Cytochromes c are electron-transfer proteins having one or several heme c

Heme c

Heme C differs from heme B in that the two vinyl side chains of heme B are replaced by covalent, thioether linkages to the apoprotein...

 groups, bound to the protein by one or, more generally, two thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochromes c possess a wide range of properties and function in a large number of different redox processes. The founding member of this family is mitochondrial cytochrome c

Cytochrome c

The Cytochrome complex, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins. Cytochrome c is a highly soluble protein, unlike other cytochromes, with a solubility of about 100 g/L and is an...

.

Ambler recognized four classes of cytC.

• Class I includes the low-spin soluble cytC of mitochondria and bacteria, with the haem-attachment site towards the N-terminus, and the sixth ligand provided by a methionine residue about 40 residues further on towards the C-terminus. On the basis of sequence similarity, class I cytC were further subdivided into five classes, IA to IE. Class IB includes the eukaryotic mitochondrial cytC and prokaryotic 'short' cyt c2 exemplified by Rhodopila globiformis cyt c2; class IA includes 'long' cyt c2, such as Rhodospirillum rubrum
  Rhodospirillum rubrum
  Rhodospirillum rubrum is a Gram-negative, purple-coloured Proteobacterium, with a size of 800 to 1000 nanometers.It is a facultative anaerobe, it can therefore use alcoholic fermentation under low oxygen conditions or use aerobic respiration in aerobic conditions. Under aerobic growth...
  
  cyt c2 and Aquaspirillum itersonii cytc-550, which have several extra loops by comparison with class IB cytC.

• Class II includes the high-spin cytC' and a number of low-spin cytochromes, e.g. cyt c-556. The haem-attachment site is close to the C terminus. The cytC' are capable of binding
  Binding (molecular)
  Molecular binding is an attractive interaction between two molecules which results in a stable association in which the molecules are in close proximity to each other...
  
   such ligands as CO, NO or CN(-), albeit with rate and equilibrium constants 100 to 1,000,000-fold smaller than other high-spin haemoproteins. This, coupled with its relatively low redox
  Redox
  Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
  
   potential, makes it unlikely that cytC' is a terminal oxidase
  Oxidase
  An oxidase is any enzyme that catalyzes an oxidation-reduction reaction involving molecular oxygen as the electron acceptor. In these reactions, oxygen is reduced to water or hydrogen peroxide ....
  
  . Thus cytC' probably functions as an electron transfer protein. The 3D structure
  Secondary structure
  In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...
  
  s of a number of cytC' have been determined. The molecule
  Molecule
  A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
  
   usually exists as a dimer
  Dimer
  A dimer is a chemical entity consisting of two structurally similar subunits called monomers joined by bonds that can be either strong or weak.- Organic chemistry :...
  
  , each monomer
  Monomer
  A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
  
   folding
  Protein folding
  Protein folding is the process by which a protein structure assumes its functional shape or conformation. It is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....
  
   as a four-alpha-helix bundle incorporating a covalently-bound haem group at the core. The Chromatium vinosum cytC' exhibits dimer
  Protein dimer
  In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
  
   dissociation
  Dissociation (chemistry)
  Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
  
   upon ligand binding.

• Class III comprises the low redox potential multiple haem cytochromes: cyt C7 (trihaem), C3 (tetrahaem), and high-molecular-weight cytC, HMC (hexadecahaem), with only 30-40 residues per haem group. The haem c groups, all bis-histidinyl coordinated, are structurally and functionally nonequivalent and present different redox
  Redox
  Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
  
   potentials in the range 0 to -400 mV. The 3D structure
  Secondary structure
  In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...
  
  s of a number of cyt C3 proteins
  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...
  
   have been determined. The proteins consist of 4-5 alpha-helices
  Alpha helix
  A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
  
   and 2 beta-strands wrapped around a compact core of four non-parallel haems, which present a relatively high degree of exposure to the solvent
  Solvent
  A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
  
  . The overall protein architecture, haem plane orientations and iron-iron distances are highly conserved.

• Class IV includes complex proteins containing other prosthetic groups besides haem c, such as flavocytochromes c and cytochromes cd.