In the image of a cell shown in Figure 1, one class of macromolecules, proteins, is almost everywhere. As we see in this chapter, proteins are so widespread in biology because they are exceedingly versatile components, exhibiting enormous variety in both structure and function (Table 1). They are the most versatile and important working molecules in the cell.
Fig1. Typical structure of a bacterial cell. The major and minor components of the various entities are as follows: pili, composed of protein; plasmid, composed of lipid, protein, and nucleic acid; ribosomes, composed of nucleic acid and protein; cytoplasm, composed of protein, small molecules, and nucleic acid; plasma membrane, composed of lipid and protein; cell wall, composed of polysaccharide and protein; capsule, composed of polysaccharide; flagellum, composed of protein; and nucleoid, composed of nucleic acid and protein. [Adapted, courtesy of Mariana Ruiz Villarreal, Wikimedia.]
Table1. Major functions of proteins.
We now know of the numerous functions that proteins fulfill. But the beginning of the protein field was not easy, especially in analyzing the proteins in the cell nucleus. Obtaining purified nuclei from many tissues was impossible at the time because the crude methods for fragmenting tough materials like muscle or organs would mix nuclear materials with the cell homogenate. A young German scientist, Friedrich Miescher, working in the lab of Felix Hoppe-Seyler, adopted, in 1871, a novel approach. He used, as a source of material, pus from bandages from the local hospital. Such a soft “tissue” allowed gentle cell lysis and isolation of nuclei. Miescher obtained a substance which we now call chromatin—the material that makes up chromosomes. It contained a phosphorus-rich component (now called DNA) and a group of proteins, the histones. Another student in the lab, Albrecht Kossel, was able to isolate the nuclear protein component in 1884, using blood cells from geese. We shall have much more to say about histones in later chapters.
All proteins are large molecules: some are very large, with masses in the millions of Daltons. Some are fibrous and highly extended and thus have mainly structural roles. Similarly, scaffold proteins connect and hold in place proteins whose functions are inter connected. A vast class of more compact proteins includes molecules that act as signals within and between cells, as transporters of small molecules, as regulators of cellular processes, and as enzymes. Enzymes are the catalysts responsible for facilitating the myriad chemical reactions that a living cell or organism utilizes in its metabolism and growth.
