Carbohydrate Metabolism Overview

Citric Acid Cycle Overview Metabolism &
Energy Summary
 Elmhurst College
Glycolysis Summary Pyruvic Acid - Crossroads  Chemistry Department
Glycolysis Reactions Glycogenesis / Glycogenolysis / Gluconeogenesis  Virtual ChemBook


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Glycolysis Reactions

Introduction to Glycolysis:

The overall reaction of glycolysis which occurs in the cytoplasm is
represented simply as:

C6H12O6 + 2 NAD+ + 2 ADP + 2 P -----> 2 pyruvic acid, (CH3(C=O)COOH + 2 ATP + 2 NADH + 2 H+

At this time, concentrate on the fact that glucose with six carbons is converted into two pyruvic acid molecules with three carbons each. Only a net "visible" 2 ATP are produced from glycolysis. The 2 NADH will be considered separately later.

The major steps of glycolysis are outlined in the graphhic on the left. There are a variety of starting points for glycolysis; although, the most usual ones start with glucose or glycogen to produce glucose-6-phosphate. The starting points for other monosaccharides, galactose and fructose, are also shown.

Glycolysis - with white background for printing

Overview of Metabolism

Link to: Great Animation of entire Glycolysis - John Kyrk
Link to: Interactive Glycolysis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.

Link to Glycolysis Aninmation 1
Link to Glycolysis Aninmation 2


Reaction 1: Phosphate Ester Synthesis

Phosphate is added to the glucose at the C-6 position. The reaction is a phosphate ester synthesis using the alcohol on the glucose and a phosphate from ATP.

This first reaction is endothermic and thus requires energy from a coupled reaction with ATP. ATP is used by being hydrolyzed to ADP and phosphate giving off energy and the phosphate for reaction with the glucose for a net loss of ATP in the overall glycolysis pathway.

Hydrolysis: ATP + H2O --> ADP + P + energy

P = PO4-3; ATP = adenine triphosphate;ADP = adenine diphosphate

This reaction is catalyzed by hexokinase.

Off-site chime link: Boyer Tutorial - Hexokinase

Reaction 1 - Chime in new window

Reaction 2: Isomerization

The glucose-6-phosphate is changed into an isomer, fructose-6-phosphate. This means that the number of atoms is unchanged, but their positions have changed.

This works because the ring forms may open to the chain form, and then the aldehyde group on glucose is transformed to the keone group on fructose. The ring then closes to form the fructose-6-phosphate.

This reaction is catalyzed by phosphoglucoisomerase.
Off-site chime link: Phosphoglucoisomerase

Reaction 2 - Chime in new window

 

 

 Reaction 3: Phosphate ester synthesis

This reaction is virtually identical to reaction 1 The fructosee-6-phosphate has an alcohol group on C-1 that is reacted with phosphate from ATP to make the phosphate ester on C-1.

Again this reaction is endothermic and thus requires energy from a coupled reaction with ATP. ATP is used by being hydrolyzed to ADP and phosphate giving off energy and the phosphate for reaction with the glucose for a net loss of ATP in the overall glycolysis pathway.

Hydrolysis: ATP + H2O --> ADP + P + energy

This reaction is catalyzed by phosphofructokinase.
Off-site chime link: Phosphofructokinase
Link to: Rodney Boyer Animation of Phosphofructokinase

Reaction 3 - Chime in new window

 

Reaction 4: Split Molecule in half

The six carbon fructose diphophate is spit into two three-carbon compounds, an aldehyde and a ketone.

The slit is made between the C-3 and C-4 of the fructose. The ring also opens at the anomeric carbon. The product on the right is the glyceraldehyde.

Technically this is called a reverse aldol condensation.

This reaction is catalyzed by aldolase.
Off-site chime link: Aldolase

Reaction 4 - Dihydroxyacetonephosphate Chime in new window
Reaction 4 - Glyceraldehyde-3-phosphate Chime in new window

 

 Reaction 4A: Isomerization

The dihydroxyacetone phosphate must be converted to glyceraldehyde-3-phosphate to continue the glycolysis reactions. This reaction is an isomerization between the keone group and an aldehyde group.

As a result of this reaction, all of the remaining glycolysis reactions are carried out a second time. The first series of reactions occurs with the first glyceraldehyde molecule from the orginal split. Then the second series of reactions occurs after the isomerization of the dihydroxyacetone into the glyceraldehyde.

This reaction is catalyzed by triose phosphate isomerase.
Off-site chime link: Triose Phosphate Isomerase (TIM)

Reaction 4A - Isomerization Chime in new window

 

 Reaction 5: Oxidation/Phosphate Ester Synthesis

This reaction is first an oxidation involving the coenzyme NAD+. The aldehyde is oxidized to an acid as an intermediate through the conversion of NAD+ to NADH + H+. Then an inorganic phosphate is added in a phosphate esteer synthesis.

This and all remaining reactions occur twice for each glucose-6-phosphate (six carbons), since there are now two molecules of 3-carbons each.

This reaction is catalyzed by glyceraldehyde-3-phosphate.
Off-site chime link: G3P Dehydrogenase

Reaction 5 - 1,3-diphosphoglycerate Chime in new window

 

 Reaction 6: Hydrolysis of Phosphate;
Synthesis of ATP

One of the phosphate groups undergoes hydrolysis to form the acid and a phosphate ion, giving off energy. This first energy producing reaction is coupled with the next endothermic reaction making ATP. The phosphate is transferred directly to an ADP to make ATP.

This reaction is catalyzed by phosphoglycerokinase.
Off-site chime link: Phosphoglycerate Kinase

Reaction 6 - 3-phosphoglycerate Chime in new window

 

 Reaction 7: Isomerization

In this reaction the phosphate group moves from the 3 position to the 2 position in an isomerization reaction.

This reaction is catalyzed by phosphoglycerate mutase.
Off-site chime link: Phosphoglycerate Mutase

Reaction 7 - 2-phosphoglycerate Chime in new window

 

 Reaction 8: Alcohol Dehydration

In this reaction, which is the dehydration of an alcohol, the -OH on C-3 and the -H on C-2 are removed to make a water molecule. At the same time a double bond forms between C-2 and C-3. This change makes the compound somewhat unstable, but energy for the final step of glycolysis.

This reaction is catalyzed by enolase.
Off-site chime link: Enolase

Reaction 8 - phosphoenol pyruvic acid Chime in new window

 

 Reaction 9:Phosphate Ester Hydrolysis;
Synthesis of ATP

This is the final reaction in glycolysis. Again one of the phosphate groups undergoes hydrolysis to form the acid and a phosphate ion, giving off energy. This first energy producing reaction is coupled with the next endothermic reaction making ATP. The phosphate is transferred directly to an ADP to make ATP.

This reaction is catalyzed by pyruvic kinase.
Off-site chime link: Pyruvate Kinase

Reaction 9 - pyruvic acid Chime in new window

Conclusion:

Starting with glucose-6-phosphate with 6 carbons, the final result of the glycolysis reactions is two molecules of pyruvic acid, since reaction 5-9 are each carried out twice.