Locomotion moduleBiochemistry

Omer M. Abdalla, PhD molecular medicine

Collagen and ground substance Objective

State the basic steps involved in collagen synthesis and their defects.

State different types of collagen and their distribution.

State how the structures of collagen and ground substance of bone facilitate the deposition of bone mineral.

Explain the mechanism of calcification.

Fibrous Proteins: Collagen Collagen is the main protein of connective tissues. Strong, tough bundles of collagen called collagen fibers are a major component of the extracellular matrix.Component cartilage, ligaments, tendons, bone and teeth.

Collagen: Structure

Collagen has an unusual amino acid sequence. Glycine (Gly) is found at almost every third

residue, and collagen contains large amounts of proline (Pro) as well as hydroxyproline and hydroxylysine.

Multiple tropocollagen molecules form collagen fibrils and multiple collagen fibrils form collagen fibers

Collagen is attached to cell membranes via several types of protein, including fibronectin and integrin.

The tropocollagen or "collagen molecule" subunit is made up of three polypeptide strands, each of which is a left-handed helix

These three left-handed helices are twisted together into a right-handed coiled coil, a triple helix, a cooperative quaternary structure stabilized by numerous hydrogen bonds.

Tropocollagen subunits spontaneously self-assemble, with regularly staggered ends, into even larger arrays in the extracellular spaces of tissues. There is some covalent crosslinking within the triple helices, and a variable amount of covalent crosslinking between tropocollagen helices, to form the different types of collagen found in different mature tissues

Crosslinking side chains in collagen

Lys Lys Lysyl oxidase Lysyl oxidase

Allysine Allysine

Allysine aldol

His

Aldol-His

5-Hydroxy-Lys

Histidinodehydrohydroxymerodesmosine

A distinctive feature of collagen is the regular arrangement of amino acids in each of the three chains of these collagen subunits. The sequence often follows the pattern Gly-X-Pro or Gly-X-Hypro, where X may be any of various other amino acid residues. Gly-Pro-Hypro occurs frequently.

Such high glycine and regular repetitions are never found in globular proteins. Chemically-reactive side groups are not needed in structural proteins.

The high content of Pro and Hypro rings, with their geometrically constrained carboxyl and (secondary) amino groups, accounts for the tendency of the individual polypeptide strands to form left-handed helices spontaneously.

Because glycine is the smallest amino acid, it plays a unique role in fibrous structural proteins. In collagen, Gly is required at every third position because the assembly of the triple helix puts this residue at the interior (axis) of the helix, where there is no space for a larger side group.

Collagen Diseases

Scurvy: results from lack of ascorbic acid required for

collagen synthesis. Leads to the formation of liver spots on the skin, spongy gums and bleeding from all mucous membranes.

Molecular Basis of DiseaseHydroxylation of Proline and Lysine Collagen

Synthesis

Prolyl and Lysyl HydroxylaseCofactor

(Acsorbic acid)

Proline Hydoxy ProlineProlyl Hydroxylase

Ascorbic Acid reduces the ferric ion and oxidized to dehydroascorbic acid

Fe+2 activates oxygen

Rheumatoid arthritis

A chronic, inflammatory autoimmune disorder that causes the immune system to attack the joints. It can lead to substantial loss of mobility due to pain and joint destruction. It is a systemic disease often affecting the skin, blood vessels, heart, lungs, and muscles.

Antibodies against Type II Collagen.

Systemic lupus erythematosus (SLE or lupus) is a chronic autoimmune disease resulting in inflammation and tissue damage. SLE can affect any part of the body including heart, joints, skin, lungs, blood vessels, kidneys and nervous system. Antibodies against Type II Collagen.

Scleroderma

A rare, chronic disease characterized by excessive deposits of collagen. Progressive systemic scleroderma or systemic sclerosis, the generalized type of the disease, can be fatal. It affects the skin, and in more serious cases the blood vessels and internal organs. The most evident symptom is the hardening of the skin and associated scarring. The skin appears reddish or scaly in appearance. Blood vessels may also be more visible. Where large areas are affected, fat and muscle wastage will weaken limbs and affect appearance.Systemic scleroderma can cause painful ulcers on the fingers or toes which are known as digital ulcers.

Causes: Overproduction of collagen is thought to

result from an autoimmune dysfunction, in which the immune system would start to attack the kinetochore of the chromosomes. This would lead to genetic malfunction of nearby genes. T cells accumulate in the skin; these are thought to secrete cytokines and other proteins that stimulate collagen deposition.

TGFβ appear to be overproduced. An intracellular pathway is responsible for the secondary messenger system that induces transcription of the proteins and enzymes responsible for collagen deposition.

Lathyrism

produces serious abnormalities of the bones, joints and large blood vessels.

Molecular Basis of Disease

Caused by regular ingestion of sweet pea Lathyrus odoratus that contains a compound that specifically inactivates Lysyl oxidase resulting in reduced crosslinking of collagen fibers

Osteogenesis imperfecta

rare heritable disorder resulting from the mutation of Type I collagen

Molecular Basis of Disease

Severity of the disease is variable:

Lethal consequences with a single amino acid change.

e.g. Gly to Ala substitution

Distortion of helix

ruptures the H-bond between the Ala (NH) and Pro (CO)