The a helix is the basic structural unit for the class of fibrous proteins known as a keratins. Besides hair and wool, they include skin, beaks, nails, claws, and most of the external protective layers possessed by vertebrates. In accordance with good ropemaking principles, the fibers in a human hair go through seven layers of organization from protein chain to complete hair. This organization is shown on the next page. The protein chain is twisted into a right-handed a helix that is held together by hydrogen bonds. Three such a helices then are given a gentle left-hand twist to group them into a triple-chain coil called a protofibril. Nine of these protofibrils are bundled into a cylinder surrounding two others to build a 9 + 2 microfibril; and several hundred microfibrils are embedded in a protein matrix to form a macrofibril bundle. The macrofibrils are packed tightly inside the keratin-producing cells of the hair, and in the final level of organization these cells make up the hair fiber itself, surrounded by protective scales.

Wool is stretchable in a way that silk is not, because pulling on an a helix stretches only the relatively weak hydrogen bonds, and not covalent bonds. There is a limit to the stretch of wool fibers-when the a helices are pulled into fully extended chains. But if this limit is not exceeded, the fiber will snap back into its original length when the tension is released ' with re-formation of the hydrogen bonds. Hence wool is not only stretchable, it is elastic. A good wool cloth has an elastic, springy feel that silk cloth lacks. The explanation lies in the way that the two fibrous proteins are constructed.

The a helix and b sheet (the sheet structure found in silk) are two of the most common structures found in fibrous proteins. There are other structures, but the basic pattern is the same: essentially endless chains of proteins held together by hydrogen bonds that extend either to different chains, or to adjacent helical turns of the same chain. These same basic structures, a helix and 8 sheet, also are found in the more compact globular proteins, of which enzymes are the most common examples.