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Biochemical intersections

Biochemical Intersections

Its not uncommon for biochemical pathways to intersect and have multiple enzymes competing for the same substrate. Glucose-6-PO4 (G6P) in glycolysis is one such intersection. In this tutorial you will see how one metabolite can be at the hub of at least 4 major pathways. What you should consider is what controls the carbon flow into each.

The case of Glucose-6-PO4


Pentose Phosphate

6-phosphoglucono--lactone

CH2OPO3=

O

OH

Glycogen

OH

HO

OH

Blood

Glucose

Glucose-1-PO4

Glucose

Fructose-6-PO4

Glycolysis

Talk about a traffic jam, glucose-6-PO4 (G6P) is at the crossroads of at least 4 biochemical pathways (click 1). Converting G6P to fructose-6-PO4 commits the molecule to glycolysis (click 1). On the other hand G6P can enter the glycogen synthesis pathway by being converted to glucose-1-PO4 (click 1). In liver G6P is the source of blood glucose (click 1). Finally, by an oxidation reaction G6P is converted into 6-phosphoglucono-d-lactone and enters the pentose phosphate pathway (click 1). The steady-state concentration of G6P depends on all these reactions plus those that form G6P. Click 1 to go on.


CH2OPO3=

6-phosphoglucono--lactone

O

OH

OH

HO

OH

Glucose-1-PO4

Glucose

Fructose-6-PO4

The steady-state concentration of G6P depends on reactions that form this molecule and those that use it. These reactions are catalyzed by enzymes (click 1). G6P is made from F6P via a reverse of the phosphoglucose isomerase reaction (click 1) and from glucose-1-PO4 by reversing the phosphoglucomutase reaction (click 1). The glucose-6-phosphatase (click 1) and glucose-6-PO4 dehydrognease reactions (click 1) are not reversible (click 1). Hexokinase (click 1) is needed to form G6P from glucose in a reaction that is not reversible (click 1). Thus, G6P levels are controlled by 5 different enzymes, 3 or which catalyze irreversible reactions. Click 1 to go on.

Glucose-6-PO4 dehydrogenase

Glucose-6-phosphatase

Phosphoglucomutase

Hexokinase

Phosphoglucoisomerase


CH2OPO3=

O

OH

OH

HO

OH

Hexokinase

(-16.7 kJ/mole

Phosphoglucomutase

(7.3 kJ/mole)

Glucose-1-PO4

Glucose

ATP ADP

Phosphoglucoisomerase

(-1.7 kJ/mol)

Fructose-6-PO4

Finally, we must decide why some reactions of glucose-6-PO4 are not reversible (click 1). The answer will be found in the DGo' for the reaction. Recall, a large negative DGo' indicates the forward direction is favored. To reverse the reaction an equivalent amount of energy must be available. For example, when ATP is hydrolyzed to ADP + PO4 , 30.5 kJ of energy per mole are released. Thus 30.5 kJ input is needed to make ATP from ADP + PO4. To be reversed, the hexokinase reaction (click 1) must use the hydrolysis of a phosphate ester (DGo'=-13.8 kJ) to form a bond that requires an input of 30.5 kJ. This can't be done (click 1). On the other hand breaking and making a phosphate ester bond as the phosphoglucomutase is practically neutral in energy (click 1) as is rearranging C-1 and C-2 in the phosphoglucose isomerase reaction (DGo'= 1.7 kJ/mol) (click 1). That's why the isomerase and mutase reactions are freely reversible and hexokinase is not.


What have you learned ?

1. Glucose-6-PO4 is clearly a molecule that has many metabolic fates. Can you think of a molecule in glycolysis that has only one? (click 1 for answer)

Glucose. Glucose has only one fate and that is to be converted into glucose-6-PO4. One must consider, however, that glucose is generally found outside the cell, blood glucose, for example.

2. How much energy is required to reverse the glucose-6-phosphatase reaction?

A phosphate group must bind directly to the -OH group in the 6th position on the glucose

molecule, replacing a water molecule. The energy requirement of that reaction is Go’=13.8 kJ/mol

3. Is the phosphorylation of glucose by ATP an anabolic or catabolic reaction?

Catabolic. It catabolic because it results in the release of a substantial amount of free energy, i.e.,

exergonic. Similarly, the reaction takes place in a catabolic pathway. But, one could also argue that

ATP hydrolysis is required and that is consistent with an anabolic reaction.

4. Would an increase in the concentration of glucose-6-PO4 favor all the forward reactions

where this molecule is a substrate?

It might. But one must be mindful that glucose-6-PO4 is also a regulator of enzyme activity and an

increase in G6P could result in the shut down of enzymes more forward in the pathways. We’ll see

allosteric effects later.


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