Measured in terms of their total mass, the largest class is the structural one. Structural proteins are fibrous proteins. The most familiar of the fibrous proteins are probably the keratins, which form the protective covering of all land vertebrates: skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks and feathers. Equally widespread, if less visible, are the actin and myosin proteins of muscle tissue. Another group of fibrous structural proteins are the silks and insect fibers. In addition, there are the collagens of tendons and hides, which form connective ligaments within the body and give extra support to the skin where needed.
There is a great variety in the structures of the various fibrous proteins, more than we can look at in this lesson. But let's take a look at one common example, the keratin structure of hair, which is shown in example 7 in your workbook. The basic unit of hair is a long strand of protein chain which forms into an alpha helical secondary structure. Three of these alpha-helical lengths of protein wind around each other to form a structure called a protofibril. In turn, a microfibril is assembled from eleven protofibrils arranged in a way that involves nine protofibrils wound around two protofibrils. Hundreds of microfibrils are then packed together with a sort of protein glue to form a macrofibril. These macrofibrils are, in turn, packed within the body of a dead hair cell. A typical hair fiber is about ten hair cells in diameter.
A typical growth rate for human hair is about one half of an inch per month. This requires that the hair follicle produce approximately ten turns of alpha helical protein every second. Ten turns, by the way, is about thirty-six amino acids.
The protein strands within hair and other alpha-keratins are crosslinked to some extent by covalent bonds between cysteine residues to form disulfide bonds. The more such disulfide bonds there are between the strands, the more rigid the protein becomes as a whole.
The alpha keratins can be classed as "soft" or "hard" according to their sulfur content, which is to say, the relative number of cysteines in the polypeptide chains. The low-sulfur keratins of skin and callous are much more flexible than the high-sulfur, hard keratins of horns, claws and hooves.
E-mail instructor: Sue Eggling
Clackamas Community College