2 values, classified each patient into one of four groups (no shock, mild shock, moderate shock, and severe shock) using conventional physiologic parameters., Results: Mean +/- SD thenar StO2 values for each group were as follows: normals, 87 +/- 6% (n = 707); no shock, 83 +/- 10% (n = 85); mild shock, 83 +/- 10% (n = 19); moderate shock, 80 +/- 12% (n = 14); and severe shock, 45 +/- 26% (n = 14). The thenar StO2 values clearly discriminated the normals or no shock patients and the patients with severe shock (p < 0.05)., Conclusion: Decreased thenar muscle tissue oxygen saturation reflects the presence of severe hypoperfusion and near-infrared spectroscopy may be a novel method for rapidly and noninvasively assessing changes in tissue dysoxia.

Background: Trends in the management of renovascular hypertension were evaluated by using a representative national database to determine whether a shift in treatment technology and outcomes has occurred., Methods: Clinical information regarding the treatment of renovascular hypertension in 5433 patients from 1988 to 2001 was derived from the Nationwide Inpatient Sample (NIS) database. Patients were classified into 3 groups: combined aortic and renal revascularization, isolated renal revascularization, and catheter-based procedures (angioplasty with or without stenting). Population-based trends were determined by using sampling weights for each year to estimate the total number of each intervention in the United States. Outcomes were compared using multivariate logistic regression analysis for risk-adjustment., Results: A 73% decrease in combined aortic and renal revascularizations ( P = .033) and a 56% decrease in isolated renal revascularizations ( P < .001) occurred during the study period. Catheter-based procedures have increased 173% from 0.4 to 1.1 procedures per 100,000 adults during this same time period ( P < .001). Predictors favoring catheter-based treatment were admission acuity, increasing age, nonwhite race, and high socioeconomic status. Predictors of mortality for all 3 treatment groups included increasing age, emergent admission, and nonwhite race., Conclusions: A significant change in the management of patients with renovascular hypertension has occurred, with a shift towards less invasive catheter-based interventions. A better understanding of the diffusion of this technology in the treatment of individuals with renovascular hypertension will influence the training and distribution of future vascular specialists responsible for these patients.

Fifty-two surgical patients with isolated aortic valve stenosis were studied preoperatively and postoperatively to determine the incidence of pulmonary hypertension and its response to surgical intervention. Pulmonary artery systolic hypertension was classified as absent (group 1, less than 30 mm Hg), mild (group 2, 30 to 39 mm Hg), moderate (group 3, 40 to 59 mm Hg), and severe (group 4, greater than 60 mm Hg). Thirty-seven of our patients (71%) had preoperative pulmonary hypertension. There was a positive correlation between left ventricular end-diastolic pressure and both systolic and diastolic pulmonary artery pressures preoperatively (p less than 0.001). After operation we found a decrease in mean systolic pulmonary pressure in group 4, from 85.8 +/- 23 mm Hg to 41.2 +/- 10.4 mm Hg (a 52% decrease, p less than 0.001), and in group 3, from 48.9 +/- 5.9 mm Hg to 32.1 +/- 7.1 mm Hg (a 34% decrease, p less than 0.001). A significant decrease in the mean diastolic pressure was found only in group 4, in which the pressure decreased from 33.7 +/- 8.7 mm Hg to 26.0 +/- 7.6 mm Hg (p less than 0.05). The operative mortality was 1.9%. Our data indicate that pulmonary artery hypertension in aortic stenosis is common, is related to end-diastolic pressure, and can be expected to improve in the early postoperative period.

The putrescine, spermidine, and spermine content of subcultured human newborn foreskin keratinocytes was determined during growth and early plateau phase and found to be highest during growth. Exposure of the cells to methylglyoxal bis(guanylhydrazone) during growth phase caused a dose-dependent fall in intracellular spermidine and spermine levels and an increase in putrescine levels at higher concentrations. These effects reflect inhibition of S-adenosyl methionine decarboxylase by the drug. At 8 X 10(-6) M the drug reduced incorporation of leucine into protein, lowered or stopped the accumulation of DNA per dish, inhibited mitotic activity, and increased the histidine/leucine incorporation into protein. The last effect is regarded as induction of keratinization. All these effects were reversible if the use of the drug was discontinued after 3 days. Inhibition of the enzymes of polyamine biosynthesis may have value in psoriasis therapy.

Following percutaneous transluminal coronary angioplasty, wire fragments retained within the patient usually can be removed without an operation. We report a case requiring surgical removal of a guidewire fragment from the distal circumflex coronary artery.

Samples of blood, anticoagulated with EDTA, from 11 patients with psoriasis and 11 individuals without psoriasis were analyzed for their polyamine content. The average spermidine level in patients with psoriasis was approximately twice that of the controls and the average spermine level was three times that of the controls. The level of spermidine and spermine in the skin of two patients with psoriasis were significantly depressed as compared to those of controls.

Lentigo maligna melanoma developed in a patient with a 41-year history of psoriasis. The patient had received substantial radiation therapy and limited tar and ultraviolet energy therapy to the site at which melanoma developed. We raise the question of whether the carcinogenic modalities used to treat the patient may have influenced the development of melanoma.

Polyamine metabolism is important in cell proliferation and may play a role in the epidermal cell hyperproliferation of psoriasis. We have determined changes in polyamine levels in skin and urine accompanying clinical improvement in psoriasis following topical therapy. Nine hospitalized patients were examined at the beginning and end of their courses of treatment. Skin biopsy specimens and portions of 24-hour urine collections were analyzed for polyamines with a modified automatic amino acid analyzer. Treatment resulted in lower cutaneous levels of putrescine (by 50%, P less than .05), spermidine (by 24%, P less than .05), and spermine (by 35%, P less than .005), and lower urinary levels of spermidine (by 20%, P less than .025) and spermine (by 35%, P less than .025). These results suggest that in psoriasis, the skin significantly contributes to the levels of spermidine and spermine in systemic fluids. Topical therapy may reduce epidermal cell proliferation in psoriasis by lowering polyamine levels.

In five cases of mycosis fungoides, previously treated with electron-beam therapy, subcutaneous nodules developed. Clinically, these lesions were thought to be epidermoid cysts or lipomas, but on biopsy were discovered to be subcutaneous infiltrates, three of which were diagnosed as mycosis fungoides. The other two specimens showed only a nonspecific subcutaneous infiltrate. There is no ready explanation for the appearance of these lesions, but it is speculated that they may be the result of inadequate penetration of the electron beam to the depth at which some atypical cells may originally have been located. Patients with mycosis fungoides who develop unusual subcutaneous nodules should be fully investigated so that appropriate and adequate therapy may be initiated.