Introduction

• Carbohydrate metabolism is the major source of energy for all living cells.
• Glucose is the central molecule in carbohydrate metabolism and actively participates in all metabolic pathways.
• In carbohydrate metabolism, carbohydrates are broken down into simpler compounds or common intermediates and released free energy.
• The released free energy is conserved in the form of ATP & NADPH. These ATP or NADPH are utilized in the biosynthetic reaction.
• Carbohydrates are converted into acetyl CoA, whose Acetyl group is then converted to CO2 and H2O through the action of the Krebs cycle and oxidative phosphorylation.

Pathway for carbohydrate metabolism

1. Glycolysis

Other Name- E.M.P. pathway (Embden, Meyerhof, Parnas pathway)

Position in cell- glycolysis occurs in the cytoplasm of cells.

• Glycolysis is the major pathway for ATP Synthesis.
• Glycolysis is an oxidative process in which one mole of glucose is partially oxidized into 2 moles of pyruvate.
• Glycolysis occurs in the absence of Oxygen it’s called an anaerobic condition, in there the end product is lactate.

2. Citric acid cycle 

Other Names – Krebs cycle, tricarboxylic acid cycle(TCA cycle)

• Krebs cycle is the common oxidative pathway for carbohydrates, fats, or amino acids through acetyl CoA
• Position in cell- TCA cycle takes place in mitochondria
• TCA cycle involves the oxidation of acetyl CoA to CO2 and H2O

3. Gluconeogenesis

• Gluconeogenesis is a universal pathway found in all animals, plants, fungi, and microorganisms.
• Gluconeogenesis occurs in the liver and to a smaller extent, in the kidney and to a smaller extent, in the kidney cortex.
• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors.
• Lactate, pyruvate, glycerol, and citric acid cycle intermediates are the precursor of gluconeogenesis.

4. Glycogenesis

• The formation of glycogen from glucose is glycogenesis.
• Location in cell- glycogenesis takes place in the cytosol of the liver and skeletal muscles, it requires ATP and UTP.
• Glycogen serves as a fuel reserve for the supply of ATP during muscle contraction.

5. Glycogenolysis

• The degradation of glycogen to glucose is called Glycogenolysis.
• Location in cell– degradation of glycogen occurs in the liver and muscle constituted for fulfilling energy requirements.
• The degradation of glycogen is not reversible.

6. Pentose phosphate pathway

• Other Names- it is also called Hexose monophosphate pathway (HMP shunt), phosphogluconate pathway
• This is an alternative pathway to glycolysis and the TCA cycle for the Oxidation of glucose.
• Location in cell– The enzymes of the HMP shunt are located in the cytosol.
• In the pentose phosphate pathway, there occurs the production of five-carbon sugars, including ribose, produced from glucose.
• In the HMP pathway, 12 molecules of NADPH are produced.

7. Uronic acid pathway

• This is an alternative pathway for glucose, also known as a glucuronic acid pathway.
• Uronic acid pathway occurs in the liver.
• It is involved in the synthesis of glucuronic acid, pentoses, vitamins, and ascorbic acid.

Significance of carbohydrate metabolism

Carbohydrate metabolism is very important for producing energy and all biochemical processes. We can easily understand the importance of carbohydrate metabolism through the significance of their pathways.

•  Significance of Glycogenolysis

Glycolysis is the major source of metabolic energy for mammalian tissue such as the renal medulla and brain.

• Significance of the TCA cycle

TCA cycles serve as the mechanism by which much of the free energy liberated during the Oxidation of carbohydrates, lipids, and amino acids is made available.

• Significance of pentose phosphate pathway

In the HMP shunt, hexoses are converted into pentoses. These pentoses are useful for the synthesis of nucleic acid and nucleosides. In there NADPH is produced which is used for the reductive biosynthesis of steroids and fatty acids.

• Significance of gluconeogenesis

Gluconeogenesis in the liver and kidney provides glucose for use by the brain, muscles, and erythrocytes.

Disease-associated with improper carbohydrate metabolism

Diseases that associate with improper carbohydrate metabolism-

1. Hypoglycemia

• Hypoglycemia is abnormally low levels of glucose in the blood.
• Decrease blood glucose level causes symptoms of sweating, shakiness, fatigue, weakness, and inability to think clearly.

2. Diabetes mellitus

• Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia.
• In this disorder, blood sugar levels are abnormally high. Resulting in disturbances of carbohydrate, fat, and protein metabolism and long-term damage, dysfunction, and failure of different organs.

3. Galactosemia

Deficiency of Galactokinase enzyme and Epimerase enzyme. Symptoms are Jaundice, hepatomegaly, seizures, and diarrhea.

4. Hereditary fructose intolerance

Aldolase B deficiency. Symptoms appear on intake of fructose. Other symptoms are Hypoglycemia, Jaundice, Hepatomegaly, vomiting

5. Some glycogen storage diseases

• Von gierke’s disease (type-1 glycogenesis)

Glycogen accumulates in hepatocytes and renal cells, enlarged liver, and kidney, and fasting hypoglycemia are some of the symptoms.

• Pompe’s disease

Symptoms are glycogen accumulates in lysosomes in almost all the tissues, the heart is mostly involved, and the nervous system is also affected.

• Tarui’s disease

Muscle cramps due to exercise, blood lactate not elevated, and hemolysis occurs are the symptoms.

• Her’s disease

Symptoms– liver enlarged, liver glycogen can not form glucose, mild Hypoglycemia, and ketosis seen.

Summary of all pathways

• Glycolysis is the first stage of glucose metabolism. In there, six-carbon sugar glucose is converted into 3 carbon molecule pyruvate.
• This pyruvate is oxidized into acetyl CoA and enters into the TCA cycle which then produces CO2 and H2O and ATP.
• Under anaerobic conditions pyruvate converts into lactic acid this is called lactic acid fermentation.
• When glucose levels are down, stored glucose does not have sufficient gluconeogenesis occurs in their pyruvate or other 3 carbon compounds produced glucose.
• In glycogenesis, glucose is stored in the form of glycogen in the liver and muscles.
• The degradation of glycogen to glucose is called Glycogenolysis.
• HMP shunt is the alternative pathway for the Oxidation of glucose. In their hexose Sugar produced pentose sugar, this sugar used in the synthesis of nucleotides and nucleic acid.
• In the Uronic pathway, glucuronic acid, pentoses, vitamins, and ascorbic acid are produced.
• All pathways are very important for all biosynthesis processes and generate energy.

Q&A

1. When carbohydrates are metabolized without oxygen is called?

Carbohydrates metabolized without oxygen are called lactic acid fermentation. In anaerobic conditions, pyruvate converts into lactic acid to fulfill energy needs.

2. Which electrolyte is responsible for carbohydrate metabolism?

Phosphorus is responsible for carbohydrate metabolism, it is an essential electrolyte. Potassium is also an essential electrolyte for carbohydrate metabolism.

3. What is carbohydrate metabolism?

Carbohydrate metabolism is a fundamental biochemical process in all living organisms that provides energy in the form of ATP or NADPH for other biosynthetic processes. 

4. Which metabolic pathway produces carbohydrates?

Carbohydrates are produced in the pentose phosphate pathway,  in there pentose sugar is produced into glucose, and in gluconeogenesis glucose is formed from non-carbohydrates precursor.

Written By: Richa Pachori

References

• Biochemical Pathways: An Atlas of Biochemistry and molecular biology. 
• 4th edition biochemistry Donald Voet & Judith G. Voet.
• Lehninger, 4th edition, Principal of biochemistry, David L. Nelson & Michael.
• Biochemistry, 4th edition, U. Satyanarayana, U. Chakrapani.