Glycolysis

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  • 16.1 Glycolysis Is an Energy-Conversion Pathway

  • 16.2 NAD+ Is Regenerated from the Metabolism of Pyruvate

  • 16.3 Fructose and Galactose Are Converted into Glycolytic Intermediates

  • 16.4 The Glycolytic Pathway Is Tightly Controlled

  • 16.5 Metabolism in Context: Glycolysis Helps Pancreatic Beta Cells Sense Glucose

Usain Bolt sprints to a world record in the 200-meter finals at the 2008 Beijing Olympics. Glucose metabolism can generate the ATP to power muscle contraction. During a sprint, when the ATP needs outpace oxygen delivery, as would be the case for Bolt, glucose is metabolized to lactate. When oxygen delivery is adequate, glucose is metabolized more efficiently to carbon dioxide and water.

Earlier, we examined how complex carbohydrates are digested into biochemically useful molecules, such as glucose (Chapter 14). Glucose is the principal carbohydrate in living systems and an important fuel. In mammals, it is the only fuel that the brain uses under nonstarvation conditions and the only fuel that red blood cells can use at all. Indeed, almost all organisms use glucose, and most process it in a similar fashion. Why is glucose such a prominent fuel, rather than some other monosaccharide? We can speculate on the reasons. First, glucose is one of several monosaccharides formed from formaldehyde under prebiotic conditions, and so it may have been available as a fuel source for primitive biochemical systems. Second, glucose is the most stable hexose because the hydroxyl groups and the hydroxymethyl group are all in the equatorial position, minimizing steric clashes. Third, glucose has a low tendency, relative to other monosaccharides, to nonenzymatically glycosylate proteins. In their open-chain forms, monosaccharides contain carbonyl groups that can covalently modify the amino groups of proteins. Such nonspecifically modified proteins often do not function effectively. Glucose has a strong tendency to exist in the ring formation and, consequently, relatively little tendency to modify proteins.

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We start this section with glycolysis, paying special attention to the regulation of this pathway. We proceed to gluconeogenesis, again with a focus on regulation. The section ends with a discussion of the regulation of glycolysis and gluconeogenesis within a cell as well as between tissues.

In this chapter, we first examine how ATP is generated in glycolysis and how ATP can be generated in the absence of oxygen. We then see how sugars other than glucose are converted into glycolytic intermediates. The chapter ends with a discussion of the regulation of glycolysis.