Type III Collagen
Collagen III is known as a fibrous protein (Scleroprotein). It is a connective tissue and can be found in the lungs, skin, bone and muscle fibre. A scleroprotein is made up of amino acid chains (polymers), that make up long protein filaments shaped like rods or wires. They are water soluble and usually inert (not chemically reactive). Therefore, their role is to be used as structural or storage proteins. Its hydrophobic side chains that protrude cause the protein to exist as an aggregate.
Biomolecular Structure
Collagen III (Col3) is a trimeric protein, made up of three monomeric pro-alpha chains (amino acid peptide sequences) that bind together covalently and intertwine forming a triple-helix structure.
The peptide chains are synthesised at the rough endoplasmic reticulum, by a ribosome, though the process of translation. The amino acid sequences usually follow the pattern Gly-Pro-X or Gly-X-Hyp, with amino acid X being various other amino acid residues. The amino acid chains or polypeptides are known as pre-procollagen. Pre-procollagen have registration peptides at the C-terminus and the N-terminus and a signal peptide. (Brandon, et al 1999). Once the pre-procollagen has been released from the rough endoplasmic reticulum, into the lumen, the cleaving of signal peptides occurs. The amino acid chains are now called pro-alpha chains. Inside the lumen, proline and lysine are hydroxylated, as well as the glycosylation (addition of a sugar molecule) of specific hydroxylysine residues. This process requires an abundance of the cofactor ascorbic acid (Vitamin C). The triple helical structure is then formed by the pro-alpha chains coiling around a central axis in a right-handed manor, to form the triple-helix structure. Every third amino acid consists of glycine, due to its small hydrogen side chain, that allows the helix to be close to the centre of the axis. Thus, producing the Tropocollagen (Fidler, et al, 2018). Tropocollagen is formed from pro collagen after modification by the cell by the cleaving of the C-terminus and N-terminus by proteinase (Fig 3). It is enzymes that complete this modification stage, by adding chemical groups to proline and lysine to form a stable structured protein, along with the cleavage of propeptides.
The collagen molecules form long thin fibrils and interact to produce cross-links with one another and other types of collagen eg type three collagen usually works alongside type one collagen. The crosslinks between lysine and hydroxylysine side chains allow for a strong and stable structure. (Meyers, 1995).
Historical Fact
In 1940, Astbury developed the first model for the primary structure of collagen. Once assuming glycine was positioned in every third amino acid residue along the peptide chain and hydroxyproline was every ninth residue (Mayne and Burgeson, 1987).
Glycine, Proline and Hydroxyproline
Glycine
-
The chemical formula of glycine is NH2‐CH2‐COOH
-
Required at every third position because the assembly of the triple helix.
-
Smallest amino acid due to no side chains.
-
Larger side chains would lead to steric or electronic hindrance
-
Approximately half of the collagen sequence is not glycine
The proline and hydroxyproline repeats are important for the strength and stability of the Col3 structure, because they are both cyclic amino acids so are inflexible.
Proline
-
Side chains have a cyclic structure allowing for remarkable conformational rigidity.
-
Has a chemical formula of C5H9NO2.
-
Compromises of around 17% of collagen.
-
The hydroxylation of proline by prolyl hydroxylase increases collagen stability.
-
Procollagen-proline dioxygenase, also commonly known as prolyl hydroxylase,
is an enzyme that catalyses the addition of oxygen (Szpak 2015). Proline hydroxylation
is therefore imperative in the formation of connective tissue.
Hydroxyproline
-
Comprises of 13.5% of mammalian collagen (Nelson, 2005).
-
The ring must face outwards in the helical structure.
-
The inflexible cyclic structure allows for stability and sharp twisting of the helix.
Figure 3 : Schematic representation of the assembly of a collagen fibre. Showing the Collagen gene COL3 in the DNA double helix, that codes for the 3 separate alpha chains. Two α1-chains shown by the blue colour and one α2-chain in green. Each chain is a specific amino acid sequence, making up a polymer. Next is showing the protocollagen helix, compromised of the three alpha chains intertwined.
The cleaving of the N and C terminus by restriction enzymes, results in the tropocollagen. The tropocollagen then binds to other collagen tropocollagen by cross-linked covalent bonds between the molecules. the fibrils being 10 to 300 nm long. Thus, forming the collagen fibril which makes the collagen fibre. Which ranges from 0.5 to 3.0 µm in diameter.
Figure 2: Shows William Thomas Astbury, alongside the X-ray diffraction photograph of the collagen fibril, showing the helical shape. Source: (Hall. K, 2010, Centre for History and Philosophy of Science, University of Leeds, Leeds, Elsevier Ltd, URL: Hall_paper.pdf)
Figure 1: Shows the schematic image of Collagen type III, alpha 1 protein.
Source: Unknown
4a
4b
4c.
Figure 4: a) Shows the chemical structure of glycine.
b) Shows the structure of Proline
c) Shows the structure of Hydroxyproline.
Source: Computer Drawing, Eleanor Summers 2018
References
Branden, Carl, and Tooze, John (1999). Introduction to Protein Structure , 2nd edition. New York: Garland Publishing.
Meyers, Robert A., ed. (1995). Molecular Biology and Biotechnology: A Comprehensive Desk Reference. New York: VCH.
Mao, J.R. Bristow, J. (2001), ‘The Ehlers-Danlos syndrome: on beyond collagens’, J Clin Invest. 107(9): 1063–1069.
Pepin, M.G. Murray, M.L., Byers, P.H. (1999), ‘Vascular Ehlers-Danlos Syndrome’, Gene Reviews [Internet]
Pepin M, Schwarze U, Superti-Furga A, Byers P. Clinical and genetic features of Ehlers-Danlos syndrome type IV, the vascular type. N Engl J Med. 2000;342:673–680.
Szpak, Paul (2011). "Fish bone chemistry and ultrastructure: implications for taphonomy and stable isotope analysis". Journal of Archaeological Science. 38 (12): 3358–3372.
Nelson, D. L. and Cox, M. M. (2005) Lehninger's Principles of Biochemistry, 4th Edition, W. H. Freeman and Company, New York.