Pont disulfide

A Pont disulfide (bond S) is a covalent Lien extremely which, by Oxydation, joins together the functions Thiol of two Cystéine S in a peptide sequence (or Protéine) (see the figure below where the R represent the remainder of the Amino-acid .
La Molécule resulting from the Liaison of two cysteins is the Cystine.

R R | | HS S oxydation-> | + 2e^- + 2:00 ⁺ HS S | | R R

The bridge disulfide is an element of the structure S primary, tertiary sector (after the folding up of protein) or quaternary (during association of proteinic sub-units) of protein. The formation of a bridge disulfide starting from two cysteins is achieved spontaneously in oxidizing condition, in particular presence of Dioxygène. The reduction of a bridge disulfide is also easy to make with soft reducers, the such 2-mercaptoéthanol or the dithiothreitol (DTT or reagent of Cleland).

Biological role

It is a connection necessary to the stabilization of the structure of certain proteins: certain small proteins like the Toxin S present of the Venom of Reptile S or scorpion S can reach active conformation only because bridges disulfide lock their structure.

In other proteins, the bridges disulfides are used to maintain the connection between the various peptide chains or sub-units. It is the case for the Anticorps produced by the cells of the Immune system. Those are composed of four chains, two doors and two light, connected by bridges disulfide. It is also the case of the Insuline which is made up of two chains comprising three bridges dislufure, necessary to the activity of this Hormone.

Lastly, in some specific cases, one finds a bridge disulfide in the active site of some Enzyme S or Protéine S implied in processes of Oxydo-réduction or transport of electron. It is the case for example Thioredoxine S which take part in the Homéostasie of the state redox of the cell.

Biogenesis and cellular localization

In proteins containing a great number of cysteins, the correct fitting of the bridges disulfides requires sometimes the intervention of a specific Enzyme having an activity protein disulfide isomérase like PDI ( Protein disulfide isomerase ). This process is carried out by proteins of the endoplasmic reticulum rough (the RER) but not in the Cytosol. The Cytosol of the cells is indeed a very reducing environment, and consequently the cytoplasmic proteins thus contain little or not bridges disulfide. One finds some especially in proteins exported in other cellular compartments or out of the cell. One also finds some in the extracellular fields of membrane proteins, they intervene in particular in the receiving oligomerization of the sub-units of some S like the receiver of insulin.

In the hair

The hair is made up to 90% of Kératine dependant between them by bridges disulfide, of which the number and the site give to the hair their form. Permanent consists of two successive chemical reactions. The first (a reduction) breaks a certain number of bridges disulfides. The structure of protein is slackened. One then rolls up the hair according to the desired form. This operation brings opposite from/to each other of cysteins which, in the beginning, were distant. The second reaction (an oxidation) thus remade bridges between cysteins which would never have met naturally: it is the process of the Permanente. The protein takes the corrugated form imposed by the hairdresser or the hairdresser.

The reactions employed to curl a smooth hair can obviously be used to stretch a curly hair. As the destruction and the formation of the bridges disulfides are two reactions easy to make; like in addition, the hair pushes according to its natural form, the curling lasts only a limited time.

N.B. This is only one simplified explanation of the structure of the hair. Indeed with other molecules are associated with keratin, in particular the pigments which give to each hair a glare and a single color.

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