Major Elements in Biological Molecules: Proteins

Proteins are chains of amino acids linked by peptide bonds. The 20 different amino acids used to make all proteins differ only in their side chains, and the properties of these side chains account for the great diversity of protein structure and function. Collagen is an example of how a protein's amino acid sequence determines its structure and function. Amino acid sequences are encoded in the DNA of genes. Collagen is the most abundant mammalian protein. It is the main component of skin, bones, and teeth. This protein is composed of three helical polypeptide chains that form a stiff, supercoiled cable. The type of helix in each polypeptide is unique to collagen and results from collagen's unusual amino acid composition. A collagen helix forms largely from the influences of two types of amino acids: proline, which introduces sharp twists in the poly-peptide, and glycine, which has a small side chain (H) that doesn't interfere with packing in the helix. Collagen contains many of these amino acids, but few bulky ones (e.g., phenylalanine). Each helix contains three amino acids per turn, with glycine located at every third position. The three polypeptides in a collagen molecule associate with their glycines all facing collagen's center. Glycine is the only amino acid small enough to allow the polypeptides to pack (by hydrogen bonding) into a tight cable. Individual collagen molecules cross link to other collagen molecules to form tough collagen fibrils. Collagen, from which gelatin is derived, is a poor source of essential amino acids—the amino acids that the body cannot manufacture and must receive from the diet.

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