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Introduction

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High-Yield Terms

  • Hexokinase/Glucokinase: glucose phosphorylating enzymes, differential tissue expression and regulatory properties, humans express 4 distinct hexokinase/glucokinase genes

  • PFK1: 6-phosphofructo-1-kinase, major rate-limiting enzyme of glycolysis

  • PFK2: bifunctional enzyme that is responsible for the synthesis of the major allosteric regulator of glycolysis via PFK1 and gluconeogenesis via fructose-1,6-bisphophatase (F-1,6-BPase)

  • Pyruvate kinase: multiple forms with tissue-specific distribution and regulation

  • PKM2: isoform of pyruvate kinase expressed in proliferating and cancer cells, participates in the Warburg effect

  • Substrate-level phosphorylation: refers to the formation of ATP via the release of energy from a catabolic substrate as opposed to via oxidative phosphorylation

  • Glucose-fatty acid cycle: describes the interrelationship between how fatty acid metabolism results in inhibition of glucose metabolism and vice versa

  • Intestinal glucose homeostasis: in addition to regulating glucose uptake from the diet and delivery to the blood, in times of fasting or compromised liver function, the small intestine provides up to 20% of blood glucose via gluconeogenesis using glutamine and glycerol as substrates

  • Renal glucose homeostasis: kidneys regulate circulating glucose levels through efficient resorption of plasma glucose as well as by being able to carry out gluconeogenesis using glutamine as a carbon source

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Importance of Glycolysis

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Glycolysis represents a major metabolic pathway for the conversion of the carbons of carbohydrates into other forms of biomass and for the production of cellular energy in the form of ATP. The physiologically significant property of glycolysis is that the pathway can provide cellular energy whether or not oxygen is present, as discussed later. All tissues have varying needs for the glycolytic pathway with the brain being particularly dependent upon glycolysis for energy production. Red blood cells, which lack mitochondria, are totally dependent upon glucose oxidation in glycolysis for their energy needs. In the context of glycolysis, the major carbohydrate entering the pathway is glucose. However, other carbohydrates such as fructose (see Chapter 11) and galactose (see Chapter 12) are utilized for energy and biomass production by being oxidized within the glycolytic pathway. Entry of carbohydrates into glycolysis can occur either from dietary sources, which can include a wide variety of mono-, di-, and polysaccharides, or from carbohydrate stores in the form of glycogen (see Chapter 14).

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Digestion and Uptake of Dietary Carbohydrate

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The details of digestive processes are discussed in Chapter 43. Digestion and absorption of carbohydrates is covered here briefly. Dietary carbohydrates enter the body in complex forms, such as mono-, di-, and polysaccharides. Through the actions of various digestive enzymes, these complex sugars are broken down into monosaccharides consisting primarily of glucose, fructose, and galactose. Intestinal absorption of carbohydrates occurs via passive diffusion, facilitated diffusion, and active transport. The primary transporter involved in the uptake of glucose is the sodium-glucose transporter 1 (SGLT1). Galactose is also absorbed from the gut via the action of SGLT1. Fructose is absorbed from the intestine via GLUT5 uptake. Indeed, GLUT5 has a much ...

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