A protein is a biological macromolecule composed of one or more chains of amino acids linked together by peptide bonds. In general, when we talk about protein chain contains a large number of amino acids, and peptide for small assemblies.

The order in which amino acids are linked together is encoded by the genome and is the primary structure of the protein. The protein folds on itself to form secondary structures, which are quantitatively the most important alpha helix and beta sheet, which creates hydrogen bonds between the atoms of carbon and nitrogen of two peptide bonds neighbors. Then, the different secondary structures are arranged against each other to form the tertiary structure, often reinforced by disulfide bonds. The forces that govern the folding forces are the classical physical. In the case of proteins formed by the arrangement of several channels, the quaternary structure describes the relative position of the subunits relative to each other.

There are several chaperone proteins [chaperone] that facilitate or are necessary to protein folding to the active state.

Protein folding is the subject of intense research in the field of structural biology, combining the techniques of molecular biophysics and cell biology mainly.

Proteins are the essential elements of the life of the cell may play a structural role (like actin), a role in mobility (eg myosin), a catalytic (enzymes), a regulatory role of compaction of DNA (histones) or expression of genes (transcription factor), etc.. In fact, the vast majority of cellular functions are performed by proteins.

The proteins were discovered by the Dutch chemist Gerhard Mulder (1802-1880). The word protein comes from the Greek protos which means first, essential. This probably refers to the fact that proteins are essential to life and they often constitute the majority share (≈ 60%) of cell dry weight. Another theory, that would make reference protein, as the adjective protean, with the Greek god Proteus who could change shape at will. Proteins indeed adopt many forms and provide multiple functions. But this was not discovered until much later in the twentieth century.

Proteins are assembled from amino acids based on the information present in the genes. Their synthesis involves two steps:

* The transcript where the DNA sequence encoding the gene associated with protein is transcribed into messenger RNA
* Translation or messenger RNA is translated into protein at the ribosome, based on the genetic code

The assembly of a protein is thus amino acid amino acid of its N-terminus at its C-terminus. Also, a gene is not necessarily associated with a single protein but often several.

Proteins are molecular objects whose precise description introduces the concept of structures (more or less hierarchical).

Protein function is conferred by three-dimensional structure, that is to say how amino acids are arranged against each other in space. That is why methods for determining three dimensional structures as well as measures of protein dynamics are important and are a very active research field. In addition to these experimental methods, many studies focus on computational methods for predicting the 3D structure from the sequence.

Proteins perform many different functions within the cell and organism:

* Protein structures that allow the cell to maintain its organization in space,
* Transport proteins, which provide for the transfer of various molecules in and out of cells,
* Regulatory proteins, which modulate the activity of other proteins,
* Signaling proteins that detect external signals, and ensure their transmission in the cell or organism,
* Motor proteins, allowing cells or organisms to move.

The plan for making proteins depends primarily on the gene. Gold sequences of genes are not strictly identical from one individual to another. Moreover, in the case of living beings diploid, there are two copies of each gene. And these two examples are not necessarily identical. A gene exists in several versions so from one individual to another and sometimes within the same individual. These different versions are called alleles. The set of alleles an individual form the genotype.

Since genes come in several versions, the proteins will also exist in different versions. These different versions of proteins will lead to differences from one individual to another, such person will have blue eyes but another has black eyes, etc.. These characteristics, visible or not, to each individual are called the phenotype. At the same individual, a group of proteins similar sequence and function is identical isoform said. Isoforms may be the result of alternative splicing of a single gene, the expression of several alleles of a gene, or the presence of several homologous genes in the genome.

Evolution (Theory)
During evolution, the accumulation of mutations have diverged genes within species and between species. Away from the diversity of proteins associated with them. However, we can define protein families, themselves corresponding to gene families. Thus, in one species can coexist genes and therefore proteins, forming a very similar family. Two closely related species are likely to have representatives of the same family of proteins.

We speak of homology between proteins when different proteins have a common origin, a common ancestral gene.

The comparison of protein sequences can reveal the degree of "kinship" between different proteins, we are talking here of sequence similarity. Protein function can diverge progressively as the similarity decreases, giving rise to families of proteins with a common origin but have different functions.

Sequence analysis and protein structures revealed that many were organized into domains, that is to say by parties acquiring a structure and performing a specific function. The existence of proteins with multiple domains may be the result of recombination in a single gene of several genes originally individual, and each protein composed of a single domain can be the result of the separation of several genes in a gene originally encoding a protein with multiple domains.

In food, proteins are broken down during digestion from the stomach. This is where proteins are hydrolyzed by proteases and cut into polypeptides and then provide amino acids for the body, including those so-called essential that the body can not synthesize. Pepsinogen is converted to pepsin when it comes into contact with hydrochloric acid. Pepsin is the only proteolytic enzyme that digests collagen, the main protein of connective tissue. The bulk of protein digestion occurs in the duodenum.

Almost all proteins are absorbed when they arrive in the jejunum and only 1% of ingested proteins are found in feces. Some amino acids remain in the epithelial cells and are used for the synthesis of new proteins, including some intestinal proteins, consistently digested and absorbed and recycled by the small intestine.

Foods rich in protein
* Dried vegetables (soybeans, lentils, dried beans, for example, also rich in iron) and whole grains;
* Dairy concentrates or dried (eg cheese, also rich in calcium and vitamin B);
* Meat (also rich in iron);
* Fish;
* Egg;
* Bread, pasta, rice, other cereals (also rich in carbohydrates, B vitamins, minerals, fiber);
* Dried fruits and nuts (almonds, peanuts (groundnuts), hazelnuts, walnuts, cashews, pine nuts, pistachios: between 15 and 30% protein).

See also Cell