Carbohydrates and Energy Metabolism Glucose

The breakdown of glucose can be divided into two major parts: the anaerobic conversion of glucose to pyruvate, known as glycolysis, and the aerobic breakdown of pyruvate to carbon dioxide and water, which involves the tricarboxylic acid cycle and the electron transport chain.

Glycolysis is the series of enzymatic steps leading to the breakdown of one molecule of glucose to produce two molecules of pyruvate (Figure 1). Gly-colysis occurs in the cytosol of different cells, and all human cells are capable of carrying out this process.

However, most glycolysis occurs in the liver, muscle, and adipose tissue.

The fate of pyruvate is determined by the cell type and the availability of oxygen. In the absence of oxygen, pyruvate is reduced to lactate in the cytosol. This occurs in the muscles during strenuous exercise, when the demands for energy are high. In cells that do not contain mitochondria, such as the erythro-cytes, the glycolysis pathway is the only mechanism of energy production.

In the presence of oxygen, pyruvate is converted to acetyl coenzyme A (acetyl CoA) in the mitochondria and thus enters the tricarboxylic acid cycle and subsequently the electron transport chain. As a result, pyru-vate is fully oxidized to carbon dioxide and water, and large amounts of energy are produced.

Fructose and Galactose

Fructose and galactose enter the glycolytic pathway through their conversion to intermediate compounds

Galactose

Glucose

Glucose 6-phosphate

Glucose 1-phosphate

Galactose 1-phosphate

Fructose

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Fructose 1-phosphate

Dihydroxy acetone phosphate

Pyruvate

Aerobic conditions

Acetyl CoA

Pyruvate

Aerobic conditions

Anaerobic conditions

Phosphoenol pyruvate

Anaerobic conditions

Lactate

Citric acid cycle

Electron transport chain

Glyceraldehyde 3-phosphate

Glyceraldehyde

2-Phospho glycerate

Bisphospho glycerate

3-Phospho glycerate

Figure 1 Outline of carbohydrate metabolism, including the points of entry of glucose, fructose, and galactose.

(Figure 1). This occurs primarily in the liver, and, as a result, these two monosaccharides are not generally available for uptake by other tissues. The end products of the catalysis of these monosaccharides are similar to glucose; however, when they are absorbed, they do not elicit the same hormonal response as glucose.

In the liver, breakdown of fructose, known as fructolysis, is initiated by the conversion of fructose to fructose 1-phosphate and subsequent hydrolysis to glyceraldehyde and dihydroxyacetone phosphate, a reaction catalyzed by fructose 1-phosphate aldolase. These products of hydrolysis can be used for further glycolytic conversion. Fructolysis in the liver bypasses the highly regulated step of phospho-fructokinase and can produce a large amount of glycolytic metabolites. In the muscle and kidney cells, fructose can enter the glycolysis pathway through its conversion to fructose 6-phosphate, prior to the highly regulated phosphofructokinase step.

In the liver, galactose enters the glycolytic pathway through its phosphorylation to galactose 1-phosphate and subsequent epimerization to glucose 1-phosphate. This metabolic intermediate can either enter glycolysis by its conversion to glucose 6-phosphate or be used in glycogen synthesis, depending on the nutritional state of the organism.

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