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Plastocyanin

February 15, 2024

Plastocyanin is a copper-containing protein that mediates electron-transfer. It is found in a variety of plants, where it participates in photosynthesis. 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".[1]

Plastocyanin
Phormidium laminosum plastocyanin, PDB: 3BQV​.
Identifiers
SymbolPlastocyanin
InterProIPR002387
CATH3BQV
SCOP23BQV / SCOPe / SUPFAM
CDDcd04219
UniProt Family

Function edit

In photosynthesis, 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.[2]

Structure edit

 
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.[3][2][4] It features an eight-stranded antiparallel β-barrel containing one copper center.[3]

Structures of the protein from poplar, algae, parsley, spinach, and French bean plants have been characterized crystallographically.[3] In all cases the binding site is generally conserved. Bound to the copper center are four ligands: the imidazole groups of two histidine 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 trigonal pyramidal’. The Cu-S (cys) contact is much shorter (207 picometers) than Cu-S (met) (282 pm) bond. The elongated Cu-thioether bond appears to destabilise the CuII state thereby enhancing its oxidizing power. The blue colour (597 nm peak absorption) is assigned to a charge transfer transition from Spπ to Cudx2-y2.[5]

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-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.

Reactions edit

Plastocyanin (Cu2+Pc) is reduced (an electron is added) by cytochrome f according to the following reaction:

Cu2+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 → Cu2+Pc + e−

The redox potential is about 370 mV[6] and the isoelectric pH is about 4.[7]

Entatic state edit

A catalyst's function is to increase the speed of the electron transfer (redox) 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 16–28 kcal/mol (67–117 kJ/mol).[8]

To study the properties of the redox reaction of plastocyanin, methods such as quantum mechanics / molecular mechanics (QM/MM) molecular dynamics simulations. This method was used to determine that plastocyanin has an entatic strain energy of about 10 kcal/mol (42 kJ/mol).[8]

 
Copper site of Plastocyanin from PDB 1AG6 showing the distorted tetrahedral geometry with the elongated Cu(I)-SMet and shortened Cu(I)-SCys bonds.[9]

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 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.[10] 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 that there is a long and weak Cu(I)-SMet 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.[11]

 
Copper site of Plastocyanin showing the large splitting of the Cu dx2-y2 and SCys dxy orbitals.[12]

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 dx2-y2 and dxy orbitals (See Blue Copper Protein Entatic State). Additionally, the structure of plastocyanin exhibits a long Cu(I)-SMet bond (2.9Å) with decreased electron donation strength. This bond also shortens the Cu(I)-SCys 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.[13] 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 [Cu(phen)2]2+/+.[10]

In the ocean edit

Usually, plastocyanin can be found in organisms that contain chlorophyll b and cyanobacteria, as well as algae that contain chlorophyll c. Plastocyanin has also been found in the diatom, 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 and iron. Other organisms that live in the ocean, such as phytoplankton, have adapted to where they do not need these low concentration metals (Fe and Zn) to facilitate photosynthesis and grow.[14]

References edit

  1. ^ Choi M, Davidson VL (February 2011). "Cupredoxins--a study of how proteins may evolve to use metals for bioenergetic processes". Metallomics. 3 (2): 140–151. doi:10.1039/c0mt00061b. PMC 6916721. PMID 21258692. (for an overview of the various types of blue copper proteins)
  2. ^ a b Redinbo MR, Yeates TO, Merchant S (February 1994). "Plastocyanin: structural and functional analysis". Journal of Bioenergetics and Biomembranes. 26 (1): 49–66. doi:10.1007/BF00763219. PMID 8027022. S2CID 2662584.
  3. ^ a b c Xue Y, Okvist M, Hansson O, Young S (October 1998). "Crystal structure of spinach plastocyanin at 1.7 A resolution". Protein Science. 7 (10): 2099–2105. doi:10.1002/pro.5560071006. PMC 2143848. PMID 9792096.
  4. ^ Freeman HC, Guss JM (2001). "Plastocyanin". In Bode W, Messerschmidt A, Cygler M (eds.). Handbook of metalloproteins. Vol. 2. Chichester: John Wiley & Sons. pp. 1153–69. ISBN 978-0-471-62743-2.
  5. ^ Gewirth AA, Solomon EI (June 1988). "Electronic structure of plastocyanin: excited state spectral features". J Am Chem Soc. 110 (12): 3811–9. doi:10.1021/ja00220a015.
  6. ^ Anderson GP, Sanderson DG, Lee CH, Durell S, Anderson LB, Gross EL (December 1987). "The effect of ethylenediamine chemical modification of plastocyanin on the rate of cytochrome f oxidation and P-700+ reduction". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 894 (3): 386–398. doi:10.1016/0005-2728(87)90117-4. PMID 3689779.
  7. ^ Ratajczak R, Mitchell R, Haehnel W (1988). "Properties of the oxidizing site of Photosystem I". Biochim. Biophys. Acta. 933 (2): 306–318. doi:10.1016/0005-2728(88)90038-2.
  8. ^ a b Hurd CA, Besley NA, Robinson D (June 2017). "A QM/MM study of the nature of the entatic state in plastocyanin". Journal of Computational Chemistry. 38 (16): 1431–1437. doi:10.1002/jcc.24666. PMC 5434870. PMID 27859435.
  9. ^ Xue Y, Okvist M, Hansson O, Young S (October 1998). "Crystal structure of spinach plastocyanin at 1.7 A resolution". Protein Science. 7 (10): 2099–105. doi:10.1002/pro.5560071006. PMC 2143848. PMID 9792096.
  10. ^ a b Solomon EI, Szilagyi RK, DeBeer George S, Basumallick L (February 2004). "Electronic structures of metal sites in proteins and models: contributions to function in blue copper proteins". Chemical Reviews. 104 (2): 419–458. doi:10.1002/chin.200420281. PMID 14871131.
  11. ^ Solomon EI, Hadt RG (2011). "Recent advances in understanding blue copper proteins". Coordination Chemistry Reviews. 255 (7–8): 774–789. doi:10.1016/j.ccr.2010.12.008. ISSN 0010-8545.
  12. ^ Bertini G (2007). Biological Inorganic Chemistry: Structure and reactivity. University Science Books. p. 253. ISBN 978-1-891389-43-6.
  13. ^ Randall DW, Gamelin DR, LaCroix LB, Solomon EI (February 2000). "Electronic structure contributions to electron transfer in blue Cu and Cu(A)". Journal of Biological Inorganic Chemistry. 5 (1): 16–29. doi:10.1007/s007750050003. PMID 10766432. S2CID 20628012.
  14. ^ Peers G, Price NM (May 2006). "Copper-containing plastocyanin used for electron transport by an oceanic diatom". Nature. 441 (7091): 341–344. Bibcode:2006Natur.441..341P. doi:10.1038/nature04630. PMID 16572122. S2CID 4379844.

Further reading edit

  • Berg JM, Lippard SJ (1994). "Blue Copper Proteins". Principles of bioinorganic chemistry. Sausalito, Calif: University Science Books. pp. 237–242. ISBN 978-0-935702-72-9.
  • Sato K, Kohzuma T, Dennison C (February 2003). "Active-site structure and electron-transfer reactivity of plastocyanins". Journal of the American Chemical Society. 125 (8): 2101–2112. doi:10.1021/ja021005u. PMID 12590538.

February 15, 2024
plastocyanin, copper, containing, protein, that, mediates, electron, transfer, found, variety, plants, where, participates, photosynthesis, protein, prototype, blue, copper, proteins, family, intensely, blue, colored, metalloproteins, specifically, falls, into. Plastocyanin is a copper containing protein that mediates electron transfer It is found in a variety of plants where it participates in photosynthesis 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 1 PlastocyaninPhormidium laminosum plastocyanin PDB 3BQV IdentifiersSymbolPlastocyaninInterProIPR002387CATH3BQVSCOP23BQV SCOPe SUPFAMCDDcd04219UniProt Family Contents 1 Function 2 Structure 3 Reactions 4 Entatic state 5 In the ocean 6 References 7 Further readingFunction editIn photosynthesis 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 2 Structure edit nbsp 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 3 2 4 It features an eight stranded antiparallel b barrel containing one copper center 3 Structures of the protein from poplar algae parsley spinach and French bean plants have been characterized crystallographically 3 In all cases the binding site is generally conserved Bound to the copper center are four ligands the imidazole groups of two histidine 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 trigonal pyramidal The Cu S cys contact is much shorter 207 picometers than Cu S met 282 pm bond The elongated Cu thioether bond appears to destabilise the CuII state thereby enhancing its oxidizing power The blue colour 597 nm peak absorption is assigned to a charge transfer transition from Spp to Cudx2 y2 5 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 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 Reactions editPlastocyanin Cu2 Pc is reduced an electron is added by cytochrome f according to the following reaction Cu2 Pc e Cu PcAfter 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 Cu2 Pc e The redox potential is about 370 mV 6 and the isoelectric pH is about 4 7 Entatic state editA catalyst s function is to increase the speed of the electron transfer redox 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 16 28 kcal mol 67 117 kJ mol 8 To study the properties of the redox reaction of plastocyanin methods such as quantum mechanics molecular mechanics QM MM molecular dynamics simulations This method was used to determine that plastocyanin has an entatic strain energy of about 10 kcal mol 42 kJ mol 8 nbsp Copper site of Plastocyanin from PDB 1AG6 showing the distorted tetrahedral geometry with the elongated Cu I SMet and shortened Cu I SCys bonds 9 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 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 10 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 that there is a long and weak Cu I SMet 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 11 nbsp Copper site of Plastocyanin showing the large splitting of the Cu dx2 y2 and SCys dxy orbitals 12 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 dx2 y2 and dxy orbitals See Blue Copper Protein Entatic State Additionally the structure of plastocyanin exhibits a long Cu I SMet bond 2 9A with decreased electron donation strength This bond also shortens the Cu I SCys bond 2 1A 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 13 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 Cu phen 2 2 10 In the ocean editUsually plastocyanin can be found in organisms that contain chlorophyll b and cyanobacteria as well as algae that contain chlorophyll c Plastocyanin has also been found in the diatom 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 and iron Other organisms that live in the ocean such as phytoplankton have adapted to where they do not need these low concentration metals Fe and Zn to facilitate photosynthesis and grow 14 References edit Choi M Davidson VL February 2011 Cupredoxins a study of how proteins may evolve to use metals for bioenergetic processes Metallomics 3 2 140 151 doi 10 1039 c0mt00061b PMC 6916721 PMID 21258692 for an overview of the various types of blue copper proteins a b Redinbo MR Yeates TO Merchant S February 1994 Plastocyanin structural and functional analysis Journal of Bioenergetics and Biomembranes 26 1 49 66 doi 10 1007 BF00763219 PMID 8027022 S2CID 2662584 a b c Xue Y Okvist M Hansson O Young S October 1998 Crystal structure of spinach plastocyanin at 1 7 A resolution Protein Science 7 10 2099 2105 doi 10 1002 pro 5560071006 PMC 2143848 PMID 9792096 Freeman HC Guss JM 2001 Plastocyanin In Bode W Messerschmidt A Cygler M eds Handbook of metalloproteins Vol 2 Chichester John Wiley amp Sons pp 1153 69 ISBN 978 0 471 62743 2 Gewirth AA Solomon EI June 1988 Electronic structure of plastocyanin excited state spectral features J Am Chem Soc 110 12 3811 9 doi 10 1021 ja00220a015 Anderson GP Sanderson DG Lee CH Durell S Anderson LB Gross EL December 1987 The effect of ethylenediamine chemical modification of plastocyanin on the rate of cytochrome f oxidation and P 700 reduction Biochimica et Biophysica Acta BBA Bioenergetics 894 3 386 398 doi 10 1016 0005 2728 87 90117 4 PMID 3689779 Ratajczak R Mitchell R Haehnel W 1988 Properties of the oxidizing site of Photosystem I Biochim Biophys Acta 933 2 306 318 doi 10 1016 0005 2728 88 90038 2 a b Hurd CA Besley NA Robinson D June 2017 A QM MM study of the nature of the entatic state in plastocyanin Journal of Computational Chemistry 38 16 1431 1437 doi 10 1002 jcc 24666 PMC 5434870 PMID 27859435 Xue Y Okvist M Hansson O Young S October 1998 Crystal structure of spinach plastocyanin at 1 7 A resolution Protein Science 7 10 2099 105 doi 10 1002 pro 5560071006 PMC 2143848 PMID 9792096 a b Solomon EI Szilagyi RK DeBeer George S Basumallick L February 2004 Electronic structures of metal sites in proteins and models contributions to function in blue copper proteins Chemical Reviews 104 2 419 458 doi 10 1002 chin 200420281 PMID 14871131 Solomon EI Hadt RG 2011 Recent advances in understanding blue copper proteins Coordination Chemistry Reviews 255 7 8 774 789 doi 10 1016 j ccr 2010 12 008 ISSN 0010 8545 Bertini G 2007 Biological Inorganic Chemistry Structure and reactivity University Science Books p 253 ISBN 978 1 891389 43 6 Randall DW Gamelin DR LaCroix LB Solomon EI February 2000 Electronic structure contributions to electron transfer in blue Cu and Cu A Journal of Biological Inorganic Chemistry 5 1 16 29 doi 10 1007 s007750050003 PMID 10766432 S2CID 20628012 Peers G Price NM May 2006 Copper containing plastocyanin used for electron transport by an oceanic diatom Nature 441 7091 341 344 Bibcode 2006Natur 441 341P doi 10 1038 nature04630 PMID 16572122 S2CID 4379844 Further reading editBerg JM Lippard SJ 1994 Blue Copper Proteins Principles of bioinorganic chemistry Sausalito Calif University Science Books pp 237 242 ISBN 978 0 935702 72 9 Sato K Kohzuma T Dennison C February 2003 Active site structure and electron transfer reactivity of plastocyanins Journal of the American Chemical Society 125 8 2101 2112 doi 10 1021 ja021005u PMID 12590538 Retrieved from https en wikipedia org w index php title Plastocyanin amp oldid 1136101869, wikipedia, wiki, book, books, library,

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