Mucoproteins and Proteoglycans

  • Definition: Mucoproteins (also known as proteoglycans) are conjugated proteins composed of carbohydrate moieties linked to core proteins.
  • Glycosaminoglycans (GAGs): These are mucopolysaccharides that are covalently linked to core proteins by
    • N-Glycosidic bonds or O-Glycosidic bonds.
  • Characteristics:
    • Mucoproteins typically have greater than 10% carbohydrate content.
    • They are viscous in nature and form highly complex, dense molecular aggregates.

Composition and Structure

  • GAG Chains:
    • Composed of up to 200 chains of GAGs linked to a core protein through serine/threonine side chains.
    • Molecular weight ranges from 105 to 107 Daltons.
    • Examples of GAGs include:
    • Hyaluronic acid
    • Chondroitin sulfate
    • Heparan sulfate
    • Dermatan sulfate
    • Keratan sulfate

Examples of Proteoglycans

  • Types:
    • Aggrecan: Features hyaluronate-core protein which is non-covalently linked (found in cartilage).
    • Other examples include biglycan, decorin, and syndecan.

Functions of Mucoproteins

  • Mucoproteins are primarily present in the extracellular matrix of connective tissues, providing:
    • Structural framework
    • Mechanical support to tissues such as bone and cartilage.

Mucopolysaccharidoses (MPS)

  • Definition: Group of inherited disorders from defective mucopolysaccharide metabolism.
  • Causes: Impaired degradation of GAGs due to defective lysosomal enzymes.
  • Lifespan of GAGs:
    • Most have a half-life of 3-10 days, except for keratan sulfate, which has a half-life of approximately 120 days.
  • Biochemical Alterations:
    • Lack of catabolism of GAGs leads to accumulation—increasing deposition of GAGs causes functional tissue disruption.
    • GAG excretion found in urine.

Common Mucopolysaccharidoses Types

  • Hurler’s Syndrome: Deficient in α-L iduronidase, leading to accumulated dermatan sulfate and heparan sulfate.
  • Hunter’s Syndrome: Deficient iduronate sulfatase.
  • Sanfilippo’s Syndrome: Deficient heparan sulfatase.
  • Morquio’s Syndrome: Galactosamine sulfatase deficiency, affecting keratan sulfate and chondroitin sulfate.
  • Scheie’s Syndrome: Defect in L-iduronidase—affects dermatan sulfate.
  • Maroteaux-Lamy Syndrome: N-acetylgalactosamine-4-sulfatase deficiency related to dermatan sulfate.
  • SLY’s Syndrome: β-glucuronidase deficiency affecting dermatan sulfate and heparan sulfate.
  • Naowicz Syndrome: Effected hyaluronidase deficiency.

Inheritance Patterns

  • Most mucopolysaccharidoses exhibit autosomal recessive inheritance, with the exception of Hunter’s Syndrome which is X-linked.

Clinical Manifestations

  • Symptoms associated with MPS disorders include:
    • Coarse facial features
    • Skeletal deformity
    • Corneal opacity
    • Hearing loss
    • Mental retardation
    • Hepatosplenomegaly
    • Cardiopulmonary defects
    • Growth deficiencies

Diagnosis of MPS

  • Utilizes:
    • Measurement of lysosomal hydrolases concentration.
    • Detection of GAG in urine.

Plant Heteropolysaccharides

Agar-Agar

  • Derived from red algae (seaweed).
  • Components:
    • Agarose (neutral galactose polymer)
    • Agaropectin (galactose and galacturonic acid units partially esterified with sulfuric acid).
  • Uses:
    • Bacteriological culture media
    • Emulsifier and thickener in foods like ice creams
    • Laxative for ulcers and chronic constipation.

Pectin

  • Sourced from apple pomace and citrus.
  • Composition:
    • Arabinose and galactose units, with galacturonic acid.
  • Average molecular weight: 100,000-250,000.
  • Uses:
    • Topical applications for burns and ulcers
    • Acts as a detoxifying agent
    • Important in treating diarrhea
    • Emulsion stabilizer in jellies and jams.

Glycoproteins

Definition and Structure

  • Glycoproteins are conjugated proteins with:
    • Oligosaccharide chains (branched or unbranched) attached to a protein backbone via O-glycosidic or N-glycosidic linkages.
  • Glycoprotein Characteristics: The carbohydrate content is typically less than 10%.
    • N-linked: Attaches to asparagine residues via amide nitrogens.
    • O-linked: Attaches to serine, threonine, or hydroxylysine hydroxyl groups.

Importance of Oligosaccharide Chains

  • Function includes:
    • Providing stability against denaturation.
    • Protecting proteins from proteolytic degradation.
    • Enhancing solubility and serving as recognition signals for cell-cell interactions.

Glycoproteins in the Human Body

  • Examples include:
    • All plasma proteins
    • Structural protein collagen
    • Certain enzymes (e.g., ribonuclease)
    • Hormonal receptors and mucins
    • Important for immune function (immunoglobulins) and clotting factors.

Biomedical Importance of Carbohydrates

Energy Aspects

  • Carbohydrates can provide dietary and calorific value, typically 4 Cal/gm.

Dietary Sources

  • Sources include:
    • Starch (grains, potatoes)
    • Glycogen (non-veg sources)
    • Sucrose (table sugar)
    • Lactose (dairy)
    • Glucose and fructose (fruits)

Dietary Fiber and Health

  • Non-digestible carbohydrates like cellulose aid in digestion and obesity management as dietary fiber.

Functional Roles

  • Carbs serve essential roles in the structural framework of mucoproteins, lubrication, and shock absorption within joints.
  • Some specific examples include:
    • Hyaluronic acid (synovial fluid)
    • Heparin (anticoagulant structure)
    • Ribose and deoxyribose (nucleotide components).

Diagnostic and Therapeutic Uses

  • Fructosan inulin used to assess glomerular filtration rate (inulin clearance test).
  • Cardiac glycosides and antibiotics derive from carbohydrate compounds for medical treatments.