Carbohydrate Metabolism

Introduction

Complex carbohydrates are consumed by living organisms and they get converted into simple glucose molecules which involve ten significant biochemical pathways. Carbohydrates are a major source of energy in our diet. They are often involved in the formation of different components of a cell as well as performing various other functions. The term sugar may be defined as Carbohydrates that are readily soluble in water and are sweet in taste. They can be broadly classified into 3 major groups- monosaccharide, disaccharide, and oligosaccharide. The monosaccharides can be further subdivided on the basis of the presence of different functional groups and the number of carbon atoms present in one molecule. Starch in the case of plants and glycogen in the case of animals is the stored form of carbohydrates. They are made up of carbon, hydrogen, and oxygen. They may be ketones or polyhydroxy aldehydes.

Carbohydrate Metabolism

Carbohydrate metabolism occurs in ten significant pathways - glycolysis, tricarboxylic acid cycle, gluconeogenesis, glycogenesis, glycogenolysis, hexose monophosphate shunt pathway, uronic acid pathway, glyoxylate cycle, fructose metabolism, and galactose metabolism.

• Glycolysis − The glycolysis pathway was first established in 1940 by two biochemists Embden and Meyerhof. This is a path-breaking discovery and the pathway is named as E.M pathway after the name of the discoverers. In aerobic conditions, 38 ATP molecules are synthesized along with the production of some amount of heat. While in anaerobic conditions only 2 ATP molecules are produced. In anaerobic conditions, plants produce ethanol and animals produce lactic acid as a by-product.

• Citric acid cycle − In this metabolic pathway oxidation of Acetyl CoA occurs with the production of carbon dioxide and water. It connects almost all metabolic pathways directly or indirectly. It is popularly known as the Tricarboxylic acid cycle. It was proposed by Hans Adolf Krebs in 1937. It is often called the Krebs cycle in the honor of scientist Krebs's outstanding discovery in the field of biochemistry.

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• Gluconeogenesis − Glucose is synthesized from noncarbohydrate precursors like lactate, amino acids, glycerol, etc. It is just the opposite of glycolysis. In glycolysis, there are three irreversible steps that are catalyzed by enzymes hexokinase, phosphofructokinase, and pyruvate kinase. In Gluconeogenesis these are the three steps bypassed by some alternate enzymes.

• Glycogenolysis − In animals, glucose is stored in the form of glycogen in liver and muscle cells. During emergency situations, the muscle glycogen breaks down to form glucose for carrying out different physical activities whereas liver glycogen breaks down to form glucose which in turn breaks down to form blood glucose. The process of degradation of glycogen into glucose is known as glycogenolysis.

• Hexose Monophosphate Shunt Pathway − The HMP pathway deals with the biosynthesis of lipids and nucleic acids and plays a vital role. It is the direct breakdown of glucose that generates NADPH and other pentose sugar for the biosynthesis of nucleic acids. The enzymes for the HMP pathway are in the cytosol. HMP occurs in the liver, adipose tissue, adrenal gland, erythrocytes, testes, and lactating mammary gland. It is popularly known as Pentose Phosphate Pathway (PPP).

• Glycogenesis − It is the biochemical pathway in which glucose is converted to glycogen. Glycogen has ${\underline{\alpha}}$-1,4 and ${\underline{\alpha}}$-1,6 glycosidic bonds and is highly branched in nature. It mainly occurs in the liver, kidney, and skeletal muscles. It utilizes some amount of energy when glucose molecule is attached during the formation of branches.

• Uronic Acid Pathway − The alternate pathway for oxidation of glucose is known as uronic acid pathway. It is involved in the synthesis of glucuronic acid with the utilization of sugar and sugar acids.

• The Glyoxylate Cycle − This pathway is prevalent in plants and microorganisms and involves the biosynthesis of fat to carbohydrates. This pathway plays a key role during the germination of seed in which triglyceride is converted to simple sugars to meet the energy requirement. It occurs in glyoxysomes.

• Fructose Metabolism − In order to enter the glycolysis pathway, fructose has to be phosphorylated. In addition, a phosphate group fructose is converted to fructose-6-phosphate, which is an intermediate of glycolysis pathways. Fructose metabolism occurs in muscles, adipose tissue, and kidney. Fructose-1-phosphate is later converted to glyceraldehyde and di-hydroxy acetone phosphate.

• Galactose Metabolism − Lactose present in milk consists of one glucose and one galactose molecule. After the lysis of lactose, galactose molecule enters the liver for conversion to glucose. Galactokinase phosphorylates galactose to galactose-1-phosphate which is then converted to glucose-1-phosphate and then to glucose-6-phosphate which enters the glycolytic pathway.

Significance

• Carbohydrates are the main source of energy. Carbohydrate metabolism produces NADPH and FADH2. The energy stored in NADPH in turn produces 1.5 to 2.5 molecules of ATP and FADH2 produces 1.5 molecules of ATP. Thus the cofactors produced in carbohydrate metabolism produce ATP which is “energy currency”.

• Hormones like insulin and glucagon maintain blood glucose level that is secreted by the beta cells of the pancreas. The amount of hormones secreted is well maintained by the number of nutrients.

• Carbohydrates are stored in the form of polymers such as chitin, cellulose, starch, and glycogen. The glucose molecules are held together by glycosidic bonds to form long-chain polymers. Chitin and cellulose give mechanical support while starch and glycogen act as storage materials in the case of plants and animals, respectively.

Disorders

Human diseases that are associated with carbohydrate metabolism are as follows:

• Diabetes mellitus is associated with high blood sugar levels. It results in frequent urination, increased thirst, and appetite.

• Lactose intolerance is a condition in which a patient cannot digest lactose or any dairy products.

• Galactosemia is the accumulation of galactose in blood. It is a rare genetic metabolic disorder due to a deficiency of galactose metabolizing enzyme.

• Fructose malabsorption is a dietary disorder in which absorption of fructose is impaired which results in increased fructose concentration in the intestine.

Conclusion

Carbohydrates are key molecules that form the main energy source. It takes part in different biochemical pathways to form essential biomolecules for nucleic acid synthesis, glucose production, glycogen synthesis, glycogen breakdown, etc. This glucose is further broken down into carbon dioxide and water with the production of NADPH which further produces ATP.

FAQs

Q1. What happens when excess carbohydrates are consumed?

Ans: When excess carbohydrates are consumed it results in high blood sugar levels, weight gain, and poor metabolic activities. It may lead to an increased risk of heart disease.

Q2. What is the role of carbohydrates in our body?

Ans: They help us by acting as fuel for our different organs like kidneys, brain, muscles, and nervous system. It keeps cholesterol in control by converting it into simple sugar by gluconeogenesis.

Q3. Why an early meal is suggested before bedtime?

Ans: When carbohydrates are consumed it increases blood glucose levels and the individual who has taken a meal gets energy from it. So it hampers sleep. Thus it is advised to eat meals four hours before sleep.

Q4. What are monosaccharides?

Ans: Carbohydrates made of a single sugar are known as monosaccharides. It is the simplest form and can be directly absorbed in the blood from the small intestine.

Q5. Which sugar is found in nuclear material?

Ans: Ribose and Deoxyribose sugar is found in the nuclear material. It is a pentose sugar with a phosphate group attached to it that forms the backbone of the genetic material.