Collagen: Difference between revisions

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As shown above tropocollagen is formed by <scene name='Collagen/One_tropocollagen/1'>three peptides</scene> twisting around each other, and in doing so the peptides make <scene name='Collagen/Peptide_3_residue_segments2/2'>one turn every ~7 three-residue repeats</scene> (Cyan colored residues mark the approximate length of one turn.).  <scene name='Collagen/One_tropocollagen2/1'>Three cyan colored residues</scene> mark the approximate distance of one turn of the peptides in a tropocollagen.  Tropocollagen displayed as <scene name='Collagen/One_tropocollagen_backbone2/1'>backbone only</scene> clearly shows both types of helical turns - the 3.3 residue/turn and ~21 residue/turn.   
As shown above tropocollagen is formed by <scene name='Collagen/One_tropocollagen/1'>three peptides</scene> twisting around each other, and in doing so the peptides make <scene name='Collagen/Peptide_3_residue_segments2/2'>one turn every ~7 three-residue repeats</scene> (Cyan colored residues mark the approximate length of one turn.).  <scene name='Collagen/One_tropocollagen2/1'>Three cyan colored residues</scene> mark the approximate distance of one turn of the peptides in a tropocollagen.  Tropocollagen displayed as <scene name='Collagen/One_tropocollagen_backbone2/1'>backbone only</scene> clearly shows both types of helical turns - the 3.3 residue/turn and ~21 residue/turn.   


Looking down the axis of a tropocollagen displayed as wireframe, <font color="#ff0000">glycine</font> can be seen <scene name='Collagen/Gly_position_tropo/2'>positioned in the center</scene> of the triple helix.  The two types of helical turns consistently positions the Gly in the center of the triple helix.   <font color="limegreen">Proline</font> and the <font color="gold">hydroxyproline</font>  are on the <scene name='Collagen/Pros_position_tropo/1'>outside</scene> of the triple helix.  With the hydroxyl group of Hyp extending to the surface of the triple helix, it can be involved in hydrogen bond formation, as will be seen in the next section. The cyclical side chains of Pro and Hyp are some what rigid, and this rigidity adds to the stability  of the collagen fiber. The primary structure of repeating Gly-Pro-Hyp along with the two types of helical turns determine the 3D positions of Gly, Pro and Hyp in the tropocollagen.  
Looking down the axis of a tropocollagen displayed as wireframe, <font color="#ff0000">glycine</font> can be seen <scene name='Collagen/Gly_position_tropo/2'>positioned in the center</scene> of the triple helix.  The two types of helical turns consistently positions the Gly in the center of the triple helix. <span style="color:limegreen;background-color:black;font-weight:bold;">Proline</span> and the <font color="gold">hydroxyproline</font>  are on the <scene name='Collagen/Pros_position_tropo/1'>outside</scene> of the triple helix.  With the hydroxyl group of Hyp extending to the surface of the triple helix, it can be involved in hydrogen bond formation, as will be seen in the next section. The cyclical side chains of Pro and Hyp are some what rigid, and this rigidity adds to the stability  of the collagen fiber. The primary structure of repeating Gly-Pro-Hyp along with the two types of helical turns determine the 3D positions of Gly, Pro and Hyp in the tropocollagen.  


In order to make a compact strong fiber the interior residues of the triple helix need to be close packed.  The <scene name='Collagen/Gly_no_hindrance/1'>Gly side chain</scene> is the only one small enough to accommodate this close packing in the interior of the triple helix. (Realize that in this model the hydrogen on the <font color="limegreen"> α carbon</font> is not displayed.)  <scene name='Collagen/Glys_close_pack/1'>Three Gly</scene>, one on each of three different chains, are close packed together.  The gray atoms of the yellow and lime Gly are the α-carbons, and only a hydrogen could fit between these carbons and the atoms of the adjacent Gly.  <scene name='Collagen/Glys_pro_close/2'>A Pro</scene> on each of the 3 chains are shown close packed to the three Gly (lime, cyan, yellow). Adding the <scene name='Collagen/Glys_pro_hyp/1'>Hyp</scene> shows that Pro and Hyp are tightly positioned around the small interior Gly leaving no space for side chains longer than the single hydrogen of Gly.  
In order to make a compact strong fiber the interior residues of the triple helix need to be close packed.  The <scene name='Collagen/Gly_no_hindrance/1'>Gly side chain</scene> is the only one small enough to accommodate this close packing in the interior of the triple helix. (Realize that in this model the hydrogen on the <font color="limegreen"> α carbon</font> is not displayed.)  <scene name='Collagen/Glys_close_pack/1'>Three Gly</scene>, one on each of three different chains, are close packed together.  The gray atoms of the yellow and lime Gly are the α-carbons, and only a hydrogen could fit between these carbons and the atoms of the adjacent Gly.  <scene name='Collagen/Glys_pro_close/2'>A Pro</scene> on each of the 3 chains are shown close packed to the three Gly (lime, cyan, yellow). Adding the <scene name='Collagen/Glys_pro_hyp/1'>Hyp</scene> shows that Pro and Hyp are tightly positioned around the small interior Gly leaving no space for side chains longer than the single hydrogen of Gly.  
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