Mucopolysaccharides : Exceptions, Important Facts

Introduction 

• Mucopolysaccharidosis, also known as glycosaminoglycans.
• They are long, straight, unbranched chains containing repetitive uronic acid and amino sugar units arranged alternatively.
• Mucopolysaccharides are sulphated or acetylated based on their location.
• All Mucopolysaccharides are glycosaminoglycans, components of the Extracellular matrix.
• In the Extracellular matrix, negative charges provide 2 advantages:
  • Provides gel-like viscous consistency tear matrix.
  • It is responsible for the selectivity of the Glomerular basement membrane.
• The matrix should be gel-like viscous and not rigid rod-like.
• If it is rigid rod-like, then, under pressure, the matrix will break.
• All matrices have an array of Mucopolysaccharides, which are negatively charged. 
• The extracellular matrix is present in between cellular and vascular compartments of the body.
• The extracellular matrix contains negatively charged mucopolysaccharides; eventually, they attract positively charged sodium ions from the circulation.
• Sodium ions are osmotically active; they drag along the water, and therefore, the matrix gets overloaded with water.
• Thus, the water provides a viscous consistency to the extracellular matrix.
• The extracellular matrix is hygroscopic; it attracts more water.
• Thus, Hyaluronic acid is used as a moisturizing agent and lubricant.

Exceptions in The Mucopolysaccharide Group

It is important to know what are the mucopolysaccharides which need a special mention:

1. Keratan Sulphate:
   • Instead of uronic acid, it has Galactose.
   • It has repetitive units of Galactose and amino sugar arranged alternatively.
2. Uronic acid:
   • Mostly Glucuronic acid.
   • Instead, Iduronic acid is present in Heparin, Heparan sulphate, and Dermatan sulphate.
   • Both Glucuronic and Iduronic acid are epimers. They change only in the orientation of carboxyl growth.
3. Amino sugar:
   • Mostly Glucosamine.
   • Instead, Galactosamine is present in Chondroitin sulphate and Dermatan sulphate.
   • Dermatan sulfate is made up of Iduronic acid and Galactosamine.

Important Facts About Mucopolysaccharides

• Mucopolysaccharides have Glucuronic acid, and Glucosamine is Hyaluronic acid.
• Mucopolysaccharides, which have Glucuronic acid and galactosamine, are called Chondroitin sulfate.
• Mucopolysaccharides with Iduronic acid and galactosamine are Dermatan sulfate.
• Mucopolysaccharides, which have no acid part, and it has galactose, are Keratan sulphate.

Tips To Identify The Structure Of Mucopolysaccharide

• The uronic acid part has a carboxyl group (COO-)
• The amino sugar part has N.

Step1. Check if there is no acid part, 

• It has galactose. 
• It is Keratan sulphate.

Step2. In the uronic acid part, find if it is Glucuronic or Iduronic acid:

• If the carboxyl group is present above the plane of the ring- Glucuronic acid
• If the carboxyl group is present below the plane of the ring- Iduronic acid.

Step3. In the amino sugar part, find if it is Glucosamine or galactosamine. In the 4th carbon atom (Glucose and galactose are epimers at the 4th carbon position).

• If OH lays below the plane of the ring- Glucosamine
• If OH lays above the plane of the ring- galactosamine.

Location of Mucopolysaccharide(MPS)

Classes of Enzymes which have MPS derivative

• Digestive enzymes (Lactose) – present in the digestive duct
• Metabolic enzymes/ Glycolytic enzymes (within the cell
  • Hexogines, Oligogines)-- present in cytoplasm
  • Citric acid cycle enzymes- Present in mitochondria
• Enzymes within the vascular tree:
  • Lipoprotein Lipase: acts on lipoproteins
  • Lipoproteins are transport forms of lipids in the blood.
  • VLDL, LDL, IDL, and HDL are present in the blood.
  • For an enzyme to act on the blood component, it should be on the vessel wall, held in place by heparan Sulfate.

Also Read: All About Vitamins - Important Information and Diseases

The Function of Mucopolysaccharides (MPS)

• All Mucopolysaccharides are covalently attached to one core protein.
• The structure in which the central core protein and Mucopolysaccharide are attached on either side is called Proteoglycan.

One-liners

Lately, examinations like FMGE and NEET PG have also been asking for one-liners in the exam. It is very important that young doctors save time on such questions and utilize it for more clinically oriented questions. Here are some one-liners that should be revised during last-minute revision as well.

1. The Mucopolysaccharide with galactose is Keratan sulphate
2. Mucopolysaccharides with Iduronic acid are Heparan sulphate and Dermatan sulphate.
3. Mucopolysaccharides with galactosamine are chondroitin sulphate and Dermatan sulphate.
4. The Mucopolysaccharide present in the Glomerular Basement membrane is Heparan sulphate.

Hurler’s(Mucopolysaccharidosis I) and Hunter’s (Mucopolysaccharidosis II) Syndrome

Let us differentiate Mucopolysaccharidosis I and Mucopolysaccharidosis II in a tabular form to make this easy to learn.

Also Read: Lipids: Classification, Pufas, Lipoproteins, Ketone Body Synthesis & Utilization

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