Proteins Types: Top 6 Types of Proteins | Biochemistry

The following points highlight the top six types of proteins. The types are: 1. Albumin 2. Glycoproteins 3. Chromoproteins 4. Scleroproteins (Albuminoids) 5. Silk Fibroin 6. Collagen.

Type # 1. Albumin:

Chemistry:

a. Albumins are simple proteins, soluble in water, coagulated by heat and precipitated by saturated salt solutions.

b. The glycine content of many albumins is low.

Sources:

They are frequently found in serum, muscle, milk and egg white. They are also widely distrib­uted in plants, especially in the seeds and fruits.

Characteristics:

a. Albumin is more readily affected by nu­tritional factors, e.g., restricted protein in­take.

b. Because of its small molecular size, it is lost from the circulating plasma more eas­ily than the other proteins by passage through capillary walls of increased per­meability as in inflammation and nephro­sis (albuminuria).

c. In sudden haemo-dilution or severe mal­nutrition, in the majority of the disease states, the plasma proteins are altered and the albumin is decreased.

d. It has the property of osmotic pressure and fatty acid transport.

Clinical importance:

a. The plasma albumin concentration falls somewhat in the later stages of normal pregnancy owing to haemo-dilution and decreased synthesis.

b. The major causes for abnormal decrease in albumin (hypoalbuminemia) are out­lined:

(i) Excessive loss in urine.

(ii) Inadequate protein supply (dietary protein, restriction, vomiting, diar­rhea etc.).

(iii) Impaired synthesis (liver dysfunction, chronic infection and severe anemia etc.).

(iv) Sudden plasma dilution (following sudden recovery from dehydration, infantile diarrhea etc.).

c. In the prominent clinical manifestation of hypoalbuminemia edema occurs as a re­sult of the decrease in plasma colloid os­motic pressure which favours retention of water in tissue spaces.

d. The investigation of albumin is of much importance not only in the case of edema but also of the state of liver function and of protein nutrition.

e. Pathological proteinuria may occur as a result of

(i) Increased glomerular permeability.

(ii) Renal tubular damage with defective reabsorption of serum albumin (ne­phrosis).

(iii) Disease of the lower urinary tract.

(iv) Abnormal protein in the plasma.

Type # 2. Glycoproteins:

a. The polypeptide chain is attached to one or more hetero-saccharide units by covalent links.

b. Some glycoproteins have many identical disaccharide units attached to the polypeptide chain.

c. In addition to the more common sugars such as glucose, mannose, N-acetyl glucosamine, and N-acetylgalactosamine, other units also occur e.g., L-fucose (a me­thyl pentose) and sialic acids (acyl deriva­tives of neuraminic acid which is formed by the condensation of pyruvic acid and mannosamine).

d. The carbohydrate part is bound to the pro­tein by a glycosidic link between N- acetyl-glucosamine and the amide group of asparagine; this link is more stable than a peptide bond.

e. The carbohydrate is in the form of disac­charide units. 600-700 such units are at­tached to the peptide chain, one per 6.4 amino acid residues.

f. They are secreted by the sub-maxillary glands of various animals.

Functions:

i. They form viscous solutions which func­tion as lubricants and protective ‘screens’ in the body.

ii. The continuously secreted mucus of the respiratory tract is a protection against in­vasion by bacteria and the uterus is pro­tected from the vaginal microbial flora by the cervical mucus.

iii. Intestinal mucus makes a protection for the intestinal cells against mechanical damage.

Type # 3. Chromoproteins:

These are conjugated proteins composed of simple proteins united with a coloured prosthetic group. Many biologically important proteins belong to this group.

They are:

a. Hemoglobins:

These are the respiratory proteins in which the prosthetic group is the iron containing porphyrin complex heme.

b. Cytochromes:

These are cellular oxida­tion-reduction proteins in which the pros­thetic group is heme.

c. Flavoproteins:

These are cellular oxida­tion-reduction proteins in which the pros­thetic group is riboflavin.

Chromo proteins of certain animal fibres such as black wool and hair in which the prosthetic group is melanin.

Visual purple of the retina, a chromo protein, in which the prosthetic group is a carotenoid pig­ment.

Catalase, a chromo protein, in which the pros­thetic group is heme. Peroxidase has a similar com­position.

Type # 4. Scleroproteins (Albuminoids):

a. These proteins are similar to albumins and globulins.

b. They are characterized by great stability and insolubility in water and salt solu­tions.

c. They form the supporting structures of ani­mals e.g., collagen in cartilage and white fibres of connective tissue and in bones and teeth, elastin in the yellow or elastic fibers, keratins in horn, hair, wool, real silk and feathers.

d. The keratins differ from the collagen-like members in having high sulphur content, chiefly in the form of cystine.

e. They are readily attacked and dissolved by alkali sulphides.

f. The collagen of animal hides can be pre­served in flexible condition giving the substance known as leather when treated with tannic acid, alum or various metal salts.

g. By long boiling with water collagen is partly hydrolysed giving a soluble prod­uct known as gelatin.

Type # 5. Silk Fibroin:

a. Fibroin, the principal protein of silk worm, fibres, consists entirely of glycine’s alter­nating with either alanine or serine. These three amino acids cover 85 per cent of the fibroin structure.

b. Silk fibres are extremely resistant to stretching and not highly flexible.

c. It also contains small quantities of “bulky” amino acids such as valine or tyrosine. These bulky residues interrupt the β-sheet regions and confer the flex­ibility essential for a successful cocoon.

Type # 6. Collagen:

a. Tropocollagen, the fundamental unit of collagen, consists of three polypeptide chains each about 1,000 residues in length. Each polypeptide forms a left- handed helix having about three residues per turn. Three left-handed helical polypeptides then entwine to form a right- hand triple helix which is stabilized by H bonds.

b. Mature Collagen fibres measuring 1.5 nm by about 300 nm reflect this stable and extended helical conformation.