The following points highlight the two main divisions of mucosubstances. The divisions are: 1. Glycoprotein 2. Mucopolysaccharides.
Mucosubstances: Division # 1. Glycoprotein:
(i) They are protein-polysaccharide compounds occurring in the tissues, particularly in mucus secretions.
(ii) They do not contain uronic acids although they contain acetyl hexosamines such as N-acetyl glucosamine and N-acetylgalactosamine. Hexoses such as mannose or galactose are also found. In addition, a methyl pentose (L-fucose) and the sialic acids commonly occur in these conjugated proteins.
The sialic acids are actually a family of compounds derived from neuraminic acid. They are widely distributed in vertebrate tissues and have also been isolated from certain strains of bacteria. N-acetylneuraminic acid is an example of sialic acid. Enzymes which are identified in the liver of the rat can accomplish the biosynthesis of N-acetylneuraminic acid.
(iii) Examples of glycoproteins are also found among the alpha, and alpha, globulins of the plasma.
(iv) They form viscous solutions which function as lubricants and protective “screens” in the body.
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. Intestinal mucus constitutes a protection for the intestinal cells against mechanical damage.
(a) The gastric mucus glycoprotein is a high molecular-weight (2 x 106) component of the viscous coat that protects the gastric mucosa from chemical, enzymatic, and mechanical damage. The protein’s resistance to pepsin digestion and its rheologic properties in solution correlates with its degree of acylation.
(b) Normally, about 20 moles of fatty acids (palmitate, stearate, and oleate) are bound in ester linkage per mole of mucus glycoprotein.
(c) In patient with cystic fibrosis, the stoichiometry of acylation is increased to 60-70 moles fatty acid/mol protein.
Hyper-acylation of mucus glycoprotein may be partially responsible for the extreme viscosity of the mucus produced by persons afflicted with the disease.
Mucosubstances: Division # 2. Mucopolysaccharides:
The heteropolysaccharide situated in extracellular matrix is called glycosaminoglycan, such as hyaluronic acid, chondroitin sulphate, heparin, kerato sulphate.
Glycosaminoglycan + Protein = Proteoglycan.
A. Hyaluronic acid:
(i) It has a molecular weight of 1 to 4 million.
(ii) It consists of alternate units of glucuronic acid and N-acetyl glucosamine linked to give a thread-like structure.
(iii) It forms viscous solutions in water and is important in the body as a lubricant.
(iv) It has an enormous capacity to hold water.
(v) It occurs in synovial fluid, in the skin and vitreous humour.
(vi) Its presence affects the rate of diffusion of materials in the skin. The enzyme reduces its viscosity and helps diffusion in tissue space.
(vii) The presence of the enzyme hyaluronidase increase the rate of diffusion of substances through tissues containing hyaluronic acid.
The richest source of the enzyme in mammals is the testis and its presence in the semen facilitates fertilization.
It is secreted by some pathogenic organisms which can, therefore, invade the tissues of the host animal more easily. Hyaluronic acid in tissue acts as a cementing substance.
B. Chondroitin sulphates:
(i) They occur in the ground substance of connective tissue and they are the components of cartilage, tendon and skin.
(ii) Chondroitin sulphates A and C consist of alternate units of glucuronic acid and 2- N-acetyl amino galactose.
(iii) Chondroitin sulphate B contains iduronic acid in place of glucuronic acid.
(iv) In all three forms, one hydroxyl group of each amino sugar is esterified with sulphuric acid and the molecules are long and linear.
(v) They have a marked capacity to bind water and contribute to the resistance to compression of connective tissue.
(vi) Since they are polyvalent anions they may also regulate the flow and concentration of cations round the cells.
C. Heparin:
(i) It occurs in most cells and is present in liver, lung and the arterial wall.
(ii) It consists of an un-branched chain of alternate units of glucosamine and glucuronic acid joined mainly by 1, 4-α- links.
Sulphate groups are linked to the hydroxyl group of carbon atom 6 and the amino group of the glucosamine and to some of the glucuronic acid units.
(iii) Its molecular weight is about 17,000 and the molecule is highly sulphated.
(iv) It is used in medicine as an anticoagulant.
D. Blood group substances:
(i) They consist of N-acetyl-glucosamine, galactosamine, galactose, fucose and sialic acid.
(ii) They are used to determine blood group.
E. Keratosulpbate:
(i) It consists of N-acetyl-glucosamine, galactose, sulphuric acid.
(ii) It is the component of cartilage and cornea.