Photosystem I P700 chlorophyll a apoprotein A2 (psaB) is a 741-residue protein from Thermosynechococcus vestitus (strain NIES-2133 / IAM M-273 / BP-1). This is its AlphaFold structure prediction, created 1 Aug 2025. UniProt accession: P0A407.
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The mean pLDDT of this model is 96.6 (very high overall). pLDDT is AlphaFold's per-residue confidence score from 0 to 100. In MolViewer, choose the B-factor color scheme to color the model by pLDDT, because AlphaFold stores it in the B-factor column.
| pLDDT band | Meaning | Share of residues |
|---|---|---|
| Above 90 | Very high: backbone and side chains are usually accurate | 97% |
| 70 to 90 | Confident: backbone generally right | 3% |
| 50 to 70 | Low: treat with caution | 1% |
| Below 50 | Very low: often disordered regions | 0% |
What pLDDT means and how to read it
PsaA and PsaB bind P700, the primary electron donor of photosystem I (PSI), as well as the electron acceptors A0, A1 and FX. PSI is a plastocyanin/cytochrome c6-ferredoxin oxidoreductase, converting photonic excitation into a charge separation, which transfers an electron from the donor P700 chlorophyll pair to the spectroscopically characterized acceptors A0, A1, FX, FA and FB in turn. Oxidized P700 is reduced on the lumenal side of the thylakoid membrane by plastocyanin or cytochrome c6
The PsaA/B heterodimer binds the P700 chlorophyll special pair and subsequent electron acceptors. PSI consists of a core antenna complex that captures photons, and an electron transfer chain that converts photonic excitation into a charge separation. The cyanobacterial PSI reaction center is composed of one copy each of PsaA,B,C,D,E,F,I,J,K,L,M and X, and forms trimeric complexes
Cellular thylakoid membrane
Compare the prediction with experimentally determined structures of the same protein:
| PDB ID | Method | Resolution | Chains and residues |
|---|---|---|---|
| 7FIX | EM | 1.97 Å | B1/B2/B3=1-741 |
| 1JB0 | X-ray | 2.5 Å | B=2-741 |
| 7M75 | X-ray | 2.75 Å | B=2-741 |
| 6TRA | EM | 2.85 Å | B=1-741 |
| 6PFY | X-ray | 2.9 Å | B/H/Z=1-741 |
| 6PGK | X-ray | 2.9 Å | B/H/Z=1-741 |
| 6TRC | EM | 2.98 Å | 2/B/b=1-741 |
| 7M76 | X-ray | 3.0 Å | B=2-741 |
| 7M78 | X-ray | 3.0 Å | B=2-741 |
| 6TRD | EM | 3.16 Å | 2/B/b=1-741 |
| 6LU1 | EM | 3.2 Å | B=1-741 |
| 9LZJ | EM | 3.4 Å | aB/bB/cB=1-741 |
| 9LZK | EM | 3.5 Å | B=1-741 |
| 1C51 | X-ray | 4.0 Å | - |
| 2PPS | X-ray | 4.0 Å | - |
| 5ZF0 | X-ray | 4.2 Å | B1/B2/B3/B4/B5/B6=2-741 |
| 4FE1 | X-ray | 4.92 Å | B=2-741 |
| 7BW2 | X-ray | 6.5 Å | B=2-741 |
| 3PCQ | X-ray | 8.98 Å | B=2-741 |
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