7.6: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways

7.6.1: Connecting Other Sugars to Glucose Metabolism

Learning Objective

Identify the types of sugars involved in glucose metabolism

Key Points

• When blood sugar levels drop, glycogen is broken down into glucose-1-phosphate, which is then converted to glucose-6-phosphate and enters glycolysis for ATP production.
• In the liver, galactose is converted to glucose-6-phosphate in order to enter the glycolytic pathway.
• Fructose is converted into glycogen in the liver and then follows the same pathway as glycogen to enter glycolysis.
• Sucrose is broken down into glucose and fructose; glucose enters the pathway directly while fructose is converted to glycogen.

Key Terms

monosaccharide

A simple sugar such as glucose, fructose, or deoxyribose that has a single ring.

glycogen

A polysaccharide that is the main form of carbohydrate storage in animals; converted to glucose as needed.

disaccharide

A sugar, such as sucrose, maltose, or lactose, consisting of two monosaccharides combined together.

You have learned about the catabolism of glucose, which provides energy to living cells. But living things consume more than glucose for food. How does a turkey sandwich end up as ATP in 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 intermediates leave for other pathways. These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways. Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways.

Glycogen Pathway

Glycogen from the liver and muscles, hydrolyzed into glucose-1-phosphate, together with fats and proteins, can feed into the catabolic pathways for carbohydrates.

Glycogen, a polymer of glucose, is an energy-storage molecule in animals. When there is adequate ATP present, excess glucose is shunted into glycogen for storage. Glycogen is made and stored in both the liver and muscles. The glycogen is hydrolyzed into the glucose monomer, glucose-1-phosphate (G-1-P), if blood sugar levels drop. The presence of glycogen as a source of glucose allows ATP to be produced for a longer period of time during exercise. Glycogen is broken down into G-1-P and converted into glucose-6-phosphate (G-6-P) in both muscle and liver cells; this product enters the glycolytic pathway.

Glycogen Structure

Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain around 30,000 glucose units.

Galactose is the sugar in milk. Infants have an enzyme in the small intestine that metabolizes lactose to galactose and glucose. In areas where milk products are regularly consumed, adults have also evolved this enzyme. Galactose is converted in the liver to G-6-P and can thus enter the glycolytic pathway. 

Fructose is one of the three dietary monosaccharides (along with glucose and galactose) which are absorbed directly into the bloodstream during digestion. Fructose is absorbed from the small intestine and then passes to the liver to be metabolized, primarily to glycogen. The catabolism of both fructose and galactose produces the same number of ATP molecules as glucose.

Fructose Metabolism

Although the metabolism of fructose and glucose share many of the same intermediate structures, they have very different metabolic fates in human metabolism.

Sucrose is a disaccharide with a molecule of glucose and a molecule of fructose bonded together with a glycosidic linkage. The catabolism of sucrose breaks it down to monomers of glucose and fructose. The glucose can directly enter the glycolytic pathway while fructose must first be converted to glycogen, which can be broken down to G-1-P and enter the glycolytic pathway as described above.

7.6.2: Connecting Proteins to Glucose Metabolism

Learning Objective

Describe the role played by proteins in glucose metabolism

Key Points

• Amino acids must be deaminated before entering any of the pathways of glucose catabolism: the amino group is converted to ammonia, which is used by the liver in the synthesis of urea.
• Deaminated amino acids can be converted into pyruvate, acetyl CoA, or some components of the citric acid cycle to enter the pathways of glucose catabolism.
• Several amino acids can enter the glucose catabolism pathways at multiple locations.

Key Terms

catabolism

Destructive metabolism, usually including the release of energy and breakdown of materials.

keto acid

Any carboxylic acid that also contains a ketone group.

deamination

The removal of an amino group from a compound.

Metabolic pathways should be thought of as porous; that is, substances enter from other pathways and intermediates leave for other pathways. These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways. Proteins are a good example of this phenomenon. They can be broken down into their constituent amino acids and used at various steps of the pathway of glucose catabolism.

Proteins are hydrolyzed by a variety of enzymes in cells. Most of the time, the amino acids are recycled into the synthesis of new proteins or are used as precursors in the synthesis of other important biological molecules, such as hormones, nucleotides, or neurotransmitters. However, if there are excess amino acids, or if the body is in a state of starvation, some amino acids will be shunted into the pathways of glucose catabolism.

Connection of Amino Acids to Glucose Metabolism Pathways

The carbon skeletons of certain amino acids (indicated in boxes) are derived from proteins and can feed into pyruvate, acetyl CoA, and the citric acid cycle.

Each amino acid must have its amino group removed (deamination) prior to the carbon chain's entry into these pathways. When the amino group is removed from an amino acid, it is converted into ammonia through the urea cycle. The remaining atoms of the amino acid result in a keto acid: a carbon chain with one ketone and one carboxylic acid group. In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the principal waste product in mammals produced from the nitrogen originating in amino acids; it leaves the body in urine. The keto acid can then enter the citric acid cycle.

When deaminated, amino acids can enter the pathways of glucose metabolism as pyruvate, acetyl CoA, or several components of the citric acid cycle. For example, deaminated asparagine and aspartate are converted into oxaloacetate and enter glucose catabolism in the citric acid cycle. Deaminated amino acids can also be converted into another intermediate molecule before entering the pathways. Several amino acids can enter glucose catabolism at multiple locations.

7.6.3: Connecting Lipids to Glucose Metabolism

Learning Objective

Explain the connection of lipids to glucose metabolism

Key Points

• Many types of lipids exist, but cholesterol and triglycerides are the lipids that enter the pathways of glucose catabolism.
• Through the process of phosphorylation, glycerol can be converted to glycerol-3-phosphate during the glycolytic pathway.
• When fatty acids are broken down into acetyl groups through beta-oxidation, the acetyl groups are used by CoA to form acetyl-CoA, which enters the citric acid cycle to produce ATP.
• Beta-oxidation produces FADH₂ and NADH, which are used by the electron transport chain for ATP production.

Key Terms

lipid

A group of organic compounds including fats, oils, waxes, sterols, and triglycerides; characterized by being insoluble in water; account for most of the fat present in the human body.

beta-oxidation

A process that takes place in the matrix of the mitochondria and catabolizes fatty acids by converting them to acetyl groups while producing NADH and FADH2.

Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways. The lipids that are connected to the glucose pathways are cholesterol and triglycerides. 

Cholesterol

Cholesterol contributes to cell membrane flexibility and is a precursor to steroid hormones. The synthesis of cholesterol starts with acetyl groups, which are transferred from acetyl CoA, and proceeds in only one direction; the process cannot be reversed. Thus, synthesis of cholesterol requires an intermediate of glucose metabolism.

Triglycerides

Triglycerides, a form of long-term energy storage in animals, 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 to glycerol-3-phosphate, which continues through glycolysis. 

Fatty acids are catabolized in a process called beta-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups, while producing NADH and FADH₂. The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle as it combines with oxaloacetate. The NADH and FADH₂ are then used by the electron transport chain.

Attributions

• Connecting Other Sugars to Glucose Metabolism
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "glycogen." http://en.wiktionary.org/wiki/glycogen. Wiktionary CC BY-SA 3.0.
  • "disaccharide." http://en.wiktionary.org/wiki/disaccharide. Wiktionary CC BY-SA 3.0.
  • "monosaccharide." http://en.wiktionary.org/wiki/monosaccharide. Wiktionary CC BY-SA 3.0.
  • "Medical Physiology/Basic Biochemistry/Sugars." http://en.wikibooks.org/wiki/Medical_Physiology/Basic_Biochemistry/Sugars%23Galactose_Metabolism. Wikibooks CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44441/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "Medical Physiology/Basic Biochemistry/Sugars." http://en.wikibooks.org/wiki/Medical_Physiology/Basic_Biochemistry/Sugars%23Galactose_Metabolism. Wikibooks CC BY-SA 3.0.
  • "Glycogen Structure." https://en.wikipedia.org/wiki/Glycogen%23mediaviewer/File:Glycogen_structure.svg. Wikipedia Public domain.
  • "Glycogen pathway.jpg." https://cnx.org/contents/ZP457F64@7/Connections-of-Carbohydrate-Pr. OpenStax CNX CC BY-SA 3.0.
• Connecting Proteins to Glucose Metabolism
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "catabolism." http://en.wiktionary.org/wiki/catabolism. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44441/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "deamination." http://en.wiktionary.org/wiki/deamination. Wiktionary CC BY-SA 3.0.
  • "keto acid." http://en.wiktionary.org/wiki/keto_acid. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways. October 16, 2013." http://cnx.org/content/m44441/latest/Figure_07_06_01.jpg. OpenStax CNX CC BY 3.0.
• Connecting Lipids to Glucose Metabolism
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "Metabolomics/Metabolites/Lipids/Energy Storage." http://en.wikibooks.org/wiki/Metabolomics/Metabolites/Lipids/Energy_Storage. Wikibooks CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44441/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "lipid." http://en.wiktionary.org/wiki/lipid. Wiktionary CC BY-SA 3.0.