Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Protein Chemistry
Chemistry of Simple Proteins

Protamines and Histones. This group of Proteins is distinguished by A number of characteristic physicochemical properties and a unique Amino Acid Composition, consisting mainly of low-molecular-weight proteins. Protamines exhibit strong basic properties due to an Arginine content ranging from 60% to 85%. For instance, salmine, isolated from salmon sperm, contains 85% arginine. Another well-studied protein with a high arginine content is clupeine, isolated from herring sperm: out of 30 Amino Acids, 21 are arginine residues. The Introduction/19.html">Primary Structure of clupeine has been successfully sequenced. Protamines are readily soluble in Water, and the isoelectric point of their aqueous solutions lies in the alkaline range. According to modern concepts, protamines are more accurately classified as Peptides rather than proteins, since their molecular weight does not exceed 5,000. They constitute the protein component in The structure of several complex proteins.

Histones are also basic proteins. Their composition includes Lysine and arginine; however, the content of these amino acids does not exceed 20–30%. The Molecular Weight of histones is significantly higher than the lower limit of protein molecular weight. These proteins are localized primarily in Cell nuclei as part of deoxynucleoproteins and play a crucial role in regulating Gene Expression (see Chapters 2 and 3).

Prolamins and glutelins. These are Plant-derived proteins characterized by unique amino acid compositions and physicochemical properties. They are found mainly in cereal seeds (wheat, rye, barley, etc.) and make up the bulk of gluten. A characteristic feature of prolamins is their solubility in a 60–80% aqueous ethanol solution, whereas all other simple proteins typically precipitate under these conditions. The most thoroughly studied among them are oryzenin (from rice), glutenin and gliadin (from wheat), zein (from corn), hordein (from barley), and others. It has been established that prolamins contain 20–25% glutamic acid and 10–15% Proline.

Albumins and globulins. These proteins are widely distributed across animal Organs and Tissues. They are most abundant in Blood serum, milk, egg white, and Muscles, among other sources. Normally, human Blood Plasma contains about 7% protein, predominantly represented by albumins and globulins. Albumins and globulins are Globular proteins that differ in their solubility (Table 1.6).

It should be noted that the very definition of "albumins" and "globulins" is based on their solubility in distilled water and a half-saturated (NH₄)₂SO₄ solution. However, as the data in Table 1.6 demonstrate, globulins are soluble only in dilute salt solutions.

Class="center">Table 1.6. Solubility of albumins and globulins

The differing solubility of serum albumins and globulins was previously widely used in clinical practice for their fractionation and quantitative determination (see Chapter 17).

Currently, the qualitative composition and content of Serum proteins are determined using paper and Polyacrylamide gel Electrophoresis on small blood serum samples. A typical electrophoretogram of serum proteins, as well as the proportions of individual fractions, is presented in Chapter 17. Albumins and globulins also differ in molecular weight—ranging from 40,000 to 70,000 and 150,000 or more, respectively.

Not only has pure albumin been isolated from blood serum, but the primary structure of its single polypeptide chain (consisting of 575 amino acid residues) has also been determined. Albumin has a relatively low isoelectric point (4.7) and a high negative charge at pH 8.6, which enables it to migrate rapidly toward the anode in an electric field. It is generally accepted that albumins account for approximately 75–80% of the Osmotic Pressure of blood serum proteins; furthermore, their primary function is considered to be The transport of Fatty acids. However, the precise function of albumins is not entirely clear. There are documented cases where certain individuals virtually lack albumins in their blood (a congenital anomaly) yet remain practically healthy.

Globulins, represented by the a₁ fraction, are present in the blood as a complex with bilirubin and high-density Lipoproteins. Globulins migrating as the a₂ fraction during electrophoresis contain globulin and an unknown glycoprotein. ß-Globulins comprise a number of functionally important proteins, notably transferrin, the protein responsible for iron transport. Ceruloplasmin, a protein that transports copper ions, is also associated with this fraction. A deficiency of this protein leads to hepatolenticular degeneration (Wilson's disease), characterized by systemic poisoning of the Organism by free copper ions. The condition stems from a congenital defect in ceruloplasmin synthesis. Finally, the ß-globulin fraction contains prothrombin, the precursor of Thrombin—the protein responsible for converting blood fibrinogen into fibrin during blood clotting.

The y-globulin fraction is the most heterogeneous. A multitude of Antibodies with distinct primary structures are known to exist. Electrophoretically, they are detected primarily in the y-globulin fraction and partially in the ß₂-globulin fraction. The Structure and function of y-globulins are discussed in greater detail below (see Chapter 2, "Glycoproteins").