Molecule of the Month: Collagen
Sturdy fibers of collagen give structure to our bodies
Your Most Plentiful Protein
The Collagen Triple Helix
Collagen on the Grocery Shelf
Ropes and Ladders
This illustration depicts a basement membrane, which forms a tough surface that supports the skin and many organs. A different collagen--"type IV"--forms the structural basis of this membrane. Type IV collagen has a globular head at one end and an extra tail at the other. The heads bind strongly together, head-to-head, and four collagen molecules associate together through their tails, forming an X-shaped complex. Using these two types of interactions, type IV collagen forms an extended network, shown here in light blue. Two other molecules--cross-shaped laminin (blue-green) and long, snaky proteoglycans (green)--fill in the spaces, forming a dense sheet.
Exploring the Structure
Collagen Triple Helix
A special amino acid sequence makes the tight collagen triple helix particularly stable. Every third amino acid is a glycine, and many of the remaining amino acids are proline or hydroxyproline. A classic triple helix is shown here on the left, and may be viewed in the PDB file 1cag . Notice how the glycine forms a tiny elbow packed inside the helix and the proline and hydroxyproline smoothly bend the chain back around the helix. In this structure, the researchers placed a larger alanine amino acid in the position normally occupied by glycine, showing that it crowds the neighboring chains.
The collagen helix shown on the right contains a segment of human collagen, and may be viewed in the PDB file 1bkv . Notice that the top half is very uniform, where the sequence is the ideal mixture of glycine and prolines. At the bottom, the helix is less regular, because many different amino acids are placed between the equally-spaced glycines. Click on the image for an interactive JSmol view of this structure.
April 2000, David Goodselldoi:10.2210/rcsb_pdb/mom_2000_4