We now enter the wonderful topic of Glcolysis , which is my favourite topic , glycolysis consists of 10 enzyme catalysed reactions. The first five reactions of glycolysis are what is referred to as the energy investment phase , since ATP is invested into the reactions , the last five reactions are known as the energy payoff phase since it is here that the ATP used is regenerated with two extra ATP molecules , giving glycolysis a net gain of two ATP per molecule of glucose used. It must be said however that the real stars of glycolysis are the enzyme since they allow the reactions to be fast and energy efficient.
This is the first Priming reaction of glycolysis and the first reaction of the energy investment phase as well. The reaction is irreversible. In this reaction Glucose is converted to Glucose-6-phosphate. The reaction uses an ATP molecule and converts it to ADP , the enzyme which catalyses the reaction is Hexokinase.
This reaction converts Glucose -6-phophate to Fructose-6-phosphate via the enzyme Phosphohexose isomerise.
This reaction is the second of two irreversible reactions which occur in the energy investment phase. This reaction is the second priming reaction and consumes an ATP molecule whoch is converted to ADP , in this reaction Fructose-6-phosphate is converted to Fructose-1.6-Bisphosphate via the enzyme Phospho-fructokinase-1 (PFK-1). It must also be noted that this is the most regulated step of glycolysis.
This is the splitting reaction , in this reaction the six carbon molecule Fructose-1.6-bisphosphate is converted to two three carbon molecules , one is called Glyceraldehyde -3-phosphate and the other is called Dihydroxyacetone phosphate. The enzyme which catalyses this reaction is Aldolase.
This is the final reaction of the energy investment phase , in this reaction the Dihydroxyacetone phosphate is converted to another molecule of glyceraldehyde-3-phosphate via the enzyme Triose phosphate isomerise. Hence when the energy payoff phase begins , there will be two molecules to glyceraldehydes-3-phosphates to be used.
ENERGY PAYOFF PHASE
In this , the first of the energy payoff reactions , an inorganic phosphate is used , as well as an NAD+ molecule which is converted to NADH and a proton. In this reaction the two molecules of glyceraldehyde-3-phosphate are converted to two molecules of 1,3-Bisphosphoglycerate via the enzyme Glyceraldehyde-3-phophate dehydrogenase.
In this reaction two ATP molecules are generated from two ADP molecules , the reaction converts two molecules of 1,3-bisphosphoglycerate to two molecules of 3-phosphoglycerate , via the enzyme Phophsglycerate kinase.
In this reaction the two molecules of 3-phosphoglycerate are converted to two molecules of 2-phophogycerate via the enzyme Phophoglycerate mutase.
In this reaction two molecules of water are removed , the two molecules of 2-phosphoglycerate are converted to two molecules of Phosphophenolpyruvate via the enzyme Enolase.
This is the final reaction in glycolysis , it generates two molecules of ATP from two molecules of ADP. In this reaction two molecules of Phosphophenolpyruvate are converted to two molecules of pyruvate via the enzyme Pyruvate kinase. It should be noted that this is the only irreversible reaction in the energy payoff phase of glycolysis.
It can be seen therefore that since two ATP molecules are used in the energy investment phase and Four ATP molecules are produced in the energy payoff phase , that there is a net gain of two ATP molecules from one glucose molecules from glycolysis.
So what happens to pyruvate after it’s produced? What’s the Fate of pyruvate?
Pyruvate has three main fates;
1) pyruvate under anaerobic conditions to produce ethanol and carbon dioxide , whereby , , pyruvate is converted to acetaldehyde via the enzyme pyruvate decarboxylase using cofactors TPP and Mg2+ ions , this reaction produces carbon dioxide as a by product . The acetaldehyde is then converted to ethanol via the enzyme alcohol dehydrogenase , this reaction also converts NADH to NAD+.
2) pyruvate under anaerobic conditions to produce lactate ,this reaction occurs via the enzyme lactate dehydrogenase and in this reaction NADH is converted to NAD+. Lactate is produced in vigorously contracting muscles because in such circumstances there is an oxygen debt present and hence anaerobic conditions would be prevalent , and in erythrocytes because they lack mitochondria to produce energy otherwise.
3) pyruvate under aerobic conditions to produce carbon dioxide and water. The first stage of this reaction is converting pyruvate to Acetyl-CoA , this reaction occurs via an enzyme complex referred to as the pyruvate dehydrogenase complex , which consists of three enzymes , this reaction produces carbon dioxide and uses CoA-SH as a cofactor , in the reaction NAD+ is converted to NADH. The second stage of the conversion occurs via the citric acid cycle where the Acetyl-CoA is converted to carbon dioxide and water.
It must also be noted that Fermentation ie. Fates 1 and 2 are used to regenerate NAD+ to be used in glycolysis.
Pictures taken from
1) The BIOCHEMJM Youtube channel
2) Windsor Education Online