Glycolytic Pathway
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Glycolysis is a Catabolic reaction
Cytoplasmic
Conversion of Glucose ( C6H12O6) to Pyruvate ( Pyruvic acid )
The reaction occurs in two parts
Energy investment( Priming ) – 2 ATP ( Steps 1-3)
Energy harvesting – 4 ATP and 2 NADH + H+ ( 4-10)
Net gain of ATP + 2 ATP
Step One – Phosphorylation of glucose to form glucose – 6 phosphate. The enzyme is
hexokinase. Kinase enzymes are involved in reactions in which there is a transfer of a
phosphate group. Mg ++ is required for kinase activity.
ATP
ADP + P
Step Two – Glucose – 6 phosphate changes to Fructose – 6 Phosphate. Fructose – 6
phosphate is an isomer. The reaction is an isomerization reaction. It requires the enzyme
Phsophoglucose isomerase.
Step 3. Fructose -6- phosphate is phosphorylated to form Fructose 1,6 – biphsophate. This
enzyme proceeds under allosteric regulation by the enzyme phosphofructokinase ( PFK).
PFK is regulated allosterically by the levels of ATP in the cell. Elevated level of citrate also
inhibit the action of PFK.
Step 4. Fructose 1,6 biphsophate is cleaved from a 6C molecule to 2 3C molecules
Dihydroxyacetone phosphate ( DHAP ) and Glyceraldehyde 3- Phosphate ( G3P).
This molecule is catalyzed by an aldolase.
Step 5. The molecule DHAP and G3P are isomers. Immediately DHAP
G3P This
conversion from DHAP to G3P occurs with the enzyme triose phosphate isomerase.
Step 6 ( x2) G3P undergoes substrate level phosphorylation with an inorganic phosphate.
In the process NAD+
NADH+ + H+ . The oxidation of the aldehyde group to a
carboxyl group is accompanied by the attachment of a phosphate. The formation of 1, 3
bipshophoglycerate is an example of the coupling of a phosphorylation to a redox reaction.
The enzyme that catalyzes this reaction is Glyceraldehyde- 3 phosphate dehydrogenase.
Step 7. The phosphorylation of the previous reaction drives this reaction. 1, 3
biphosphoglycerate
3, Phosphoglycerate. ADP is converted to ATP.
Step 8. The Phosphate group is shifted from the 3 position to the 2 position by a mutase to
from 2, Phosphoglycerate.
Step 9. 2, Phosphoglycerate undergoes a dehydration reaction( loss of H2O) with an
enolase to form Phosphoenol pyruvate. This reaction is catalyzed by pyruvate kinase. In
the process , 2 ATP are formed.
Glycolysis results in a net gain of 2 ATP and 2 NADH + H+.
Glycolysis is an Amphibolic Pathway. Some of the reactions of glycolysis are reversible and
may be used in the synthesis of glucose. The process by which pyruvate is converted to
glucose is referred to as Gluconeogenesis