Plastocyanin
Encyclopedia
Plastocyanin is an important copper-containing protein
involved in electron-transfer. The protein
is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants. It is a member of the plastocyanin family of copper-binding proteins.
, plastocyanin functions as an electron transfer agent between cytochrome f
of the cytochrome b6f complex
from photosystem II
and P700+ from photosystem I
. Cytochrome b6f complex
and P700+ are both membrane-bound proteins with exposed residues on the lumen-side of the thylakoid
membrane of chloroplasts. Cytochrome f acts as an electron donor while P700+ accepts electrons from reduced plastocyanin.
to be characterised by X-ray crystallography
. The tertiary structure is a beta-barrel — common in proteins which bind to other proteins.
Although the molecular surface of plastocyanins differs for plants, algae, and cyanobacteria, the structure of the copper binding site is generally conserved. The copper binding site is described as a ‘distorted trigonal pyramidal’. The trigonal plane of the pyramidal base is composed of two nitrogen atoms (N1 and N2) from separate histidines and a sulfur (S1) from a cysteine. Sulfur (S2) from an axial methionine forms the apex. The ‘distortion’ occurs in the bond lengths between the copper and sulfur ligands. The Cu-S1 contact is shorter (207 picometers) than Cu-S2 (282 pm).
The elongated Cu-S2 bonding destabilises the CuII form and increases the redox potential of the protein. The blue colour (597 nm peak absorption) is due to the Cu-S1 bond where Spπ to Cudx2-y2 charge transfer occurs.
In the reduced form of plastocyanin, His-87 will become protonated with a pKa
of 4.4. Protonation prevents it acting as a ligand and the copper site geometry becomes trigonal planar.
While the molecular surface of the protein near the copper binding site varies slightly, all plastocyanins have a hydrophobic surface surrounding the exposed histidine of the copper binding site. In plant plastocyanins, acidic residues are located on either side of the highly conserved tyrosine
-83. Algal plastocyanins, and those from vascular plants in the family Apiaceae
, contain similar acidic residues but are shaped differently from those of plant plastocyanins—they lack residues 57 and 58. In cyanobacteria, the distribution of charged residues on the surface is different from eukaryotic plastocyanins and variations among different bacterial species is large. Many cyanobacterial plastocyanins have 107 amino acids. Although the acidic patches are not conserved in bacteria the hydrophobic patch is always present. These hydrophobic and acidic patches are believed to be the recognition/binding sites for the other proteins involved in electron transfer.
After dissociation, Cu+Pc diffuses through the lumen until recognition/binding occurs with P700+. P700+ oxidizes Cu+Pc according to the following reaction:
The redox potential is about 370 mV and the isoelectric pH is about 4.
Copper proteins
Copper proteins are proteins that contain one or more copper ions as prosthetic groups. The metal centres in the copper proteins can be classified into several types:...
involved in electron-transfer. The protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants. It is a member of the plastocyanin family of copper-binding proteins.
Function
In photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, plastocyanin functions as an electron transfer agent between cytochrome f
Cytochrome f
Cytochrome f is the largest subunit of cytochrome b6f complex . In its structure and functions, the cytochrome b6f complex bears extensive analogy to the cytochrome bc1 complex of mitochondria and photosynthetic purple bacteria...
of the cytochrome b6f complex
Cytochrome b6f complex
The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, catalyzing the transfer of electrons from plastoquinol to plastocyanin...
from photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
and P700+ from photosystem I
Photosystem I
Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
. Cytochrome b6f complex
Cytochrome b6f complex
The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, catalyzing the transfer of electrons from plastoquinol to plastocyanin...
and P700+ are both membrane-bound proteins with exposed residues on the lumen-side of the thylakoid
Thylakoid
A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as...
membrane of chloroplasts. Cytochrome f acts as an electron donor while P700+ accepts electrons from reduced plastocyanin.
Structure
Plastocyanin was the first blue copper proteinsCopper proteins
Copper proteins are proteins that contain one or more copper ions as prosthetic groups. The metal centres in the copper proteins can be classified into several types:...
to be characterised by X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
. The tertiary structure is a beta-barrel — common in proteins which bind to other proteins.
Although the molecular surface of plastocyanins differs for plants, algae, and cyanobacteria, the structure of the copper binding site is generally conserved. The copper binding site is described as a ‘distorted trigonal pyramidal’. The trigonal plane of the pyramidal base is composed of two nitrogen atoms (N1 and N2) from separate histidines and a sulfur (S1) from a cysteine. Sulfur (S2) from an axial methionine forms the apex. The ‘distortion’ occurs in the bond lengths between the copper and sulfur ligands. The Cu-S1 contact is shorter (207 picometers) than Cu-S2 (282 pm).
The elongated Cu-S2 bonding destabilises the CuII form and increases the redox potential of the protein. The blue colour (597 nm peak absorption) is due to the Cu-S1 bond where Spπ to Cudx2-y2 charge transfer occurs.
In the reduced form of plastocyanin, His-87 will become protonated with a pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
of 4.4. Protonation prevents it acting as a ligand and the copper site geometry becomes trigonal planar.
While the molecular surface of the protein near the copper binding site varies slightly, all plastocyanins have a hydrophobic surface surrounding the exposed histidine of the copper binding site. In plant plastocyanins, acidic residues are located on either side of the highly conserved tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
-83. Algal plastocyanins, and those from vascular plants in the family Apiaceae
Apiaceae
The Apiaceae , commonly known as carrot or parsley family, is a group of mostly aromatic plants with hollow stems. The family is large, with more than 3,700 species spread across 434 genera, it is the sixteenth largest family of flowering plants...
, contain similar acidic residues but are shaped differently from those of plant plastocyanins—they lack residues 57 and 58. In cyanobacteria, the distribution of charged residues on the surface is different from eukaryotic plastocyanins and variations among different bacterial species is large. Many cyanobacterial plastocyanins have 107 amino acids. Although the acidic patches are not conserved in bacteria the hydrophobic patch is always present. These hydrophobic and acidic patches are believed to be the recognition/binding sites for the other proteins involved in electron transfer.
Reactions
Plastocyanin (Cu2+Pc) is reduced by cytochrome f according to the following reaction:- Cu2+Pc + e- → Cu+Pc
After dissociation, Cu+Pc diffuses through the lumen until recognition/binding occurs with P700+. P700+ oxidizes Cu+Pc according to the following reaction:
- Cu+Pc → Cu2+Pc + e-
The redox potential is about 370 mV and the isoelectric pH is about 4.