1. Classification Based on Functions i. Catalytic proteins, e.g. enzymes
ii. Structural proteins, e.g. collagen, elastin, keratin iii. Contractile proteins, e.g. myosin, actin, flagellar
proteins
iv. Transport proteins, e.g. hemoglobin, myoglobin, albumin, transferrin
v. Regulatory proteins or hormones, e.g. ACTH, insulin, growth hormone
vi. Genetic proteins, e.g. histones
vii. Protective proteins, e.g. immunoglobulins, clotting factors.
2. Classification Based on Solubility
Proteins may be divided into three major groups;
simple, conjugated and derived.
Fig. 2.25: Denaturation of protein
Native protein with
functional amino acids (A, B, C) are nearby; protein is functional
Denatured protein; three-
dimensional structure is lost; A, B, C are far apart; function is lost. But primary structure is intact.
A. Simple Proteins
According to definition, they contain only amino acids. But they also contain very small quantity of carbohydrates.
i. Albumins: They are soluble in water and coagulated by heat. Human serum albumin has a molecular weight of 69,000.
ii. Globulins: These are insoluble in pure water, but soluble in dilute salt solutions. They are also coagulated by heat. Examples are egg globulin and serum globulins.
iii. Protamines: These are soluble in water, dilute acids and alkalies. They are not coagulated by heating. They contain large number of arginine and lysine residues, and so are strongly basic. Hence they can combine with other acidic proteins. Protamine zinc insulinate is a common commercial preparation of insulin.
iv. Scleroproteins: They are insoluble in water, salt solutions, organic solvents and soluble only in hot strong acids. They form supporting tissues. Examples are collagen of bone, cartilage and tendon; keratin of hair, horn, nail and hoof.
B. Conjugated Proteins
They are combinations of protein with a non-protein part, called prosthetic group (Table 2.3). Conjugated proteins may be classified as follows:
i. Glycoproteins: These are proteins combined with carbohydrates. Hydroxyl groups of serine or threonine and amide groups of asparagine and glutamine form linkages with carbohydrate residues. When the carbohydrate content is more than 10% of the molecule, the viscosity is correspondingly increased; they are sometimes known as mucoproteins or proteoglycans. Blood group antigens and many serum proteins are glycoproteins.
ii. Lipoproteins: These are proteins loosely combined with lipid components. They occur in blood and on cell membranes. Serum lipoproteins are described in Chapter 11.
iii. Nucleoproteins: These are proteins attached to nucleic acids, e.g. Histones. The DNA carries negative charges, which combines with positively- charged proteins.
iv. Chromoproteins: These are proteins with coloured prosthetic groups. Hemoglobin (Heme, red); Flavoproteins (Riboflavin, yellow), Visual purple (vitamin A, purple) are some examples of chromoproteins.
v. Phosphoproteins: These contain phosphorus. Casein of milk and vitellin of egg yolk are examples. The phosphoric acid is added to the hydroxyl groups of serine and threonine residues of proteins.
vi. Metalloproteins: They contain metal ions. Examples are hemoglobin (iron), cytochrome (iron), tyrosinase (copper) and carbonic anhydrase (zinc).
C. Derived Proteins
They are degradation products of native proteins. Denaturation is the first step, which has been discussed previously. Progressive hydrolysis of protein results in smaller and smaller chains: protein peptones peptides amino acids. 3. Classification Based on Nutritional Value A. Nutritionally Rich Proteins
They are also called as complete proteins or first class proteins. They contain all the essential amino acids in the required proportion. On supplying these proteins in the diet, the young individuals will grow satisfactorily. A good example is casein of milk.
B. Incomplete Proteins
They lack one essential amino acid. They cannot promote body growth in young individuals; but may be able to sustain the body weight in adults. Proteins from pulses are deficient in methionine, while proteins of cereals lack in lysine. If both of them are combined in the diet, good growth could be obtained.
Table 2.3: Examples of conjugated proteins
Conjugated Protein Prosthetic
protein part group
Hemoglobin Globin Heme
Nucleoprotein Histones DNA
Rhodopsin Opsin 11-cis-retinal
Ferritin Apoferritin Iron
Ceruloplasmin Apoceru- Copper
C. Poor Proteins
They lack in many essential amino acids and a diet based on these proteins will not even sustain the original body weight. Zein from corn lacks tryptophan and lysine.
Related Topics
Plasma proteins are described in Chapter 13; RIA and ELISA tests are described in Chapter 29.
A QUICK LOOK
• Most amino acids in the body are alpha amino acids. • Amino acids can be classified based on their: (i) structure (ii) m etabolic fate (iii) nutritional requirements.
• In solution, amino acids exist as ‘Zwitter ions’ or ‘Ampholytes’ at their characteristic Isoelectric pH (pI). In this state they carry no net charge. • Glycine has no asymmetric carbon atoms and
therefore has no optical activity.
• Alpha carboxyl group of one amino acid combines with the alpha amino group of another amino acid to form a peptide bond. Proteins are polymers of amino acids linked adjacently by peptide bonds. • Nitrogen content of ordinary proteins is on the
average 16% by weight.
• Protein structure can be defined and studied at four levels viz. primary, secondary, tertiary and quarternary.
• All proteins have a N-terminal (amino) and a C- terminal (carboxy).
• Cysteine forms disulfide linkages between two polypeptide chains in oligomeric proteins. • Primary structure determines the biological activity
of the protein. Alterations lead to loss of functional capacity. For example, sickle cell hemoglobin (HbS).
• Secondary, tertiary and quaternary structures of proteins are stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions and v an der Waals forces.
• Secondary structure could be an alpha-helix or a beta-pleated sheet.
• Examples of oligomeric proteins with quarternary structure are hemoglobin, myoglobin and creatine kinase.
• Solubility of a protein is dependent on the ionic concentration of the medium. Hence, proteins may be ‘salted out’.
• Denaturation of proteins results in loss of biological activity but not the primary structure. Denaturation may be reversible.
• Proteins can be classified based on: (i) functions (ii) composition and (iv) nutritional value.
CHAPTER AT A GLANCE
The reader will be able to answer questions on the following topics:
1. Classification of enzymes 2. Coenzymes
3. Fischer's template theory 4. Koshland's induced fit theory 5. Michaelis constant, Km value, Vmax 6. Factors influencing enzyme activity 7. Inhibition, competitive, noncompetitive 8. Allosteric inhibition
9. Isoenzymes
10. Lactate dehydrogenase and creatine kinase 11. Alkaline phosphatase and acid phosphatase Once upon a time there was a rich merchant. In his last will and testament, he put aside his 17 white horses to his 3 sons to be shared thus; 1/2 for the 1st son, 1/3 for the 2nd son and 1/9 for the 3rd son. After his death, the sons started to quarrel, as the division could not produce whole number. Then their brother- in-law told them that they should include his black horse also for the sharing purpose. Thus now they had 17 + 1 = 18 horses, and so division was possible; 1st son got one-half or 9 horses; 2nd son got 6 and 3rd son had 2 horses. Now all the 17 white horses were correctly divided among the sons. The remaining black horse was taken back by the brother-in-law. Catalysts are similar to this black horse. The reaction, althou gh theoretically probable, becomes practically possible only with the help of catalysts. They enter into the reaction, but come out of the reaction without any change. Catalysts are substances which accelerate the rate of chemical reactions, but do not change the equilibrium.
Enzymes are biocatalysts. Life is possible due to the coordination of numerous metabolic reactions inside the cells. Proteins can be hydrolyzed with hydrochloric acid by boiling for a very long-time; but inside the body, with the help of enzymes, proteolysis takes place within a short-time at body temperature. Lack of enzymes will lead to block in metabolic pathways causing inborn errors of metabolism.
The substance upon which an enzyme acts, is called the substrate. The enzyme will convert the substrate into the product or products.
Almost all enzymes are proteins. Enzymes follow the physical and chemical reactions of proteins. They are heat labile, soluble in water, precipitated by protein precipitating reagents (ammonium sulfate or trichloroacetic acid) and contain 16% weight as nitrogen.