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

Chemical Foundations of Life

Any Organism must synthesize all the chemical compounds necessary for cellular maintenance and reproduction. In the following chapters, we will examine the kinetics, energetics, and Regulation of the principal biochemical pathways involved in these syntheses. However, before proceeding to these topics, it is essential to become familiar with the reactants, reaction products, catalysts, and chemical regulators that participate in the complex network of chemical transformations occurring within The Cell.

This chapter focuses primarily on the predominant macromolecular compounds found in the cell, as well as the corresponding small monomeric molecules from which these polymers are constructed. Cellular polymers are divided into four main classes: fats and Lipids; Polysaccharides (Cellulose, starch, etc.); informational macromolecules—polydeoxyribonucleic and polyribonucleic acids (DNA, RNA); and Proteins. The PHYSICOCHEMICAL PROPERTIES OF these compounds are crucial both for understanding cellular Functions and for the rational design of technological processes involving living Cells.

Depending on their Structure, biological polymers are suitably classified into homopolymers and copolymers. Biological homopolymers are built from monomer units of a single type; consequently, polymers containing single-type monomer units differ from one another primarily in molecular weight and the degree of polymer chain branching. The primary function of homopolymers in the cell is to provide structural elements possessing the requisite mechanical strength, chemical inertness, and adequate permeability. In addition, homopolymers frequently serve as energy and nutrient reserves within cells; for example, a 1 M glucose solution can be stored as Glycogen—a characteristic reserve polysaccharide of the cell that reduces the molar concentration of the solution 10,000-fold or even more. Polymers provide a particularly convenient form for nutrient storage whenever a cell needs to accumulate reserves without significantly altering intracellular osmotic pressure.

Copolymers are built from several different monomer units, the number of which can reach up to 20. Each such polymer has a defined molecular weight and a characteristic monomer composition; furthermore, the monomer residues are linked in a strictly defined, genetically programmed sequence.

Class="center">Table 2.1. Elemental Composition of E. coli

^a Data from: Luria S. E., in The Bacteria, Gunsalus I. C., Stanier R. Y. (eds.), vol. 1, chap. 1, Academic Press, Inc., New York, 1960.

The elemental composition of E. coli presented in Table 2.1 illustrates the set of chemical elements from which Biopolymers are constructed. The predominant elements (hydrogen, oxygen, nitrogen, and carbon) form chemical bonds by completing their outer electron shells with one, two, three, and four electrons, respectively. These are the lightest elements in the periodic table; with the exception of hydrogen, all of them are also capable of forming multiple chemical bonds. A vast multitude of compounds—including all four Major Classes of biopolymers—is built from these elements, and to a lesser extent from phosphorus and sulfur.

Biochemical compounds constructed from these elements are characterized not only by great diversity, but also by high stability; as a rule, they react very slowly with one another, with Water, or with other cellular components. Chemical Reactions Involving these compounds are accelerated by biological catalysts—specialized proteins called Enzymes (recall that a catalyst is defined as a substance that increases The rate of a reaction without undergoing permanent chemical change itself). Thus, the cell can regulate both The Nature and the rate of its chemical reactions by altering enzyme concentrations. The details of these regulatory mechanisms will be discussed in Chapter 6.

Phosphorus and sulfur are constituents of the organic matter of All living organisms, albeit in relatively small amounts. Sodium, potassium, magnesium, calcium, and chloride ions are likewise invariably present in cells, while trace amounts of manganese, iron, cobalt, copper, and zinc are essential for the activation of specific enzymes. Minute quantities of boron, aluminum, vanadium, molybdenum, iodine, silicon, fluorine, and tin are also required for the normal physiological function of certain organisms. In general, at least 24 different chemical elements are essential for life.

Cells inhabit an aqueous environment. Water exhibits A number of highly unusual properties—a high heat of vaporization, a high dielectric constant, The ability to dissociate into acids and bases, and a strong tendency to form Hydrogen Bonds—making it an exceptionally important reactant in numerous enzyme-catalyzed reactions. Furthermore, the properties manifested by biopolymers depend to a large extent on the CHARACTERISTICS OF THE solvent in which they are dissolved; many Separation processes, in particular, rely on this principle. The interactions of water with other common cellular components are discussed by Blum [12].