A protein , also called protid , is a Macromolécule composed by one or more chain (S) (or sequence (S)) of amino-acid dependant between them by peptide connections. In general, one speaks about protein when the chain contains more than 100 amino-acids. In the contrary case, one speaks about Peptide S and of Polypeptide S. This use tends to disappear with the profit from the term small protein . The sequence of the amino-acids is coded by the Génome and constitutes the primary Structure.

Etymology

The proteins were discovered by the Chemist Dutch Gerhard Mulder (1802-1880). The term protein comes from the old Greek prôtos which means first , essential . This probably referred to the fact that the proteins are essential to the life and that they often constitute the majority share of the dry weight of the cells. Another theory, less probable, would like that protein refers to the Greek god Protée which could change form at will. The proteins adopt multiple forms indeed and ensure of multiple functions. But this was discovered only well later, during the 20th century.

Synthesis

See also: Synthesis of the proteins

The proteins are assembled starting from the Amino-acid according to information present in the Gène S. Their synthesis is done in two stages:

• the transcription where DNA coding gene associated with protein is transcribed in ARN messenger
• the translation where the ARN messenger is translated into protein according to the genetic Code

The assembly of a protein is done thus amino-acid by amino-acid of its N-final end at its C-final end. It should also well be noted that a gene is not inevitably associated with only one protein but very often with several.

Structure

See also: Structure of the proteins

The proteins are molecular objects of which precise description introduces the concept of structures (in a more or less hierarchical way).

The function of proteins is conferred by their three-dimensional structure, i.e. the way in which the amino-acids are arranged the ones compared to the others in space. This is why the methods of determination of the three-dimensional structures as well as measurements of dynamics of proteins are important and constitute a very active research field. In addition to these experimental methods, many studies relate to data-processing methods of prediction of the structure 3D starting from the sequence.

Function

See also: Function of the proteins

The proteins fill of the very diverse functions:

• Catalysis (e.g.: many a Enzyme S - the biological Catalyst S - is proteins)
• Transport (e.g.: the Hémoglobine transports the Oxygène of the Poumon S to the bodies (see figure 1))
• Communication (e.g.: many a Hormone S - as the Insulin - is proteins and can transport a message through all the organization)
• Signalisation (e.g.: proteins are implied in the Chimiotactisme)
• Reconnaissance (e.g.: the Immune system has special proteins - the Immunoglobuline S - which allow the molecular recognition of “foreign” forms, i.e. not belonging to the molecular forms of the organization which manufactures them)
• Structure (e.g.: the proteins of the Cytosquelette allow the consolidation and the mobility of the cells, as it is the case for the Flagelle S Bactérie NS)
• etc

The proteins can be regarded as the major tools of the world living with the molecular scales.

Phenotype

The manufacturing drawing of proteins thus depends initially on the Gène. However the genes are not identical of one individual to the other. In the case of the man, each individual has well a Génome with him (we are not clones). Moreover, in the case of the living beings Diploïde S, there exist two specimens of each gene. And these two specimens are not necessarily identical. A gene thus exists in several versions of one individual to the other and sometimes at the same individual. These various versions are called Allèle S . The whole of the alleles of an individual forms the Génotype.

Since the genes exist in several versions, the proteins also will exist in various versions. These various protein versions will cause differences from one individual to another: such individual will have the blue eyes but such other will have the black eyes, etc . These characteristics, visible or not, specific to each individual are called the Phénotype. At the same individual, a group of proteins with similar sequence and identical function are known as isoforme. The isoformes can be the result of the alternate épissage of the same gene, the expression of several alleles of a gene, or the presence of several homologous genes in the genome.

Evolution

During the evolution, accumulations of changes made diverge the Gène S between the Espèce S. From there comes diversity from the proteins which are associated for them. One can however define families of proteins, themselves corresponding to families of genes. Thus, two close species are highly likely to have genes, and consequently proteins, very similar. This similarity can be measured by comparing the sequence proteins. One can thus classify a group of proteins by Homologie, of most similar to the least similar. Thus, the function of proteins will diverge as the similarity decreases.

The analysis of the sequences and protein the structures made it possible to note that much was organized in fields, i.e. in parts acquiring a structure and fulfilling a function independently of the remainder of protein. According to the theory of the mosaic genes, the existence of proteins to several fields is the result of the single gene recombination several originally individual genes.

Food

In the food, the proteins are disaggregated during the Digestion starting from the Estomac. They is there that the proteins are hydrolized in Protéose S and Polypeptide S to provide Amino-acid for the organization, including those which the organization is not able to synthesize. The pepsinogene is converted into Pepsine when it arrives at the contact with the Hydrochloric acid . The Pepsine is only the proteolytic Enzyme which digests the Collagène, the principal protein of the conjunctive Tissu. The majority of the digestion of proteins take place in the Duodénum.

Almost all the proteins are absorbed when they arrive in the Jéjunum and only 1% of introduced proteins are found in the Fèce S. Certains amino-acids remain in the epithelial cells and are used for the synthesis of new proteins, including certain proteins Intestin ales, constantly digested, recycled and absorptive by the Small intestine .

Rich foods in proteins

See also: Food

• Milk and Dairy products (also rich in Calcium and Vitamin B)
• Meat, Poisson, egg (also rich in Iron)
• Bread, pastes, Rice, dry vegetables (soya, lenses…), others Cereal S (also rich in Glucid S, Vitamin B, Mineral, fibers)