GLYCOLYSIS

Glycolysis

• Glycolysis  derives its name  from the Greek words "glycos"- sugar and "lysis" - splitting.
• In this process , Glucose  is converted into two molecules of Pyruvate or pyruvic acid, an organic acid.
• Apart from preparing substrate for Citric acid cycle, the other function of this is to yield chemical energy in the form of ATP and NADH.
• Glycolysis occurs in all living organisms (prokaryotes and eukaryotes).
• Generally ,the commom type of glycolysis studied is the EMBDEN MEYERHOF PARNAS PATHWAY (EMP).
• The word ,Fermentation is generally used for anaerobic degradation of glucose or other organic nutrients to obtain energy, conserved as ATP.
• In the early steps of glycolysis, sucrose is broken down into two monosaccharides Glucose  and Fructose  which can readily enter the glycolytic pathway.
• The reaction of glycolysis takes place in cytosol. All of the enzymes of glycolysis are found in cytosol.
• Certain trypanosomes carried out the first seven reactions of glycolysis in an organized cytoplasmic organelle called the Glycosomes .Three reactions are inconvertible  and are catalyzed  by hexokinase , phosphofructokinase and pyruvate kinase .

GLYCOLYSIS HAS TWO PHASES

1-PREPARATORY PHASE

• The breakdown of the 6 carbon glucose molecule into two molecules of 3 carbon pyruvate occurs in ten steps. The first five reactions  constitutes the Preparatory Phase. In this -
• Step 1- At first ,Glucose is phosphorylated at hydroxyl group on Carbon -6.
• Step-2-The D Glucose-6-phosphate ,then converted into D Fructose-6-phosphate.
• Step-3- It is again phosphorylated ,this time at Carbon-1position  to yield D Fructose-1,6-bisphosphate
•  (for both of the above phosphorylation ATP is phosphorylated group donor)
• Step-4- The Fructose- 1,6-bisphosphate is split to yield 3 carbon molecules Dihydroxyacetone phosphate and glyceraldehyde -3-phosphate
•  (this is the lysis step that gives its name)
• Step-5- the dihydroxyacetone Phosphate is isomerized to a second molecule  of glycerol aldehyde-3-phosphate.
• (First phase ends here 2 molecules of ATP are invested in the cleavage of glucose)

2-PAYOFF PHASE

• The next five steps contributes the Payoff phase. The energy gain comes from this phase.
• Step-6- Each molecule of glyceraldehyde 3-phosphate is oxidized and phosphorylated by inorganic phosphate (not by ATP) to form 1,3-biphosphoglycerate.
• Step-7- 1,3-biphosphoglycerate are converted to 3-phosphoglycerate by releasing energy (2ADP changes to 2ATP).
• Step -8- 3-phosphogylceratechanges to 2 -phosphoglycerate
• Step-9- 2-phosphoglycerate changes to phosphoenol pyruvate
• Step-10- The phosphoenol pyruvate converted to pyruvate or the pyruvic acid by releasing energy (2ADP changes to 2ATP).
• The energy released in the step 7 and 10  is conserved by coupl phosphorylation of 4ADP molecules to 4ATP molecules.
• The net yield in glycolysis is two ATP molecules per molecule of glucose used ,because two molecules of ATP were invested in the Preparatory phase.
• Energy is also conserved in two molecules of NADH per molecule of glucose.