Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Biosynthesis of Carbohydrates in Animal Tissues

We have now reached a turning point in our Structure/133.html">Discussion of cellular METABOLISM. Up to this point, we have seen how The major types of nutrients—CARBOHYDRATES, Fatty acids, and Amino Acids—are degraded and channeled through converging Catabolic pathways into The Citric Acid Cycle to deliver their energy-rich electrons to the Respiratory Chain. As these electrons flow along the respiratory chain to oxygen, they provide the energy required for ATP synthesis. We now turn our attention to anabolic pathways. In these pathways, chemical energy in the form of ATP and NADPH is used to synthesize cellular components from simple precursors. Catabolism and anabolism occur simultaneously, maintaining a dynamic steady state in which The breakdown of cellular components, which supplies The Cell with energy, is balanced by the biosynthetic processes that create and preserve the inherent Organization of living Cells.

At this point, it is appropriate to recall (Chapter 10) and re-emphasize several fundamental organizational principles of Biosynthesis.

1. The biosynthetic pathways and degradation pathways for specific Biomolecules are generally not identical. While these pathways may share a single reversible reaction or even several such reactions, they always differ by at least one enzymatic step. If Catabolic and anabolic reactions were catalyzed by the exact same set of reversible Enzymes, no biological structure, regardless of its complexity, could possibly exist, because the population of cellular macromolecules would fluctuate wildly in response to any shift in the concentrations of precursor molecules.

2. Biosynthetic pathways and their corresponding catabolic pathways are controlled by distinct regulatory enzymes. Typically, The regulation of corresponding biosynthetic and catabolic pathways is coordinated and reciprocal, such that The stimulation of a biosynthetic pathway is accompanied by the suppression of the catabolic pathway, and vice versa. Furthermore, biosynthetic pathways are usually regulated at one of their earliest steps. This spares the cell from wasteful expenditure, preventing it from investing precursors into intermediates that it does not currently need. This example once again demonstrates that THE PRINCIPLE OF economy lies at The Heart of the molecular logic of living cells.

3. Energy-requiring biosynthetic processes are invariably coupled with energy-yielding ATP Cleavage, making the overall process virtually irreversible, much like catabolism as a whole. Consequently, the total amount of ATP (or NADPH) energy consumed in a given biosynthetic pathway always exceeds the theoretical minimum of Free energy required to convert the precursor into the biosynthetic product.

We begin our study of biosynthetic processes with the central biosynthetic pathway that leads to The formation of various carbohydrates from non-carbohydrate precursors in animal Tissues. In all higher animals, The biosynthesis of D-glucose is an absolute necessity, because Blood D-Glucose serves as the sole or primary fuel source for The Nervous system (including the Brain), as well as for the Kidneys, Testes, erythrocytes, and all embryonic tissues. In humans, the brain alone consumes more than 120 g of glucose per day. In the animal body, D-glucose is continuously synthesized through strictly regulated reactions from simpler precursors, such as Pyruvate and Certain amino acids, and is subsequently released into the bloodstream. Other important carbohydrates are also synthesized from non-carbohydrate precursors (Fig. 20-1). Of particular significance is Glycogen biosynthesis, which takes place in The Liver and Muscles. Liver glycogen acts as a glucose reservoir, supplying glucose to the bloodstream. Muscle glycogen, broken down during Glycolysis, serves as an ATP energy source for Muscle contraction. In animals, the formation of D-glucose from non-carbohydrate precursors is known as Gluconeogenesis (the generation of "new" sugar). Important precursors of D-glucose include lactate, pyruvate, glycerol, Most amino acids, and Intermediates of the citric acid cycle (Fig. 20-1). In animals, gluconeogenesis occurs primarily in the liver and to a significantly lesser extent in the renal cortex.

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Fig. 20-1. The pathway leading from phosphoenolpyruvate to glucose-6-phosphate is common to The conversion of numerous precursors into various carbohydrates in animal tissues.

We know that in the plant kingdom, massive quantities of glucose and other carbohydrates are produced via the reduction of carbon dioxide during Photosynthesis (Chapter 23). Unlike plants, animals are incapable of the net conversion of $\text{CO}_2$ into new glucose molecules.