I will therefore try to explain what proteins are not from a biochemical point of view, but by highlighting their type of structure and relative classification.
, hormones, transport, storage and structure proteins;- simple proteins: composed only of amino acids
- compound or conjugated proteins: they are joined to other molecules (eg sugars, lipids, nucleic acids, metals, etc.).
Simple proteins give rise to eight groups of subdivisions: protamines, histones, albumins, globulins, glutelines, prolamins, phosphoprotides and scleroprotids.
mainly and lysine. They therefore have a low molecular weight.Soluble in water, of a strongly basic character, protamines are found in nature only in the animal kingdom, combined with nucleic acids to form nucleoprotides (especially in tissues with reproductive function, for example in the spermatozoa of many fish). They are not found free in nature
Protamines are free of sulfur amino acids, tryptophan and tyrosine; they are instead very rich in basic amino acids (in particular arginine).
and to originate smaller quantities of exonic bases in the cleavage (arginine, histidine and lysine), of which they are however rich.
Like protamines, histones are not found free in nature, but combined with other substances to form protides. They are found in red blood cells, in leukocytes, in the heads of spermatozoa; globin is important, which constitutes the protein group of hemoglobin.
and cell secretions. Some of their characteristic properties are: solubility in water, coagulability with heat and the possibility of giving rise to all amino acids by splitting (they are therefore complete proteins of good biological value). They have a high content of leucine (about 10-14%) and glutamic acid (7-13%); they also contain good amounts of arginine (6-10%) and lysine (6-8%).
The main animal albumins are: le ovalbumin (or egg albumin) and the serum albumin (or milk albumin). They are also found in many plants, although their properties are not perfectly known. A characteristic of animal albumin is to have a high content of sulfur, and a considerable percentage of amino acids cystine and methionine, while the vegetable ones contain modest quantities. Some plant albumins are poisonous; this is the case with the ricin in castor oil.
diluted (NaCl) neutral. The most common are: blood globulins (α β, γ), lactoglobulin (milk), ovoglobulin (eggs), myosin and myoglobin (muscle). Vegetable globulins are found in particular in the seeds of many plants, especially in the oily ones of legumes; very rich in globulins are the proteins of soybeans and peanuts, where they form almost all of the protein substances. While animal globulins do not have major deficiencies in amino acids, those of the plant world are severely deficient in methionine (not surprisingly, the limiting amino acid of soy and other legumes).
The Gluteline and the prolamine (or gliadins) represent two groups of exclusively vegetable proteins, typically associated. Together, they make up the largest percentage of the cereal protein reserve (90-95%).
and they are very rich in glutamic acid, however present in lower concentrations than prolamins. They are insoluble in water, in saline solutions and in alcohol; they coagulate under heat and are soluble in diluted acids and bases. Wheat gluteline, called glutenin, forms a protein complex with gliadin which constitutes gluten, essential for bread making and partly for the plasticization of flours. In rice the glutelin present is called orizenine. ; the zein of corn. They are insoluble in water and soluble in 60-80% alcohol. They do not coagulate in the heat.
Prolamines are rich in glutamic acid, which represents 20-30% of the amino acids of the seeds of cereals; proline and leucine are also abundant, while sulfur amino acids, lysine (which is not surprisingly the typical limiting aminoacid of cereals) and tryptophan (deficient in corn) are scarce. These amino acid deficiencies are responsible for the low protein efficiency of cereals. Congenital intolerance to gliadin is known as celiac disease.
, therefore rich in phosphorus in the form of ortho-phosphoric acid, bound to esterify the alcoholic group of amino acids (such as serine). They have acidic properties due to the hydrogens of the phosphoric acid which does not participate in the esterification. Phosphoproteins should not be considered as conjugated proteins, nor should they be confused with nucleoproteins, capable of hydrolysis of phosphoric acid. The phosphoprotides are mainly present in proteins of animal origin, where there are two important representatives: the caseins of milk and the vitelline of the egg yolk (the vitelline is one of the fundamental substances of the yolk and is a protein particularly rich in phosphorus). it also recalls the ittulin of fish eggs. The main constituents of these proteins are gluttamic acid (15-20%), serine (which is mainly abundant in egg proteins), proline (5-10%) and lysine (5-7%) Cystine, on the other hand, is scarce.chemical, insoluble in water and common solvents, which dissolve only in acids and even resist most proteolytic enzymes. By virtue of their exceptional chemical resistance, they perform mechanical functions of coating, protection and support, while they have little nutritional value. The most important scleroprotids in animal organisms are: collagen (fundamental constituent of connective, cartilage and bone tissue), elastin (basic constituent of the elastic fibers of tendons and vessel walls) and keratins (constituents of nails, hair and of hair, but also of scales, horns and feathers). Scleropeptides are composed of few amino acids: keratin abounds in cystine (therefore sulfur), while collagen is rich in glycine (25%), proline and hydroxyproline, and deficient of sulfurates, tryptophan and tyrosine. Glycine and leucine abound in elastin, while cystine is scarce. Keratin is not attackable by gastric juices, therefore digestibility and intestinal absorption are very low; it is of little importance for food. Boiling collagen with diluted acids increases its digestibility, transforming it into gelatin.
(proteins with lipids)The above description only wants to explain, albeit briefly, how proteins are classified, in order to make it clear that these substances, beyond their athletic function intended to "build" muscles, are fundamental for the life of all the organs of the our body, each of them having a specific task.
