5.7 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways

Learning Objectives

By the end of this section, you will be able to:

  • Discuss the way in which carbohydrate metabolic pathways, glycolysis, and the citric acid cycle interrelate with protein and lipid metabolic pathways
  • Explain why metabolic pathways are not considered closed systems

You have learned about the catabolism of glucose, which provides energy to living cells. But living things consume more than just glucose for food. How does a turkey sandwich, which contains protein, provide energy to your cells? This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways.  Metabolic pathways should be thought of as porous—that is, substances enter from other pathways, and other substances leave for other pathways. These pathways are not closed systems. Many of the products in a particular pathway are reactants in other pathways.

Connections of Other Sugars to Glucose Metabolism

Glycogen, a polymer of glucose, is a short-term energy storage molecule in animals. When there is adequate ATP present, excess glucose is converted into glycogen for storage. Glycogen is made and stored in the liver and muscle. Glycogen will be taken out of storage if blood sugar levels drop. The presence of glycogen in muscle cells as a source of glucose allows ATP to be produced for a longer time during exercise. Glycogen in muscle and liver cells is mobilized as glucose-1-phosphate and isomerized to glucose-6-phosphate for glycolysis.

Sucrose is a disaccharide made from glucose and fructose bonded together. Sucrose is broken down in the small intestine, and the glucose and fructose are absorbed separately. Fructose is one of the three dietary monosaccharides, along with glucose and galactose (which is part of milk sugar, the disaccharide lactose), that are absorbed directly into the bloodstream during digestion. The catabolism of both fructose and galactose produces the same number of ATP molecules as glucose.

Connections of Proteins to Glucose Metabolism

Proteins are broken down by a variety of enzymes in cells. Most of the time, amino acids are recycled into new proteins. If there are excess amino acids, however, or if the body is in a state of famine, some amino acids will be shunted into pathways of glucose catabolism (Figure 5.23). Each amino acid must have its amino group removed prior to entry into these pathways. The amino group is converted into ammonia. In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the principal waste product in mammals from the nitrogen originating in amino acids, and it leaves the body in urine.

This illustration shows that the amino acids alanine, glycine, threonine, cysteine, and serine can be converted into pyruvate. Leucine, lysine, phenylalanine, tyrosine, tryptophan, and isoleucine can be converted into acetyl upper case C lower case o upper case A. Arginine, proline, histidine, glutamine, and glutamate can be converted into alpha-ketoglutarate. Isoleucine, valine, methionine, and threonine can be converted into succinyl upper C lower o upper A. Tyrosine and phenylalanine can be converted into fumarate, and aspartate and asparagine can be converted into oxaloacetate.
Figure 5.23 This illustration shows that the amino acids alanine, glycine, threonine, cysteine, and serine, and tryptophan can be converted into pyruvate. Leucine, lysine, phenylalanine, tyrosine, tryptophan, and isoleucine can be converted into acetyl CoA. Arginine, proline, histidine, glutamine, and glutamate can be converted into alpha-ketoglutarate. Isoleucine, valine, methionine, and threonine can be converted into succinyl CoA. Tyrosine and phenylalanine can be converted into fumarate, and aspartate and asparagine can be converted into oxaloacetate. (Credit: modification of work by Mikael Häggström)

Connections of Lipids to Glucose Metabolism

The lipids that are connected to the glucose pathways are cholesterol and triglycerides. Cholesterol is a lipid that contributes to cell membrane flexibility and is a precursor of steroid hormones. The synthesis of cholesterol starts with acetyl CoA and proceeds in only one direction. The process cannot be reversed, and ATP is not produced.

Triglycerides are a form of long-term energy storage in animals. Triglycerides store about twice as much energy as carbohydrates. Triglycerides are made of glycerol and three fatty acids. Animals can make most of the fatty acids they need. Triglycerides can be both made and broken down through parts of the glucose catabolism pathways. Glycerol can be phosphorylated and proceeds through glycolysis. Fatty acids are broken into two-carbon units by beta-oxidation that enter the citric acid cycle.

Section Summary

The breakdown and synthesis of carbohydrates, proteins, and lipids connect with the pathways of glucose catabolism (Figure 5.24). The carbohydrates that can also feed into glucose catabolism include galactose, fructose, and glycogen. These connect with glycolysis. The amino acids from proteins connect with glucose catabolism through pyruvate, acetyl CoA, and components of the citric acid cycle. Cholesterol synthesis starts with acetyl CoA, and the components of triglycerides are picked up by acetyl CoA and enter the citric acid cycle.

This illustration shows that glycogen, fats, and proteins can be catabolized via aerobic respiration. Glycogen is broken down into glucose, which feeds into glycolysis. Fats are broken down into glycerol, which is processed by glycolysis, and fatty acids, which are converted into acetyl CoA. Proteins are broken down into amino acids, which are processed at various stages of aerobic respiration, including glycolysis, acetyl CoA formation, and the citric acid cycle.
Figure 5.24 Glycogen from the liver and muscles, together with fats, can feed into the catabolic pathways for carbohydrates.

Exercises

Media Attribution

  • Figure 5.23 modification of work by Mikael Häggström

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Introduction to Biology Copyright © 2023 by Natasha Ramroop Singh, PhD is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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