Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

The Chemical Foundations of Life
Amino Acids and Proteins

Proteins are the most abundant Organic compounds in The Cell; they typically account for 30 to 70% of the cellular dry weight. All proteins are composed of the four most common biological elements: carbon, hydrogen, nitrogen, and oxygen. On average, proteins contain 50% C, 7% H, 23% O, and 16% N. In addition, proteins contain up to 3% sulfur, which plays a crucial role in stabilizing the three-dimensional Structure of almost all proteins through The formation of disulfide (S—S) bonds between sulfur atoms located in different PARTS OF THE polymer chain. The molecular weights of these non-repetitive polymers vary widely, from 6,000 to over a million. Fig. 2.14 illustrates the two MAIN TYPES OF protein Spatial Structure—fibrillar and globular.

The predominance of proteinaceous substances in the cell is not surprising, given The Diversity of their biological Functions (Table 2.6). The primary function of proteins is catalysis. Protein catalysts, known as Enzymes, determine the rates of Chemical Reactions occurring within the cell. Enzymes are localized in various Regions of the cell; some are dissolved or suspended in the Cytoplasm, thereby being uniformly distributed throughout the cell volume. Other Enzymes are membrane-bound or exist in association with other substances, forming supramolecular aggregates. Certain membrane-bound proteins, called permeases, facilitate The transport of specific nutrients into the cell.

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FIG. 2.14. Two main types of protein structures and their variants—fibrillar (a) and globular (b).

Other proteins serve as Structural elements of cell membranes, while A number of proteins perform motor functions. Many unicellular organisms possess small, Hair-like structures called flagella. These flagella move under the action of contractile proteins, thereby enabling the movement of the entire cell. Other filamentous and tubular appendages, known as fimbriae, are involved in initiating the binding of pathogenic Bacteria to susceptible Tissues.

Table 2.6. Various BIOLOGICAL FUNCTIONS OF proteins

Proteins are isolated, purified, and characterized using various Physical and Chemical Methods. Protein Separation Methods (Ch. 11) are based on differences in their molecular properties, which in turn are partly determined by The Nature of their constituent Amino Acids; it is to the latter that we devote the next section.