A protein is a biological macromolecule composed of one or more chains of amino acids linked by peptide bonds (polypeptide chain). In general, one speaks of protein when the string contains at least 100 implicated acids and peptide for smaller assemblies.
Proteins are essential elements of the life of the cell: they can play a structural role (such as actin), a role in mobility (such as myosin), a catalytic role (enzymes), a role of regulation of compaction (transcription factor) gene expression and DNA (histones), etc. In short, the vast majority of cellular functions are carried out by proteins.
The structure of proteins is complex and affects the role they play in the life of the cell.
Proteins were discovered from 1835 in the Netherlands by the chemist Johannes Gerardus Mulder2 (1802-1880), under the name of wortelstof. In his illustrious Swedish colleague, Jöns Jacob Berzelius, who suggested in 1838 the name of protein.
The word protein comes from the Greek former protos meaning first, essential. This probably refers to the fact that proteins are essential to life and it is often the majority share (approx. 60%) of the dry weight of cells. Another theory, would like to that protein should refer, as the adjective protean, to the Greek god Proteus who could change shape at will. Proteins are in fact multiple forms and perform multiple functions. But this was discovered until much later, during the twentieth century.
Proteins are assembled from amino acids based on the information contained in the genes. Their synthesis is done in two steps:
The transcript where the sequence of DNA encoding the gene associated with the protein is transcribed into messenger RNA
The translation where the messenger RNA is translated into protein, ribosome level, based on the genetic code
The Assembly of a protein is therefore amino acid by amino acid N-terminal to its C-terminus. After its synthesis by the ribosome, the protein can undergo post-translational modifications, cleavages, maturations. Finally, in some organisms of the alternative messenger RNA Splicing process can result in the production of several different forms of a protein from the same gene.
Beginning of 2012, more than 3000 genomes of living organisms have been sequenced and more than 7000 are under sequencing.
The genomes of model organisms key such as the bacterium Escherichia coli, the Yeast Saccharomyces cerevisiae, the plant Arabidopsis thaliana and many other genomes which that having been decrypted, it is recognized that almost all of the genes could be set.
By against the inventory of active proteins (proteome) in an organization is far from being established. Indeed, because of the variability of the process of activation and regulation of proteins, this inventory is not the result of the simple translation of each gene that would give an active protein: for example some genes can give several different forms of a protein, or proteins must be modified to be active.
The proteins are molecular objects whose description introduces the concept of structures (in a more or less hierarchy).
For the first time in 1957, John Kendrew and Max Perutz, crystallography and x-ray diffraction, were able to describe the structure in three dimensions of Myoglobin and hemoglobin.
The function of the proteins is conferred by their three-dimensionality, that is the way structure in which amino acids are arranged to each other in space. This is the reason why the methods for the determination of the three-dimensional structures as well as measures of the dynamics of proteins are important and is a lookup field from very asset. In addition to these experimental methods, many studies focus on computational methods of 3D from the sequence structure prediction.
The order in which the amino acid chain is encoded by the genome and is the primary structure of the protein. The protein folds back on itself to form secondary structures, of which the most important quantitatively is the alpha helix and the beta sheet, allowing you to create links h-bonding between atoms of carbon and nitrogen of two neighboring peptide bonds. Then, the various secondary structures are arranged each other to form tertiary structure, often enhanced by disulfide bridges. The forces that govern this folding is the classic physical forces. In the case of proteins through the combination of several channels, the Quaternary structure describes the relative position of the subunits each other.
There are several chaperone proteins that facilitate, or are required, to the folding of proteins to the active state. Protein folding is the subject of intense research in the field of structural biology, combining the Molecular Biophysics and cell biology techniques mainly.
Proteins perform functions very different within the cell and the organism:
structural proteins, which allow the cell to keep his organization in the space. They are the constituents of the cytoskeleton
transport proteins that transfer of the different molecules in and out of cells
regulatory proteins which modulate the activity of other proteins or that control the expression of genes
signaling proteins that capture the external signals and ensure their transmission in the cell or the organism, there are
+ for example: hormonal proteins, which help to coordinate the activities of an organization acting as signals between cells.
receptor proteins: detect Messenger molecules and other signals that the cell act accordingly.
+ sensory proteins, they detect environmental signals (ex: light) and respond to signals in the cell.
+ hormone receptors, they detect the hormones and send signals to the cell so that it acts as a result (ex: insulin is a hormone that when she is about to be captured, will report to the cell to absorb and use the sugar)
motor proteins, allowing the cells or organisms or elements (lashes) of move or deform (ex: actin and myosin allows the muscle to contract)
the defense proteins, protecting the cell from viruses (ex: antibodies)
storage proteins, allowing storage of amino acids to be able to create other proteins
enzymes, they change the speed of almost all chemical reactions in the cell without be transformed in the reaction
Protein manufacturing plan depends in the first place of the gene. However the sequences of the genes are not identical from one individual to another. For more, in the case of human beings living diploid, there are two copies of each gene. And these two examples are not necessarily identical. A gene is in several versions of an individual to the other, and sometimes in the same individual. These versions are called alleles. The set of alleles an individual forms genotype.
Since genes exist in several versions, the proteins will also exist in different versions. These different versions of proteins will cause differences from one individual to another: such an individual will have blue eyes but another will have black eyes, etc. These characteristics, visible or not, specific to each individual are referred to as the phenotype. In the same individual, a group of proteins with similar sequence and re-assigning says isoform. The isoforms can be the result of alternative splicing of the same gene, the expression of several alleles of a gene, or the presence of several homologous genes in the genome.
During evolution, the accumulations of mutations made diverge genes within species and between species. Thence comes the diversity of proteins that are associated with them. One can however define protein families, themselves belonging to families of genes. Thus, in a species can coexist genes and therefore very similar proteins forming a family. Two closely related species are likely to have representatives of same family of proteins.
Talking of homology between proteins when different proteins have a common origin, a common ancestral gene.
Protein sequence comparison allows to highlight the degree of ‘relationship’ between different proteins, one speaks here of sequence similarity. The function of the proteins can diverge that similarity decreases, thus giving birth to families of proteins having a common origin but with different functions.
Protein structures and sequences analysis identified that many organized into domains, that is, parties acquiring a structure and performing a specific function. The existence of multidomain proteins can be the result of recombination in a single gene of several originally individual genes and conversely proteins consisting of a single domain can be the result of multiple genes from a gene originally separation encoding a protein in several areas.
In nutrition, proteins are disaggregated during digestion from the stomach. It is there that the proteins are hydrolyzed by proteases and cut into polypeptides to then provide amino acids for the organism, including those so-called essential, that the body is unable to synthesize. Pepsinogen is converted to pepsin when it comes in contact with hydrochloric acid. Pepsin is the only proteolytic enzyme which digests collagen, the main protein of connective tissue. The major part of digestion of proteins takes place in the duodenum.
Almost all proteins are absorbed when they arrive in the jejunum and only 1% of the ingested proteins are found in the feces. Some amino acids remain in epithelial cells and are used for the synthesis of new proteins, including some intestinal proteins, constantly digested, recycled and absorbed by the small intestine.