The Metabolic processes in carbohydrates for Competitive Exam

i. Glycolysis

  • Glycolysis, also called as Embden-Meyerhof-Parnas pathway (EMP pathway), consists of a series of reactions through which glucose is converted to pyruvate with the concomitant production of relatively small amounts of adenosine triphosphate (ATP).
  • It is derived from the Greek stem ‘glykys’ meaning sweet and ‘lysis’ meaning splitting. It is the primary pathway occurring in the cytoplasm of all the tissues of biological systems.
  • All the enzymes responsible for the catalysis are found in the extra-mitochondrial soluble fraction of the cells (cytoplasm). In plants, glucose and fructose are the main monosaccharides catabolized by glycolysis although others are also converted into these sugars.
  • Glucose entering the glycolysis is derived from starch or sucrose, and fructose is derived from sucrose.
  • The starch is either from seeds or chloroplasts of matured plants.
  • Glycolysis normally takes place in the presence of O2 in higher plant cells.
  • The enzymes in the cytoplasm catalyze the reactions involved in the conversion of glucose to pyruvate.
The series of reactions indicated take place in 3 stages.
Stage 1: Conversion of glucose to fructose 1,6-bisphosphate
  • The formation of fructose 1,6-bisphosphate takes place in three steps catalyzed by enzymes.
  • The purpose of these reactions is to form a compound that can be readily cleaved into phosphorylated three carbon units from which, through a series of reactions, ATP is formed. After the first phosphorylation reaction to form glucose 6-phosphate, isomerization of glucose 6-phosphate to fructose-6-phosphate occurs which is conversion of an aldose into a ketose.
  • A second phosphorylation reaction follows the isomerization, catalyzed by phosphofructokinase resulting in the formation of fructose 1,6-bisphosphate.
  • Phosphofructokinase is the key enzyme in the control of glycolysis.
Stage 2:Conversion of fructose 1,6-bisphosphate to 3-phosphoglycerate.
  • The splitting of fructose 1,6-bisphosphate occurs in the second stage of glycolysis resulting in the formation of a molecule of glyceraldehyde 3-phosphate and a molecule of dihydroxyacetone phosphate catalyzed by aldolase.
  • The dihydroxyacetone phosphate is isomerized to glyceraldehyde 3-phosphate by phosphotriose isomerase.
  • The isomerization reaction is rapid and reversible. In the next step, glyceraldehyde 3- phosphate is oxidized to 1,3-bisphosphoglycerate catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
  • The product is further converted into 3-phosphoglycerate and a molecule of ATP is formed.
  • The phosphorylation of ADP to ATP is called substrate level phosphorylation since the phosphate group from a substrate molecule is transferred to ADP.
Stage 3: Formation of pyruvate
  • An intramolecular rearrangement of the phosphoryl group occurs resulting in the formation of 2-phosphoglycerate from 3-phosphoglycerate catalyzed by phosphoglycerate mutase.
  • The 2-phosphoglycerate formed undergoes dehydration forming phosphoenolpyruvate which gives rise to pyruvate and a molecule of ATP (substrate level phosphorylation).
  • The reaction is irreversible and catalyzed by pyruvate kinase.
  • The net reaction in the transformation of glucose to pyruvate is
Glucose + 2 Pi + 2ADP + 2 NAD→ 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + H2O
  • Once pyruvate is formed, further degradation is determined by the presence or absence of oxygen.
  • Under anaerobic conditions, in one of the pathways, pyruvate undergoes reduction yielding lactic acid.
  • The formation of lactic acid is very rare in plants with exception of potato tubers maintained under anaerobic condition and some green algae.
  • In the second pathway, pyruvate is converted to ethyl alcohol and carbon dioxide.
  • The alcoholic fermentation is the basis of the beer and wine-making industries.
  • Under aerobic conditions, pyruvate is oxidatively decarboxylated to acetyl CoA which is then completely oxidized to CO2 and water through the citric acid cycle.

ii. The citric acid cycle

  • It is the final common oxidative pathway for carbohydrates, fats and proteins. It is also a source of precursors for biosynthesis of various biomolecules.
  • The acetyl CoA that enters in this pathway is completely oxidized to carbon dioxide and water with concomitant production of reducing equivalents, namely NADH and FADH2.

iii. The hexose monophosphate shunt

  • It is an alternative pathway to the glycolytic pathway and the citric acid cycle for the oxidation of glucose to carbon dioxide and water with the generation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) molecules and ribose 5-phosphate.

iv. Gluconeogenesis

  • It is a biosynthetic pathway that generates glucose from non-carbohydrate precursors.

v. Glycogenesis:

  • It is a pathway by which glycogen is synthesized from glucose.

vi. Glycogenolysis

Glycogenolysis is the biochemical pathway in which glycogen breaks down into glucose-1-phosphate and glucose. The reaction takes place in the hepatocytes and the myocytes. The process is under the regulation of two key enzymes: phosphorylase kinase and glycogen phosphorylase.

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