Chapter 20 Introduction to Metabolic Pathways

The metabolic activities of cells are dictated by two major concerns:

1. The cell has to synthesize its macromolecules. Proteins have to be synthesized from amino acids; complex carbohydrates from monosaccharides; membrane lipids from fatty acids and other building blocks; and nucleic acids from nucleotides. These biosynthetic processes are called anabolic. They require metabolic energy, usually in the form of adenosine triphosphate (ATP).

2. The cell has to generate metabolic energy. Nutrients must be oxidized to supply energy in the form of ATP for biosynthesis, active membrane transport, cell motility, and muscle contraction. Degradative processes are called catabolic. Most ATP is produced during the end-oxidation of metabolic intermediates to CO₂ and H₂O in the mitochondria (pathway

in Fig. 20.1).

Figure 20.1 Major types of metabolic activity in the cell. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; CoA, coenzyme A; P_i, inorganic phosphate.

Therefore an unsuspecting nutrient molecule entering a cell has two alternative fates: either it becomes incorporated into a cellular macromolecule or it is oxidized for the generation of ATP.

The biosynthesis of cellular macromolecules has to be balanced by their degradation (pathway in Fig. 20.1). Under steady-state conditions, the rates of synthesis and degradation are balanced and the amount of the macromolecule remains constant. Different nutrients can also be interconverted through metabolic intermediates. For example, most amino acids can be converted to glucose, and glucose can be converted to amino acids and fatty acids (pathways and in Fig. 20.1).