The proteins are organic macromolecules found in large quantities in the cell structure. They are present in all cells, as well as in viruses and in special infectious units called prions.
They are constituents of several structural components of most tissues; participate actively in the control of cellular metabolism, acting as enzymes ; act as defense molecules of the body, antibodies; carry substances such as hemoglobin; among other functions.
How proteins form
Proteins are formed by the chaining of special units called amino acids , linked together by peptide bonds.
An amino acid, in turn, is formed by a carbon where they bind: a hydrogen, an amine group (NH 2 ), basic character, a carboxyl group (COOH), acidic character, (hence the name amino acid) and a variable portion, a radical with 20 different types of chains, since there are 20 different types of amino acids in living beings.
Although there are only 20 types of amino acids, the number of different proteins in an organism is very large because they can vary in number of amino acids. Thus, two proteins can differentiate according to the order in which these amino acids are arranged in the protein molecule.
These amino acids make special bonds called peptide bonds, which correspond to the union of the carboxyl group of a peptide with the amino group of another peptide. This leads to the release of a water molecule forming dipeptides, tripeptides, to polypeptide chains, also called proteins .
Scheme showing a blue peptide bond between two amino acids, with release of one molecule of water:
Structures of a protein
Proteins vary in number, types and amino acid sequence in their structure. This order and arrangement of the amino acids along the protein chain is what is termed the primary structure.This arrangement is of utmost importance to the function that the protein will exert. Often, the simple inversion or alteration of one type of amino acid in the chain is sufficient for the protein to lose its normal function.
After the formation of the primary structure, the different radicals present in the amino acids initiate rotations and attractions among themselves, which promotes a twist in the molecule, characterizing the secondary (or helical) structure of the protein. This structure is maintained mainly by bonds between hydrogens.
New folds still occur on the helical form, characterizing the tertiary structure of the protein. Such a structure is the folding and the final form of the functional protein. The tertiary structure is maintained by several types of bonds; the most common are those of hydrogen and those occurring between the sulfur atoms.
Some proteins are formed by the association of two or more polypeptide chains, grouped into a single molecule and termed the quaternary structure .
Examples of proteins and their functions
Proteins have many functions in organisms, the main one being structural. For example, keratinpresent in the hair and nail structure and collagen in the skin structure.
Some proteins act as carriers; the hemoglobin , for example, Respiratory oxygen leads to other body tissues. The myoglobin have similar role, but in the muscles.
In the muscles, there are contractile structural proteins, such as actin and myosin . When the muscle is stimulated, these proteins slide over each other, causing shortening of muscle cells.
In animals, there are proteins that act on blood clotting . The fibrinogen is one of the proteins involved in this phenomenon that prevents major bleeding in case of injury. There are proteins that participate in immune defense, such as antibodies , or immunoglobulins , that are able to fight against infectious agents such as viruses and other microorganisms.
Some proteins, called hormones , are chemical messengers; distributed through the blood, can modify the functioning of organs or cells. The insulin and prolactin are two examples of proteins with hormone function.
There are also other proteins, called enzymes , that act as catalysts, because they increase the speed of chemical reactions, facilitating their occurrence.