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Classification and function of proteins. Denaturation. Proteins are divided into two broad classes : fibrous proteins, which are insoluble in water, and globular proteins, which are soluble in water or aqueous solutions of acids, bases, or salts. (Because of the large size of protein molecules, these solutions are colloidal). The difference in solubility between the two classes is a result of a difference in molecular shape, which is indicated in a rough way by their names.

Molecules of fibrous proteins are long and threadlike, and tend to lie side by side to form fibres ; in some cases they are held together at many points by hydrogen bonds. As a result, the intermolecular forces that must he overcome by a solvent are very strong.

Molecules of globular proteins are folded into compact units that often approach spheroidal shapes. Hydrogen bonds are internal, and areas of contact between molecules are small. Intermolecular forces here are comparatively weak.

Molecular and intermolecular structure determines not only the solubility of a protein but also the general kind of function it performs.

Fibrous proteins serve as the chief structural materials of animal tissues, a function to which their insolubility and fibreforming- tendency suit them. They make up : keratin, in skin, hair, nails, wool, horn, and feathers ; collagen, in tendons ; tnyosin, in muscle ; fibroin, in silk.

Globular proteins serve a variety of functions related to the maintenance and regulation of the life process, functions that require mobility and hence solubility. They make up : all enzymes ; many hormones, as, for example, insulin (from the pancreas), thyroglobulin (from the thyroid gland), ACTH (from the adrenal cortex) ; antibodies, responsible for allergies. and for defense against foreign organisms ; albumin in eggs ; hemoglobin, which transports oxygen from the lungs to the tissues; fibrinogen, which is converted into the insoluble, fibrous protein fibrin, and thus causes the clotting of blood.

Within the two broad classes, proteins are subdivided on the basis of physical properties. especially solubility : for example, albumins (soluble in water, coaLTulated by heat). globulins (insoluble in water, soluble in dilute salt solutions), etc.

Irreversible precipitation of proteins, called denaturation, is caused by heat, strong acids or bases, or various other agents.

Coagulation of egg white by heat, for example, is denaturation of the protein egg albumin. The extreme ease with which many proteins are denatured makes their study difficult. Denaturation causes a fundamental change in a protein, in particular destroyingany physiological activity.

Only one other class of compounds, the nucleic acids, shows the phenomenon of denaturation. Although closely related to the proteins, polypeptides do not undergo denaturation, presumably because their molecules are smaller and less complcv