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.