Triple helix, side chains not shown. The individual beads that are connected by rods, idea is that each sphere is an amino acid, and the chains are extended. The prolines, which cannot for alpha helix, contribute to stability by hydroxylating prolines, contributes more with having the elongated structure. Glycines by being in every third position provide minimum bulk in the interior of this structure. The point at which the strands have the side chains facing one another, has a glycine every time. That minimizes the interior bulk and allows three strands close to one another. Hydrogen binding between the strands. Not extensible, already stretched out. There is 3.1 angstroms per residue, compared to 1.5 for alpha helix. Double the length of alpha helix before you get to a structure as extended as this. The lysines are hydroxylated then glycosylated, with disaccharide of glucose and galactose. Degree of glycosylation of these side chains determine when you aggregate triple helices if you get bundles or sheets.
there is aggregation of these, the way keratin aggregates into protofibrins. But first, production of procollagen. In the cell before secreted, there is a portion of the individual strands, so that the depiction is intended to represent extended strand. These are like the three strands from the beginning, yellow red and blue, but now there is this additional portion at C terminal that has disulfide links. This allows them to get close enough together to form the energetically unfavorable long extended structure. This keeps them together, do form the triple helix, assembles into procollagen, once this structure has formed, then procollagen peptidase removes the portion at the end, tropocollagen molecule. Start with procollagen, has portions required for assembly into triple helix, remove them proteolytically, end up with tropocollagen
Tropocollagen spontaneously aggregates into structures that are shown here. If there is staggered array, and space between the tropocollagen from right to left. They look like this on EM. Some overlap vertically. Cross linking gives strength. Variable degrees of cross liking depending on function of collagen. Once they are cross linked, this is an example of type I collagen in bond dentin and cementum. By a mechanism that is not fully understood, at least the location is identified the biomineralization occurs in gaps between tropocollagen and collagen structures. What appears as holes, will remain as holes, that is a nucleation center for biomineralization. That is where it starts, then as mineralization proceeds you get degradation of collagen to various degrees depending on bone, cementum, enamel
Khuram AzizM Phil BIOCHEMISTRY Junior Scientist by IBC
Fibrous Proteins Overview Collagen and Elastin Existance
OVER VIEW Collagen and Elastin are the examples of fibrous proteins. These are basic structural elements. These proteins have special mechanical properties. They are found as components of skin, connective tissue, blood vessels, sclera and cornea of eye.
Amino Acid Sequence: Collagen is a glycoprotein containing galactose and glucose as the carbohydrate content.
Glycine is one - third of total amino acid content of collagen followed by hydroxyproline and proline account for another one-third of amino acid content of collagen.
Proline - facilitate the formation of helical conformation of α- chain, because its ring structure causes kink in the peptide chain. Glycine- found in every third position of the polypeptide chain. It fits into the restricted spaces where the three chains of the helix come together.
Glycine is the part of the repeating sequence. Gly- X-Y X- is frequently proline Y- hydroxy proline or hydroxylysine.
Triple- helical structure: Amino acids side chains are on the surface of the triple helical molecule. This allows bond formation between the exposed R- groups of neighboring collagen monomers- This leads to aggregation into fibrils.
Collagen Triple Helix Prolines, especially hydroxylated prolines, keep the individual chains extended, and increase Tm (keep it above body temperature). The small size of the glycine sidechain in every third position allows the three strands to come close together. There are interstrand hydrogen bonds. It is not very extensible because it is already extended (3.1 D per residue vs 1.5 D for α-helix). Glycosylation of hydroxylysines appear to modulate fibril or sheet formation by the triple helices.13
Hydroxyproline & Hydroxylysine: Hydroxylation of Proline & lysine residues after their incorporation into the polypeptide chains. Thus called post translational modification. Causes stabilization of triple helical structure.
Glycosylation: Hydroxyl group of hydroxylysine residues of collagen are enzymatically glycosylated. Most commonly glucose and galactose are attached.
BIOSYNTHESIS OFCOLLAGENPrecursors: Collagen is one of the proteins that functions outside the cell. Polypeptide Precursors of the collagen molecule are formed in Fibroblast, osteoblasts and chondroblasts. These are secreted into the extracellular matrix.
Formation of Pro- α-chains Hydroxylation: Glycosylation Assembly and Secretion Extracellular cleavage of Procollagen molecules Formation of collagen fibrils Cross-link formation
1. Formation of Pro- α-chains: Pre-pro α-chains- contain a special amino acid sequence at their N-terminal. This sequence acts as a signal that the newly synthesized polypepetide is destined for function out side the cell.
This sequence facilitate the binding of ribosomes to the rough endoplasmic reticulum (RER), and direct the Pre-pro α- chain into the lumen of the RER.
This sequence is cleaved in the lumen of RER and after its cleavage Precursor of collagen is formed. This precursor is called Pro α-chain.
2. Hydroxylation: Processing of Pro α-chains occur by a number of enzymic steps in the lumen of RER, while the polypeptides are still being synthesized. Proline and lysine residues are hydroxylated. This reaction requires O2 and vitamin C.
Enzymes are prolyl hydroxylase and lysyl hydroxylase. In Vit C deficiency, collagen fibers cannot cross link- and tensile strength is decreased (scurvy).
3. Glycosylation: Modified by glycosylation with glucose or galactose residues.
4. Assembly and Secretion: After hydroxylation and glycosylation- Pro α-chains are converted to Pro-collagen. Pro-collagen has a central region of triple helix and its ends have non-helical regions of amino and carboxyl terminal extensions . These extensions are called Propeptides.
In the formation of procollagen interchain disulfide bonds are formed between the C- terminal extensions of the pro α-chains. This alignment of pro α-chains is favorable for helix formation. Then pro-collagen chains are translocated to Golgi- apparatus.
In the golgi they are packaged in secretory vesicles. These vesicles fuse with the membrane and release the pro-collagen molecule into the extracellular space.
5. Extracellular cleavage of Procollagen molecules: After their release the Procollagen molecules are cleaved by N- and C Procollagen peptidases. These remove the terminal Propeptides. Triple helical structure is released as Tropocollagen.
6. Formation of collagen fibrils: Tropocollagen spontaneously associate with each other and form collagen fibrils.
7. Cross-link formation: The fibrils that are formed become a substrate for lysyl oxidase. It contains copper. It oxidatively deaminates lysyl and hydroxlysyl residues in collagen. Reactive aldehydes- Allysine and hydroxylysine are formed.
These aldehydes the react with the neighboring lysyl and hydroxlysyl and covalent cross links are formed. This cross-linking leads to the formation of mature collagen.
Ehlers-Danlos syndrom (EDS) Heterogeneous group of generalized connective tissue disorder Inheritable defect in metabolism of fibrillar collagen Deficiency of lysyl hydroxylase or procollagen peptidase enzyme Or mutation in the amino acid sequence of type I, III, or IV collegen
Osteogenesis Imperfecta (OI) Also known as brittle bone disease Heterogeneous group of inherited disorder distinguished by bones which can easily bend and fracture Defect Presentation Sign and Symptoms Types
Elastin Introduction Occurrence Structure--- insoluble protein polymer synthesized from a precursor tropoelastin (700 A.A) primarily small and non-polar Tropoelastin deposits on glycoprotein (fibrillin)
DesmosineDesmosine is formed from 4 lysines, 3 of which are oxidised. CO NH CO NH αCH αCH Allysine CH2 CH2 CH2 CH2 CH2 CH2 NH NH NH Allysine CHO Allysine C NH αCH CH2 CH2 C C CH2 CH2 α CH αCH CH2 CH2 CH2 CHO CHO CH2 CH2 CH2 αCH C C CO CO NH3+ N+ CO CO CH2 CH2 CH2 Desmosine CH2 Lysine CH2 CH2 CH2 CH2 αCH α CH NH CO NH CO Lect. 6-47
Elastin Marfan syndrome--- a connective tissue disorder characterized by1. Impaired structural integrity in the skeleton2. The eye3. And the cardiovascular system The defect is abnormal fibrillin protein (functional microfibrils are not formed)
Elastin Degradation Alpha 1-Antitrypsin Role in the lungs Deficiency