Recombinant Synechococcus elongatus Photosystem Q (B) protein 2
Description
Protein Identification and Classification
Photosystem Q(B) protein 2 is encoded by the psbA2 gene in Synechococcus elongatus and is classified as a key component of Photosystem II (PSII) . The protein is officially identified in databases with the UniProt ID P0A447 and is synonymously referred to as Photosystem II protein D1 2, PSII D1 protein 2, or simply D1 protein . As an intrinsic membrane protein, it constitutes one of the five major proteins in the PSII core complex, alongside CP47, CP43, D2, and cytochrome b-559 .
Recombinant Protein Production
The recombinant version of Photosystem Q(B) protein 2 is typically expressed in Escherichia coli bacterial systems with an N-terminal histidine tag that facilitates purification through affinity chromatography . The full-length protein (1-344 amino acids) is produced to maintain all functional domains present in the native form.
Electron Transport and QB Binding
The D1 protein serves as the binding site for the plastoquinone QB, which functions as the secondary electron acceptor in PSII . The electron transport pathway involves the initial excitation of chlorophyll molecules, followed by electron transfer to the primary electron acceptor QA, and subsequently to QB. After receiving two electrons and two protons in sequential photochemical events, QB is reduced to plastoquinol (QBH2), which then detaches from the D1 protein and diffuses into the thylakoid membrane . A fresh plastoquinone molecule from the membrane pool then occupies the vacated QB site, continuing the electron transport process.
Integration in Photosystem II Complex
In the native environment, the D1 protein is not isolated but functions as an integral component of the PSII core complex. This complex exists as a dimer with a molecular mass of approximately 580 kDa, comprising the five major intrinsic membrane proteins mentioned earlier, three extrinsic proteins (33 kDa, 12 kDa, and cytochrome c-550), and several low molecular mass membrane proteins . The intact complex is capable of remarkable oxygen evolution rates, reaching up to 3,400 μmol (mg Chlorophyll)⁻¹ h⁻¹ at 45°C when using ferricyanide as an electron acceptor .
Redox Potentials of QB
The thermodynamic properties of the QB molecule bound to the D1 protein have been investigated using electron paramagnetic resonance (EPR) spectroscopy. Research on Thermosynechococcus elongatus, a related thermophilic cyanobacterium, has determined the midpoint potentials (Em) of the QB redox couples:
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Em(QB/QB- −) ≈ 90 mV
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Em(QB- −/QBH2) ≈ 40 mV
Binding Affinity and Electron Transfer Dynamics
The redox potential measurements reveal that plastoquinone (PQ) binds approximately 50 times more tightly to the QB site than plastoquinol (PQH2) . This differential binding affinity is crucial for the functional cycle, as it promotes the release of the reduced product (QBH2) and facilitates the binding of fresh substrate (PQ). Additionally, the significant difference (approximately 234 meV) between the midpoint potentials of QB/QB- − and QA/QA- − establishes the driving force for electron transfer from QA- − to QB, ensuring efficient forward electron flow and minimizing backward reactions .
QB Redox Status as a Stress Sensor
Recent research has uncovered an intriguing role for the QB site in stress sensing mechanisms. Studies on Synechococcus elongatus PCC 7942 have shown that the redox status of the plastoquinone bound at the QB site may serve as a sensory cue for responding to diverse environmental stresses, including high light, low temperature, high salinity, and oxidative stress .
Interaction with Stress Response Pathways
Investigations into the stress sensing mechanism of NblS, a conserved histidine kinase in Synechococcus elongatus, revealed that molecular responses induced by various stresses were suppressed in the presence of 2,6-dichloro-1,4-benzoquinone (DCBQ), a compound that specifically accepts electrons from the QB site . This finding strongly suggests that the redox state of QB, rather than other factors such as membrane fluidity or direct stress effects, acts as the primary sensory cue for activating stress response pathways .
Thermal and Temporal Stability
The PSII core complex containing the D1 protein from thermophilic cyanobacteria exhibits remarkable stability. Research has shown that isolated complexes maintain their protein composition and function with minimal degradation even after extended storage periods. Specifically, studies have documented less than 10% loss in oxygen evolution capacity after dark incubation at 20°C for eight days, with no detectable release of extrinsic proteins or proteolytic degradation of subunits .
Insights into Photosynthetic Efficiency
Research on the D1 protein and its QB binding site has provided crucial insights into the thermodynamic optimization of photosynthetic electron transport. The fine-tuning of redox potentials ensures efficient forward electron flow while minimizing energy-wasting back-reactions and electron leakage to oxygen . This knowledge contributes to our understanding of how photosynthetic organisms maximize energy conversion efficiency under varying environmental conditions.
Product Specs
Note: We will prioritize shipping the format currently in stock. However, if you have specific format requirements, please indicate them during order placement, and we will accommodate your request.
Note: All our proteins are shipped with standard blue ice packs by default. If you require dry ice shipping, please contact us in advance, as additional fees will apply.
Generally, liquid formulations have a shelf life of 6 months at -20°C/-80°C. Lyophilized formulations typically have a shelf life of 12 months at -20°C/-80°C.
Target Background
KEGG: syf:Synpcc7942_0893
STRING: 1140.Synpcc7942_1389
Q&A
What is Synechococcus elongatus Photosystem Q(B) protein 2 and what is its role in photosynthesis?
Synechococcus elongatus Photosystem Q(B) protein 2 (also known as psbA2, PSII D1 protein 2) is a crucial component of Photosystem II (PSII) in cyanobacteria. This protein contains the binding site for the exchangeable plastoquinone molecule (QB) that accepts electrons from QA during photosynthetic electron transport.
The protein functions as part of the electron transfer chain in PSII, where it facilitates the sequential reduction of plastoquinone to plastohydroquinone (PQH2). This occurs through a two-step process where QB first accepts one electron to form a semiquinone (QB- −) and then a second electron coupled with protonation to form PQH2, which subsequently dissociates from the binding site and enters the plastoquinone pool in the thylakoid membrane .
The full amino acid sequence of the protein consists of 344 amino acids with multiple transmembrane domains, forming an integral part of the PSII reaction center .
What are the structural characteristics of recombinant Synechococcus elongatus Photosystem Q(B) protein 2?
Recombinant forms of this protein are typically expressed with an N-terminal His-tag to facilitate purification. The commercially available recombinant protein has the following specifications:
| Parameter | Specification |
|---|---|
| Protein Length | Full Length (1-344 amino acids) |
| Tag | N-terminal His tag |
| Expression System | E. coli |
| Form | Lyophilized powder |
| Purity | >90% as determined by SDS-PAGE |
| Storage Conditions | -20°C to -80°C |
| Storage Buffer | Tris/PBS-based buffer, 6% Trehalose, pH 8.0 |
The amino acid sequence is highly conserved among various strains of Synechococcus, with slight variations between species . The protein contains multiple transmembrane helices that anchor it within the thylakoid membrane, positioning the QB binding site in an optimal orientation for electron transfer from QA.
How can researchers optimize the storage and handling of recombinant Photosystem Q(B) protein 2?
For optimal results with recombinant Photosystem Q(B) protein 2, researchers should follow these methodological guidelines:
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Initial preparation: Briefly centrifuge the vial before opening to bring contents to the bottom.
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Reconstitution protocol: Reconstitute the lyophilized protein in deionized sterile water to achieve a concentration of 0.1-1.0 mg/mL.
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Storage optimization: Add glycerol to a final concentration of 5-50% (with 50% being optimal for long-term storage) and aliquot for storage at -20°C/-80°C.
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Working aliquots: Store working aliquots at 4°C for no more than one week.
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Stability considerations: Avoid repeated freeze-thaw cycles as they significantly compromise protein integrity .
These handling protocols are critical for maintaining the structural integrity and functional properties of the protein for experimental applications.
What are the redox properties of the QB site in Photosystem II and how do they influence electron transport dynamics?
The QB site in Photosystem II exhibits specific redox properties that have been carefully characterized using electron paramagnetic resonance (EPR) spectroscopy. Research on Thermosynechococcus elongatus has revealed the following midpoint potentials:
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Em(QB/QB- −) ≈ 90 mV
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Em(QB- −/QBH2) ≈ 40 mV
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Average Em for both couples ≈ 65 mV
These values have significant functional implications. The semiquinone (QB- −) is thermodynamically stable with a relatively high potential, which minimizes both back-reactions and electron leakage to molecular oxygen. The energy gap between the QA/QA- − couple and the QB redox couples (approximately 234 meV) provides sufficient driving force for the forward electron transfer from QA- − to QB .
How does the differential binding affinity between quinone and quinol at the QB site impact PSII function?
Research using redox titrations has demonstrated that quinone (PQ) binds to the QB site approximately 50 times more tightly than quinol (PQH2). This differential binding affinity has profound functional implications:
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It ensures preferential binding of the substrate (PQ) over the product (PQH2), allowing PSII to function efficiently even when the plastoquinone pool is significantly reduced.
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The thermodynamic driving force (ΔE ≈ 50 meV) facilitates the release of the product (PQH2) into the membrane pool.
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This optimization allows PSII to maintain function across a wide range of plastoquinone pool reduction states, an essential adaptation for efficient photosynthesis under variable environmental conditions .
This preferential binding regime is similar to that observed in purple bacterial reaction centers, suggesting evolutionary conservation of this functional feature. The preferential binding characteristics challenge earlier assumptions of equal binding constants for quinone and quinol and provide a mechanistic explanation for efficient PSII operation under varying physiological conditions .
What experimental approaches are most effective for studying the energetics of electron transfer at the QB site?
Several complementary experimental approaches have proven effective for investigating the energetics of electron transfer at the QB site:
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Electron Paramagnetic Resonance (EPR) Spectroscopy: This technique allows direct measurement of the semiquinone radical (QB- −) formation and has been successfully used to determine the midpoint potentials of both QB redox couples.
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Thermoluminescence: This approach provides a functional estimate of the energy gap between QA and QB redox couples, offering valuable insights into the driving force for electron transfer.
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Fourier Transform Infrared (FTIR) Spectroscopy: This method enables monitoring of QB- − formation upon illumination as a function of applied potential, providing information about the redox properties of the QB site.
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Electrochemical Redox Titrations: These experiments allow determination of the redox potentials of the QB couples under controlled conditions .
Each of these approaches offers unique advantages, and a comprehensive understanding of QB energetics typically requires a combination of these techniques. The choice of method depends on the specific research question and available instrumentation.
How can the psbA2 promoter be utilized for enhanced recombinant protein expression in cyanobacteria?
The psbA2 promoter has emerged as a valuable tool for recombinant protein expression in cyanobacteria due to its stress-responsive properties. Recent research has demonstrated several key strategies for optimizing this system:
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Gene Integration: The psbA2 promoter can be successfully used for the integration of recombinant genes in Synechococcus elongatus PCC 7942, as demonstrated with the ZsGreen1 reporter gene.
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Magnetic Field Application: Exposure to moderate magnetic fields (30 mT) has been shown to enhance gene transcription under the psbA2 promoter, likely due to stress-induced shifts in gene expression and enzyme activity.
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Photosystem Modulation: The 30 mT magnetic field positively impacts Photosystem II without disrupting the electron transport chain, providing a non-invasive method to enhance recombinant protein production.
This approach offers several advantages over traditional methods, including elimination of costly exogenous inducers and reduction of potential cell stress. Furthermore, the use of native promoters such as psbA2 allows for more physiologically relevant expression patterns in photoautotrophic microorganisms .
What is the relationship between PsbQ protein and the assembly of fully functional PSII complexes in cyanobacteria?
The PsbQ protein plays a critical role in defining the fully assembled and optimally active form of PSII in cyanobacteria. Research comparing PsbQ-associated PSII complexes with those isolated using histidine-tagged CP47 has revealed several important insights:
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PsbQ co-purifies with other extrinsic proteins: PsbQ is present in combination with PsbO, PsbU, and PsbV proteins in cyanobacterial PSII.
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Enhanced activity and stability: PsbQ-associated PSII complexes exhibit higher oxygen-evolution activity compared to CP47-tagged PSII complexes.
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Subpopulation enrichment: PsbQ-tagged PSII complexes represent a smaller subpopulation (approximately 25-30% on a per-Chl basis) of CP47-containing PSII complexes, but this subpopulation consists of fully assembled and highly active complexes.
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Manganese stabilization: PsbQ plays a role in stabilizing the manganese cluster in the oxygen-evolving center on the luminal side of PSII .
These findings suggest that while PSII complexes exist in heterogeneous populations within the cell (due to ongoing assembly and disassembly resulting from light-mediated damage), the presence of PsbQ can be used as a marker for isolating fully assembled and optimally functional PSII complexes for research purposes.
How does the thermodynamic stability of the semiquinone state (QB- −) contribute to PSII efficiency?
The semiquinone intermediate (QB- −) exhibits remarkable thermodynamic stability in PSII, with several functional consequences:
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Minimized back-reactions: The relatively high potential of QB- − reduces the likelihood of back-reactions, thereby enhancing the forward electron transfer efficiency.
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Reduced electron leakage: The stability of QB- − minimizes electrons leaking onto O2, which would otherwise form harmful reactive oxygen species.
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Optimized electron transfer: The energy gap between QA- − and QB (≥180-234 meV) provides sufficient driving force for efficient forward electron transfer while avoiding excessive energy losses.
This intricate redox tuning of QB with respect to neighboring redox partners (QA and free plastoquinone) represents an evolutionary optimization that balances the need for efficient forward electron transfer against the risks of back-reactions and side reactions with oxygen .
What methods can be employed to enhance the experimental utility of recombinant Photosystem Q(B) protein 2?
Several advanced methodological approaches can enhance the experimental utility of recombinant Photosystem Q(B) protein 2:
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Reconstitution into liposomes: This approach can restore the native membrane environment, potentially enhancing protein stability and functionality for biophysical studies.
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Site-directed mutagenesis: Strategic amino acid substitutions can be introduced to probe structure-function relationships within the QB binding site.
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Isotopic labeling: Expression of the protein with specific isotopic labels (15N, 13C, 2H) can facilitate structural studies using NMR spectroscopy or mass spectrometry.
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Co-expression with other PSII components: This approach can enable studies of protein-protein interactions and complex assembly.
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Integration of redox-active cofactors: Ensuring proper incorporation of essential cofactors during reconstitution experiments is critical for functional studies .
These methodological refinements can significantly expand the range of experimental applications for recombinant Photosystem Q(B) protein 2, enabling more sophisticated investigations of its structure, function, and interactions.
What are the key considerations for designing experiments with recombinant Synechococcus elongatus Photosystem Q(B) protein 2?
When designing experiments with recombinant Synechococcus elongatus Photosystem Q(B) protein 2, researchers should consider several critical factors:
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Membrane protein handling: As an integral membrane protein, special consideration must be given to maintaining its structural integrity through appropriate detergent selection or membrane reconstitution.
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Redox environment: Controlling the redox environment is essential for studies of electron transfer properties, requiring careful buffer composition and potentially the use of redox mediators.
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Light sensitivity: The photosynthetic nature of this protein necessitates attention to light exposure during experimental procedures, with appropriate dark adaptation or controlled illumination as required.
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Species-specific variations: While the core function is conserved, researchers should be aware of subtle differences between Photosystem Q(B) protein 2 from different cyanobacterial species or strains.
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Integration with other PSII components: For functional studies, consideration of the protein's interactions with other PSII components may be necessary to reproduce native activity .
