Classification by biological functions

It is preferable to classify the proteins according to their biological function. Such a classification is far from ideal, however, because one protein can have more than one function. Another problem with functional classification is that the definite function of a protein frequently is not known.

The number of existing proteins is immense. We would like to mention only some of them.

Structural (fibrous) proteins. Scleroprotein,В any of several fibrous proteins of cells and tissues once thought to be insoluble but now known to be dissolved by dilute solutions of acids such as citric and acetic. The two most important classes of scleroproteins are the collagens and the keratins.

ВThe muscle proteins. The total amount of muscle proteins in mammals, including man, exceeds that of any other protein. Thus, the human body contains about five to six kilograms (11 to 13 pounds) of muscle protein. In contrast to the scleroproteins, the contractile proteins are soluble in salt solutions and susceptible to enzymatic digestion.

Fibrinogen, the protein of the blood plasma, is converted into the insoluble protein fibrin during the clotting process. The fibrinogen-free fluid obtained after removal of the clot, called blood serum, is blood plasma minus fibrinogen.

Collagen is the structural protein of bones, tendons, ligaments, and skin. For many years collagen was considered to be insoluble in water.

Keratin is the structural protein of epithelial cells in the outermost layers of the skin. It has been isolated from hair, nails, hoofs, and feathers. Keratin is completely insoluble in cold or hot water.

Milk proteins. Milk contains the following: an albumin, О±-lactalbumin; a globulin, beta-lactoglobulin; and a phosphoprotein, casein. If acid is added to milk, casein precipitates.

Egg proteins. About 50 percent of the proteins of egg white are composed of ovalbumin, which is easily obtained in crystals. Its molecular weight is 46,000 and its amino acid composition differs from that of serum albumin. Other proteins of egg white are conalbumin, lysozyme, ovoglobulin, ovomucoid, and avidin.

Hemoglobin is an iron-containing protein in the blood of many animals – in the red blood cells (erythrocytes) of vertebrates – that transports oxygen to the tissues.

Proteins are considered to be macromolecular polypeptides – i.e., very large molecules composed of many peptide-bonded amino acids. Most of the common ones contain more than 100 amino acids linked to each other in a long peptide chain.

Peptide is any organic substance of which the molecules are structurally like those of proteins, but smaller. The class of peptides includes many hormones, antibiotics, and other compounds that participate in the metabolic functions of living organisms. Peptide molecules are composed of two or more amino acids joined through amide formation involving the carboxyl group of each amino acidand the amino group of the next. The chemical bond between the carbon and nitrogen atoms of each amide group is called a peptide bond. Some or all of the peptide bonds, which connect the consecutive triplets of atoms in the chain regarded as the backbone of the molecule, can be broken by partial or complete hydrolysis of the compound. This reaction, producing smaller peptides and finally the individual amino acids, is commonly used in studies of the composition and structure of peptides and proteins.

The number of amino-acid molecules present in a peptide is indicated by a prefix: a dipeptide contains two amino acids; an octapeptide, eight; an oligopeptide, a few; a polypeptide, many.

Peptides are distinguished from proteins on the basis of size, and as an arbitrary benchmark can be understood to contain approximately 50 or fewer amino acids. Proteins consist of one or more polypeptides arranged in a biologically functional way, often bound to ligands such as coenzymes and cofactors, or to another protein or other macromolecule (DNA, RNA, etc.), or to complex macromolecular assemblies. Finally, while aspects of the lab techniques applied to peptides versus polypeptides and proteins differ (e.g., the specifics of electrophoresis, chromatography, etc.), the size boundaries that distinguish peptides from polypeptides and proteins are not absolute. Long peptides such as amyloid beta have been referred to as proteins, and smaller proteins like insulin have been considered peptides.

REVISION EXERCISES

Ex.1. Answer the following questions:

1. What is protein? What does the Greek work proteios mean? 2. How many amino acids occur naturally in proteins? 3. Can animals and humans synthesize all of the amino acids essential for life? What must they obtain proteins from? 4. What do enzymes allow the organism to do? 5. What organic substances perform important regulatory functions in human’s organism? 6. What do biochemists often refer to? 7. Haw many classes can proteins be informally divided into? 8. What are the most important physicochemical properties of proteins? 9. What classification of proteins is today widely used? 10. What are peptides?

Ex.2. Match the words with their definitions:

1. organ-specific	a. The change of folding structure of a protein (thus of physical properties) caused by heating, changes in pH, or exposure to certain chemicals;
2. species-specific	b. produced by the injection of the cells of a certain organ or tissue;
3. liver	c. a number of things arranged in a line or row;
4.string, alignment	d. the largest glandular organ in vertebrate animals, located in the upper or anterior part of the abdomen: it secretes bile, has an important function in the storage and metabolism of carbohydrates, fats, and proteins, and helps detoxify many poisonous substances that may be ingested;
5. provide	e. to let go, to set free;
6. legume	f. Limited to or found only in one species;
7. denaturation	g. the pod or seed of some members of this order, used for food;
8. release	h. to supply or make available.

Ex.3. Say whether the following statements are true or false:

1. Proteins of similar function differ gratly in amino acid composition and sequence.В 2. Protein molecules are produced in cells and are allowed to be put into circulation within the organism only after synthesis is complete. 3. Enzymes are crucial for accomplishing all metabolic reactions. 4. The tercial structure controls the basic function of the protein. 5. Fibrous (structural) proteins serve regulatory, maintenance and catalytic roles in living organisms. 6. Ultracentrifugation is often used to determine the molecular weight of proteins. 7. A prefix shows how many amino-acid molecules present in a peptide.

Ex.4. Insert the necessary word:

1. Hemoglobin is … in the blood of many animals and humans that … oxygen to the tissues. 2. Hemoglobin forms an unstable, … with oxygen. 3. In the oxygenated state it is called oxyhemoglobin and is …; in the reduced state it is …. 4. Each hemoglobin molecule is … of four heme groups surrounding a globin group, forming a tetrahedral structure. 5. There are four iron atoms in each molecule of hemoglobin, which accordingly can … four atoms of oxygen. (made up, an iron-containing protein, bright red, transports, bind, purplish blue, reversible bond)