Plastocyanin is a copper-containing protein that mediates
electron-transfer. It is found in a variety of plants, where it participates in
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. The protein is a prototype of the
blue copper proteins, a family of intensely blue-colored
metalloproteins. Specifically, it falls into the group of small type I blue copper proteins called "cupredoxins".
Function
In
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
, plastocyanin functions as an electron transfer agent between
cytochrome f of the
cytochrome ''b''6''f'' complex from
photosystem II and P700+ from
photosystem I
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the Light-dependent reactions, photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane ...
.
Cytochrome ''b''6''f'' complex and P700
+ are both membrane-bound proteins with exposed residues on the lumen-side of the
thylakoid
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacterium, cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a #Membrane, thylakoid membrane surrounding a #Lumen, ...
membrane of
chloroplasts. Cytochrome f acts as an electron donor while P700+ accepts electrons from reduced plastocyanin.
Structure
The copper site in plastocyanin, with the four amino acids that bind the metal labelled.
Plastocyanin was the first of the blue copper proteins to be characterised by
X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
.
It features an eight-stranded
antiparallel β-barrel
In protein structures, a beta barrel (β barrel) is a beta sheet (β sheet) composed of Protein tandem repeats, tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrog ...
containing one
copper
Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
center.
Structures of the protein from poplar,
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
,
parsley
Parsley, or garden parsley (''Petroselinum crispum''), is a species of flowering plant in the family Apiaceae that is native to Greece, Morocco and the former Yugoslavia. It has been introduced and naturalisation (biology), naturalized in Eur ...
, spinach, and French bean plants have been characterized crystallographically.
In all cases the binding site is generally conserved. Bound to the copper center are four
ligands
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...
: the
imidazole
Imidazole (ImH) is an organic compound with the formula . It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. It can be classified as a heterocycle, specifically as a diazole.
Many natural products, ...
groups of two
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an Amine, α-amino group (which is in the protonated –NH3+ form under Physiological condition, biological conditions), a carboxylic ...
residues (His37 and His87), the thiolate of
Cys84 and the thioether of
Met92. The geometry of the copper binding site is described as a ‘distorted tetrahedral’. The Cu-S (Cys) contact is much shorter (207
picometer
The picometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: pm) or picometer ( American spelling) is a unit of length in the International System of Units (SI), equal to , or one trillionth o ...
s) than Cu-S (Met) (282 pm) bond. The elongated Cu-thioether bond appears to destabilise the Cu
II state thereby enhancing its oxidizing power. The blue colour (597
nm peak absorption) is assigned to a charge transfer transition from S
pπ to Cu
dx2-y2.
In the reduced form of plastocyanin, His-87 becomes protonated.
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 or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
-83.
Algal
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, s ...
plastocyanins, and those from vascular plants in the family
Apiaceae
Apiaceae () or Umbelliferae is a family of mostly aromatic flowering plants named after the type genus ''Apium,'' and commonly known as the celery, carrot, or parsley family, or simply as umbellifers. It is the 16th-largest family of flowering p ...
, contain similar acidic residues but are shaped differently from those of plant plastocyanins—they lack residues 57 and 58. In
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
, the distribution of charged residues on the surface is different from
eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
plastocyanins and variations among different bacterial species is large. Many cyanobacterial plastocyanins have 107 amino acids. Although the
acidic
An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.
The first category of acids are the ...
patches are not conserved in bacteria, the
hydrophobic
In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, thu ...
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 (Cu
2+Pc) is reduced (an electron is added) by cytochrome f according to the following reaction:
:Cu
2+Pc + e
− → Cu
+Pc
After dissociation, Cu
+Pc diffuses through the lumen space until recognition/binding occurs with P700
+, at which point P700
+ oxidizes Cu
+Pc according to the following reaction:
:Cu
+Pc → Cu
2+Pc + e
−
The redox potential is about 370 mV and the isoelectric pH is about 4.
Entatic state
A catalyst's function is to increase the speed of the electron transfer (
redox
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
) reaction. Plastocyanin is believed to work less like an enzyme where enzymes decrease the transition energy needed to transfer the electron. Plastocyanin works more on the principles of
entatic states where it increases the energy of the reactants, decreasing the amount of energy needed for the redox reaction to occur. Another way to rephrase the function of plastocyanin is that it can facilitate the electron transfer reaction by providing a small
reorganization energy, which has been measured to about .
To study the properties of the redox reaction of plastocyanin, methods such as
quantum mechanics
Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...
/
molecular mechanics
Molecular mechanics uses classical mechanics to model molecular systems. The Born–Oppenheimer approximation is assumed valid and the potential energy of all systems is calculated as a function of the nuclear coordinates using Force field (chemi ...
(QM/MM)
molecular dynamics
Molecular dynamics (MD) is a computer simulation method for analyzing the Motion (physics), physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamics ( ...
simulations. This method was used to determine that plastocyanin has an entatic
strain energy
In physics, the elastic potential energy gained by a wire during elongation with a tensile (stretching) or compressive (contractile) force is called strain energy. For linearly elastic materials, strain energy is:
: U = \frac 1 2 V \sigma \v ...
of about .

Four-coordinate copper complexes often exhibit
square planar geometry, however plastocyanin has a trigonally distorted
tetrahedral geometry. This distorted geometry is less stable than ideal tetrahedral geometry due to its lower ligand field stabilization as a result of the trigonal distortion. This unusual geometry is induced by the rigid “pre-organized” conformation of the ligand donors by the protein, which is an
entatic state. Plastocyanin performs electron transfer with the
redox
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
between Cu(I) and Cu(II), and it was first theorized that its entatic state was a result of the protein imposing an undistorted tetrahedral geometry preferred by ordinary Cu(I) complexes onto the oxidized Cu(II) site.
However, a highly distorted tetrahedral geometry is induced upon the oxidized Cu(II) site instead of a perfectly symmetric tetrahedral geometry. A feature of the entatic state is a protein environment that is capable of preventing ligand dissociation even at a high enough temperature to break the metal-ligand bond. In the case of plastocyanin, it has been experimentally determined through
absorption spectroscopy
Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons ...
that there is a long and weak Cu(I)-S
Met bond that should dissociate at physiological temperature due to increased entropy. However, this bond does not dissociate due to the constraints of the protein environment dominating over the entropic forces.

In ordinary copper complexes involved in Cu(I)/Cu(II) redox coupling without a constraining protein environment, their ligand geometry changes significantly, and typically corresponds to the presence of a
Jahn-Teller distorting force. However, the Jahn-Teller distorting force is not present in plastocyanin due to a large splitting of the d
x2-y2 and d
xy orbitals (See
Blue Copper Protein Entatic State). Additionally, the structure of plastocyanin exhibits a long Cu(I)-S
Met bond (2.9Å) with decreased electron donation strength. This bond also shortens the Cu(I)-S
Cys bond (2.1Å), increasing its electron donating strength. Overall, plastocyanin exhibits a lower reorganization energy due to the entatic state of the protein ligand enforcing the same distorted tetrahedral geometry in both the Cu(II) and Cu(I) oxidation states, enabling it to perform electron transfer at a faster rate. The reorganization energy of blue copper proteins such as plastocyanin from 0.7 to 1.2 eV (68-116 kJ/mol) compared to 2.4 eV (232 kJ/mol) in an ordinary copper complex such as
2">u(phen)2sup>2+/+.
In the ocean
Usually, plastocyanin can be found in organisms that contain
chlorophyll b and
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
, as well as algae that contain
chlorophyll c. Plastocyanin has also been found in the
diatom
A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
, ''Thalassiosira oceanica'', which can be found in oceanic environments. It was surprising to find these organisms containing the protein plastocyanin because the concentration of copper dissolved in the ocean is usually low (between 0.4 – 50 nM). However, the concentration of copper in the oceans is comparatively higher compared to the concentrations of other metals such as
zinc
Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
and
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
. Other organisms that live in the ocean, such as other phytoplankton species, have adapted to where they do not need as high of concentrations of these low concentration metals (Fe and Zn) to facilitate photosynthesis and grow.
References
Further reading
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{{refend
Photosynthesis
Coordination complexes
Copper proteins
Electron-transfer proteins