Plastoquinol-plastocyanin reductase

 

Plastocyanin (PC) is a small (approx 10 kDa), type 1 (blue) copper protein that functions as an electron donor to Photosystem I (PSI) from cytochrome (cyt) f in both chloroplast systems and in some strains of cyanobacteria.

 

Reference Protein and Structure

Sequence
P50057 UniProt IPR023511 (Sequence Homologues) (PDB Homologues)
Biological species
Prochlorothrix hollandica (Bacteria) Uniprot
PDB
2b3i - NMR SOLUTION STRUCTURE OF PLASTOCYANIN FROM THE PHOTOSYNTHETIC PROKARYOTE, PROCHLOROTHRIX HOLLANDICA (19 STRUCTURES) (solution nmr Å) PDBe PDBsum 2b3i
Catalytic CATH Domains
2.60.40.420 CATHdb (see all for 2b3i)
Cofactors
Copper(2+) (1) Metal MACiE
Click To Show Structure

Enzyme Reaction (EC:7.1.1.6)

plastoquinol
CHEBI:62192ChEBI
+
hydron
CHEBI:15378ChEBI
+
copper(2+)
CHEBI:29036ChEBI
→
plastoquinone
CHEBI:17757ChEBI
+
hydron
CHEBI:15378ChEBI
+
copper(1+)
CHEBI:49552ChEBI
Alternative enzyme names: Cytochrome b6/f complex, Plastoquinol/plastocyanin oxidoreductase, Cytochrome b6f complex,

Enzyme Mechanism

Introduction

Cu is coordinated in a skewed tetrahedral cage of Cys82, His39, weakly bound Met90 and His85. One side of His85 is exposed to the solvent in a region of hydrophobic residues which interact with the electron donor and acceptor. Cyt f donates one electron to oxidised PC at the Cu(II) centre forming reduced Cu(I) PC by through bond electron transfer involving His85. Reduced PC then binds to oxidised PSI where it donates an electron, reducing PSI and reforming oxidised PC.

Catalytic Residues Roles

UniProt PDB* (2b3i)
His119 His85A Provides a medium for the transfer of electrons to and from the Cu centre. Also forms part of the copper binding site. single electron relay, metal ligand, activator, single electron acceptor, single electron donor, electrostatic stabiliser
Cys116, Met124, His73 Cys82A, Met90A, His39A Form part of the skewed tetrahedral cage that binds copper. metal ligand, electrostatic stabiliser
*PDB label guide - RESx(y)B(C) - RES: Residue Name; x: Residue ID in PDB file; y: Residue ID in PDB sequence if different from PDB file; B: PDB Chain; C: Biological Assembly Chain if different from PDB. If label is "Not Found" it means this residue is not found in the reference PDB.

Chemical Components

electron transfer, redox reaction, cofactor used, overall reactant used, intermediate formation, electron relay, overall product formed

References

  1. Babu CR et al. (1999), Biochemistry, 38, 4988-4995. NMR Solution Structure of Plastocyanin from the Photosynthetic Prokaryote,Prochlorothrix hollandica†,‡. DOI:10.1021/bi983024f. PMID:10213601.
  2. Hass MA et al. (2007), Biochemistry, 46, 14619-14628. Kinetics and Mechanism of the Acid Transition of the Active Site in Plastocyanin†. DOI:10.1021/bi701446u. PMID:18020375.
  3. Modi S et al. (1992), Biochim Biophys Acta, 1102, 85-90. Reactivity of cytochromes c and f with mutant forms of spinach plastocyanin. DOI:10.1016/0005-2728(92)90068-d. PMID:1324731.
  4. Guss JM et al. (1986), J Mol Biol, 192, 361-387. Crystal structure analyses of reduced (CuI) poplar plastocyanin at six pH values. DOI:10.1016/0022-2836(86)90371-2. PMID:3560221.

Step 1. Cytochrome b6f donates one electron to the oxidised form of plastocyanin through the surface exposed His85. Multiple redox partners have been identified for plastocyanin [PMID:10213601].

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
His85A activator, electrostatic stabiliser, metal ligand
Cys82A electrostatic stabiliser, metal ligand
His39A electrostatic stabiliser, metal ligand
Met90A electrostatic stabiliser, metal ligand
His85A single electron relay, single electron acceptor, single electron donor

Chemical Components

electron transfer, redox reaction, cofactor used, overall reactant used, intermediate formation, electron relay, overall product formed

Step 2. Reduced plastocyanin binds to oxidised Photosystem I, initiating electron transfer and regeneration of the plastocyanin active site. Electron transfer to oxidised Photosystem I occurs through the same solvent exposed His85

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
His85A activator, electrostatic stabiliser, metal ligand
Cys82A electrostatic stabiliser, metal ligand
His39A electrostatic stabiliser, metal ligand
Met90A electrostatic stabiliser, metal ligand
His85A single electron acceptor
His85A single electron relay, single electron donor

Chemical Components

electron transfer, redox reaction, cofactor used, overall reactant used, intermediate formation, electron relay, overall product formed
Download: Image, Marvin File

Step 1. Cytochrome b6f donates one electron to the oxidised form of plastocyanin through the surface exposed His85. Multiple redox partners have been identified for plastocyanin [PMID:10213601].

Step 2. Reduced plastocyanin binds to oxidised Photosystem I, initiating electron transfer and regeneration of the plastocyanin active site. Electron transfer to oxidised Photosystem I occurs through the same solvent exposed His85

Products.

Contributors

Sophie T. Williams, Gemma L. Holliday