Simple Proteins – representatives, properties and role

Albumins (A)are the value proteins, with molecular mass from 50000 up to 70000D. They are precipitated by boiling. They are good dissolved in water, solutions of alkalis and neutral salts. They are salted out by neutral salts at complete saturation. There are a lot of monoaminodicarboxylic acids in their primary structure, because they have acidic properties and isoelectric point equal 5, owing to what they have large negative charge and high mobility. They are good hydrated and have high absorbic ability. A. are more than 50% of all proteins of plasma of blood where they create oncotic pressure of plasma what hold the water into blood vessels. They take part in transfer of medicines and bad dissolved substances, carry out protective function. They carry out absorbtion of toxic product of endogenic (fatty acids, bilirubine) and exogenic (heavy metals, poisons) origin.

A. are widely distributed in a nature and they are called accordingly on a source of detection: serum, muscular, dairy, egg… .

A. increase of volume of circulating blood, detoxical properties of blood, they are used in restoration of oncotic pressure of plasma.

Globulins [G]

It is a very large group of value proteins. Their molecular mass is from 75000 D up to 150000D. They are precipitated by boiling, are not dissolved in water but dissolved in salt’s solution and alkalis. There are a lot of neutral aminoacids (a/a) in their primary structure, they have small negative charge and low mobility. IEP (isoelectric point) is equal 6,9.

G are weak hydrated proteins and they are salted out in half saturated solutions of neutral salts.

There are few types of G in blood. They differ from each other on their charge , mol. mass and mobility in electrical field. They are marked by Greek words: α1, α2, β, γ. There are many representatives of globulins in our body (immunoglobulines (γ), haptoglobin, transferrin, ceruloplasmin etc).

G-s are widely distributed in a nature and are called depending on their localization – dairy, serum, muscular, eggs… .

G increase immunological reactivity of organism and that is why they are widely used in medicine.

G are increased during inflammation and diseases. The analysis of quantity and fractions of G have huge diagnosis significance.

Histones (H)

H are basic non value proteins. Localized in nucleus with mol. Mass (mm) 10000-20000 D. They contain of 30% diaminomonocarboxylic acids and have positive charge. Their IEP is equal 10.

They are precipitated by boiling and in alkalis. They are divided into 5 groups in dependence on the ratio of lys, arg, his in their primary structure and these groups are marked as H1, H2α, H2β, H3, H4. They regulate of the function of DNA in animal, they are contained in nucleus.

Protamines

are not fullvalue proteins, very small proteins with Mm 6000 D. They contain of 80% diaminomonocarboxylic acid, have very large positive charge and IEP more than 10. They are soluble in salts, acids but are precipitated in alkalis. They are contained in nucleus of fish, reptiles, plants and they regulate of the function of DNA.

Scleroproteinsare unfulvalue proteins, they don not contain all essential a/acids. Second name is proteinoids because their properties sharply differ from all proteins. They are not dissolved in water, weak solutions of neutral salts, are not precipitated (curtailed) by heating, the majority of them is not splitted by enzymes of gastrointestinal tract. They contain of a lot disulfide bonds (bridges).

The p. of connective tissues are proteinoids. In organism of the man and animals there are three kinds of scleroproteins (sp): collagens, elastins, keratins, which differ from each other by structure of molecules and their aminoacid composition. All of them concern to fibrous p. and are constructed from braided as twist α helix or from β-pleated sheet cross-linked by disulfide bonds.

Collagen [C] makes 25-35% of p-s of a body, the unfulvalue protein, formed in fibroblastes, molecular weight 28500 D. It does not dissolve in water. Its molecules have the original aminoacids composition. Everyone third aminoacid is subunitted glycine, 11% - are on share alanine, 21% - proline and oxyproline.

Molecule of collagen have some quantity of the disaccharides but quantity of carbohydrates small and consequently they concern to simple proteins.

The molecule of collagen has the form of a core of length 3000 Å and diameter 15. It is one of the longest proteins. The primary structure of collagen refers to tropocollagen and represents polipeptide chain from 1100 aminoacids.

Secondary structure is a stretched α helix because in the molecule of collagen there are a lot of proline and oxyproline, which prevent the formation and which do their spiral steady for the different action.

Tertiary structure of collagens is a superspiral three polipeptide chains connected by hydrogen bonds.

Quaternary structure is submitted by several three fold spirals, which are stacked up as parallel bunches for a type “a head to tail”. The heads are moved from each other ¼ length that causes striations of their molecules. Quaternary structure is stabilized due to inside molecular and intermolecular covalent bonds between two residues lysine of the parallel chains. Quantity and type of cross bonds depend on the function and age of collagen. High degree of elasticity is caused by quantity of cross linkage. The collagen fibrils have surprising durability: for break of a fibrils by a diameter of 1 mm it is necessary to apply (put) force in 10 kgs.

4 types of C are known. Collagen of the first type is contained in organism in the greatest quantity, meets in skin, tendons, bones, cornea of an eye. The second type of collagen meets in cartilages, glassoid body of an eye. Collagen of the third type is met in intima of vessels and cardiavascullar system, fourth type – in basal membranes. Collagen is the basic component of scar after wound.

At the boiling with acid water collagen is converted to gelatin – easily digested peptide of low nutrition value. Collagen breaks up under action of collagenase with formation hydroxyproline, which level in blood reflects speed of metabolism of collagen.

Elastins are unfulvalue proteins in tendons, sinews, walls of large vessels. They are capable to stretch in some times in length and at removal of loading quickly to restore the initial form. Basic subunit of elastin is tropoelastin with molecular weight 72000 D. There are a lot of alanine and glycine in elastine, especially a lot of lysine.

The spiral of polypeptide chains of tropoelastin differs both from α-helix and from a spiral of collagen. There are sites In tropoelastin in which the residues lysine, participating in formation of cross linkages prevall. Besides in elastine are found out desmosin and isodesmosin in which formation four molecules of lysine participate.

This formation enable of elastin is convertible to stretched in all directions.

Keratins have received the name from greek words and are two kinds of keratins – α and β. Alfa-keratins are proteins of a hair, wool, feathers, horns, nails, needles, scale, hoofs, shell and also significant part of an outside layer of skin. In the structure of α-keratin there is much cysteine. α-keratin consist of three polypeptide chain with spiral characteristic for each one. Besides the bonds between polypeptide chains there are many disulfide “bridges” between cysteins of various chains. Therefore keratins have the large durability. Strongest and rigid α-keratins (in the shell of a turtle) contain up to 18% cystine. They are not dissolved in water.

Β-keratins are proteins of fibroin (fibers of silk and spider’s webbs). Though they are more flexible but hardly give in to a stretching. They have β-pleated sheet. In β-keratin there is a lot of glycine and alanine, for example in fibroin of silk everyone second aminoacids is glycine.

Table 1 “The properties of globular simple proteins”