Cytochromes

Cytochrome b

Cytochrome b5

Cytochrome b5 (CB) functions as an electron transport carrier for several membrane-bound oxygenases. CB is heme-containing protein. The microsomal and mitochondrial CB are membrane-bound while bacterial and other animal tissue CB are soluble. Cytochrome b562 is the the b-type cytochrome from E. coli.^[1] (PDB entry 1b5m^[2]) is shown.

Cytochrome bc1 complex

Cytochrome bc1 (Cbc1) functions as the central pump which transfers protons across the cell membrane. The protons are used to power the rotation of ATP synthase. Cbc1 binds ubiquinol which carries hydrogen atoms. Cbc1 separates the protons and the electrons. The protons are released in the inner side of the membrane for use by ATP synthase and the electrons are transferred to cytochrome c or to the outer side of the membrane. Plants use cytochrome b6f in the same manner binding plastoquinol as a hydrogen carrier. Stigmatellin inhibits the Cbc1 electron transfer by binding to its quinone oxidation site. Antimycin inhibits Cbc1 by binding to its quinone reduction site.^[3]

More details in Complex_III_of_Electron_Transport_Chain.

Structural highlights

Cbc1 is a composed of 11 proteins and cofactors which include heme-carrying proteins like and and iron-sulfur cluster proteins like . The iron containing moieties are , (where vinyl side chain of heme are replaced by thioether) and . ^[4]

Cytochrome c

Structural and kinetic studies of imidazole binding to two members of the cytochrome c6 family reveal an important role for a conserved heme pocket residue^[5]

       is a member of the class I family of c-type cytochromes with a distinctive and a . They function in the photosynthetic electron transport chain of cyanobacteria where they shuttle an electron from the cytochrome b6f complex to photosystem I. Structures of numerous cytochrome c₆ proteins have been determined and all have the . In the present work we have solved the structure of the Q51V site-directed variant of Phormidium laminosum cytochrome c₆. This project is part of a study that is aimed at gaining insight into protein factors which modulate the heme mid-point redox potential in the cytochrome c₆ family. The Q51V variant has been shown to tune over 100 mV of heme redox potential, which for a single heme pocket mutation is very significant and has consequences for function.

      The Q51V structure confirms that the has the same side-chain orientation in the heme pocket as found in other cytochrome c₆ proteins, that naturally have a Val at this position. The significance of this structure is that the and an . Two other structures of imidazole cyt c-adducts have been reported, but neither appear to undergo the . Both protein and heme structural changes are observed, with the later centered on a accompanied by the and the .

      Protein (un)folding studies on cytochrome c have revealed that (un)folding involves structural units called 'foldons'. The regions in the Q51V imidazole-adduct where structural changes occur map well to the two foldons predicted to unfold first in cytochrome c. Thus , leading to the formation of an early unfolding intermediate that is stabilised by , enabling it to be captured in the crystalline form.

Structural model of the [Fe]-hydrogenase/cytochrome C553 complex combining NMR and soft-docking^[6]

The shows the specific interaction of the hydrogenase (light blue) with the cytochrome (pink), revealing the path of electron transport from the , through three iron-sulfur clusters, and ending in the cytochrome heme (colored red). Two , CYS 38 in the hydrogenase and CYS10 in the cytochrome, are thought to provide the electron transfer pathway between the two proteins (these scenes were created by Jaime Prilusky, David S. Goodsell, and Eran Hodis).

Conformational control of the binding of diatomic gases to cytochrome c’ ^[7]

The cytochromes c′ (CYTcp) are found in denitrifying, methanotrophic and photosynthetic bacteria. These proteins are able to form stable adducts with CO and NO but not with O2. The binding of NO to CYTcp currently provides the best structural model for the NO activation mechanism of soluble guanylate cyclase. Ligand binding in CYTcps has been shown to be highly dependent on residues in both the proximal and distal heme pockets. Group 1 CYTcps typically have a phenylalanine residue positioned close to the distal face of heme, while for group 2, this residue is typically leucine. We have structurally, spectroscopically and kinetically characterised the CYTcp from Shewanella frigidimarina , a protein that has a distal phenylalanine residue and a lysine in the proximal pocket in place of the more common arginine (monomer A is colored in red, monomer B in green, and heme group in yellow). in a similar manner to CYTcps previously characterised.

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SFCP exhibits biphasic binding kinetics for both NO and CO as a result of the high level of steric hindrance from the aromatic side chain of residue Phe 16. The binding of distal ligands is thus controlled by the conformation of the phenylalanine ring.

• of SFCP (in green;4ulv), RCCP (R. capsulatus; in magenta; 1cpq), RSCP (R. sphaeroides; in red; 1gqa) and RGCP (R. gelatinosus; in cya); 2j8w).
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Only a proximal 5-coordinate NO adduct, confirmed by structural data, is observed with no detectable hexacoordinate distal NO adduct.