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Opening Image: The triple helix of a collagen-like model peptide.
The collagen triple-helix and the a-helical coiled coil represent the two basic supercoiled protein motifs. The collagen triple-helix consists of a repeating Gly–X–Y sequence. It is a major domain of all collagen proteins.
The collagen triple-helix is defined by the supercoiling of three polypeptide chains. The repeating Gly–Pro–Hyp triplets cause each chain to twist into an extended polyproline-like helix. Three such helices are twisted together to form a triple-helix.
A closer look at the backbone of the triple-helix (no side chain or hydrogen atoms). The chains are held together by noncovalent interactions (van der Waals forces, hydrophobic interactions, and hydrogen-bonds).
Every third residue passes through the center of the triple-helix, which is so crowded that only a Gly side chain can fit there.
with all Gly residues spacefilled. Gly α-carbon atoms are shown in dark gray.
Now all the backbone atoms are shown in spacefill. .
The α carbons of Gly are colored dark gray. The b carbons of Pro and Hyp are indicated by green dots. Note the absence of a hydrogen atom on each imino nitrogen of the Pro and Hyp between chains.
The three polypeptide chains are staggered, so that the Gly, X, and Y residues from the three chains occur at the similar levels. The staggered peptide groups are oriented such that the N–H of each Gly makes a hydrogen-bond with the carbonyl oxygen atom of the X residue on a neighboring chain. Therefore, the collagen triple-helix has one hydrogen-bond per three residues.
to see the entire network of interchain hydrogen bonds.
Replacing Gly-15 with Ala forces the three chains apart. between spacefilled models of the backbone atoms.
The gaps are partially filled by four immobilized water molecules. Each water forms a cross-link between two chains through hydrogen bonds to an N–H group of one chain and a C=O group of a neighboring chain. Only the oxygen atoms, colored red, of the water molecules are shown. [ The exact positions of the hydrogen atoms of the water molecules could not be located in this model. ]
on the main backbone. The α-carbons of Ala are colored dark gray. The four water molecules are shown in purple. Once again, note the lack of a hydrogen atom on the backbone nitrogen of Pro and Hyp.
Now for some recontructive surgery...
, the Ala α-carbons (dark gray).
, the β-carbons.
, the complete backbone.
Now we return to a spacefilling model of the wild-type collagen triple-helix. The resides are colored by group: Gly, Pro and Hyp.
The relatively inflexible Pro and Hyp residues confer rigidity on the entire assembly. Collagen's well-packed, rigid triple-helix accounts for its amazing tensile strength.
Collagen functions by aggregating side-by-side into microfibrils, which assemble into larger supramolecular arrays large enough to be seen in the electron microscope. the assembly of a microfibril.