In most cells, over 90% of the total mass (excluding water) is represented by very large molecules called macromolecules.
The macromolecules vary in size from several hundred to several hundred million molecular weight units. Four major classes of macromolecules may be identified; these are the proteins, polysaccharides, lipids, and nucleic acids.
The relative amounts of these macromolecules in the “typical” cell are given in Table 4-1.
Of all macromolecules found in the cell, the proteins are the most chemically and physically diverse. It is to the Dutch chemist Gerard Johannes Mulder (I802- 1880) that we attribute the origin of the term protein. Mulder clearly recognized the primary importance of this substance to living things, for the term itself (derived from the Greek “proteios”) means “of the first rank.”
In Mulder’s time, it was generally believed that protein was a single substance, essentially the same in all cells and organisms. This, of course, is not the case, and today it is recognized that there are hundreds of thousands of chemically distinct proteins. Indeed, a single human cell may contain more than 10,000 different protein molecules.
Classes of Proteins:
Several different schemes can be used to classify proteins, no scheme being entirely satisfactory. For example, proteins may be classified according to their biological functions, being either structural or dynamic. Structural proteins include keratin, the major protein component of hair, and collagen, abundant in skin, bone, and cartilage. The dynamic proteins include the enzymes that serve as catalysts of metabolism, hormonal proteins, respiratory pigments, and other proteins whose principal role is not structural.
Proteins may also be classified according to their shape or conformation. Again, two major categories may be distinguished: fibrous proteins and globular proteins. Fibrous proteins are proteins whose overall shape is threadlike, such as myosin of muscle tissue and the fibrin threads of coagulated blood.
Globular proteins are quasi-spherical, a shape characteristic of enzymes and the respiratory pigments. Generally speaking, the conformations of fibrous proteins are simpler than those of globular proteins and are more amenable to study. Although it is convenient to classify proteins according to their function or conformation, the student should recognize that all systems of classification are at best artificial.