Elementary plant Biochemistry and Biotechnology (2+1)

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 catabolised 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 catalyse 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 catalysed 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, isomerisation 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, catalysed 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 catalysed by aldolase.
  
• The dihydroxyacetone phosphate is isomerised to glyceraldehyde 3-phosphate by phosphotriose isomerase.
  
• The isomerisation reaction is rapid and reversible. In the next step, glyceraldehyde 3- phosphate is oxidised 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 oxidised to CO2 and water through the citric acid cycle.