Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Glycolysis: The Central Pathway of Glucose Catabolism
Glycolysis is a central metabolic pathway in the majority of organisms

Now that we have explored the principles underlying cellular METABOLISM and Bioenergetics, we can examine how the chemical energy stored within the glucose molecule is released in a usable form to drive various biological activities of The Cell. Recall that Glucose serves as the primary fuel for most organisms, that it is energy-rich, and that its reserves, stored as Glycogen, can be rapidly mobilized whenever the Organism experiences a sudden demand for energy.

In this chapter, we focus on Glycolysis, a pathway in which a six-carbon glucose molecule is enzymatically broken down through ten sequential reactions into two three-carbon Pyruvate molecules. Throughout this sequence of reactions, a significant portion of The energy released from glucose is conserved in the form of ATP. Glycolysis (from the Greek glykys, meaning sweet, and lysis, meaning rupture or decomposition) is better understood than any other central metabolic pathway, and we will therefore examine it in considerable detail; the mechanisms and regulation of this process illustrate General Principles characteristic of all metabolic pathways. We will also discuss the pathways that feed into glycolysis, namely those leading from glycogen, Disaccharides, and Monosaccharides.

Glycolysis is nearly universal as a central pathway of Glucose Catabolism, playing this role not only in animal and plant Cells but also in many microorganisms. The sequence of glycolytic reactions varies among organisms only in The regulation of its rate and the metabolic fate of the resulting pyruvate.

The product of glycolysis—pyruvate—can be utilized in three different ways. In aerobic organisms, glycolysis constitutes merely the initial stage of the complete aerobic breakdown of glucose to CO₂ and Water (Fig. 15-1). The pyruvate generated during glycolysis then undergoes oxidative decarboxylation, meaning it loses CO₂, and the remaining two-carbon fragment, in the form of an acetyl group, is incorporated into acetyl-CoA (see Fig. 10-8). Subsequently, this acetyl group is fully oxidized to CO₂ and H₂O in The Citric Acid Cycle with the participation of molecular oxygen (Fig. 15-1). This is the pathway followed by pyruvate in aerobic animal and plant cells.

The second pathway involves the reduction of pyruvate to lactate. When certain animal Tissues must function under anaerobic conditions—which is particularly typical, for instance, of vigorously exercising Skeletal Muscle—the pyruvate derived from glucose cannot undergo further oxidation simply due to a lack of oxygen. Under these conditions, the glycolytic product pyruvate is reduced to form lactate. In skeletal muscle, this process, known as anaerobic glycolysis, serves as an essential source of ATP energy during intense physical exertion. In anaerobic microorganisms that carry out Lactic acid Fermentation, lactate is also the end product of glycolysis (Fig. 15-1). Lactic acid, produced from sugar through the action of lactic acid Bacteria, causes milk to sour, and this same acid gives sauerkraut its characteristic mildly tart flavor.

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Fig. 15-1. The end product of glycolysis, pyruvate, follows various Catabolic pathways depending on the organism and the metabolic conditions.

The third pathway of pyruvate metabolism culminates in The formation of ethanol. Certain microorganisms (such as brewer's Yeast) convert the pyruvate produced from glucose during glycolysis into ethanol and CO₂. This process is known as Alcoholic Fermentation (Fig. 15-1). Fermentation is a general term denoting the anaerobic breakdown of glucose or other organic nutrients to harvest energy in the form of ATP. Different Types of fermentation yield distinct products characteristic of the organisms performing them. Because the first living organisms appeared on Earth when the atmosphere was still devoid of oxygen, the anaerobic breakdown of glucose is considered the most ancient biological mechanism for extracting energy from organic foodstuffs.