Question

One pathway for the metabolism of D-glucose 6-phosphate is its enzyme- catalyzed conversion to D-fructose 6-phosphate. Show that this transformation can be accomplished as two enzyme-catalyzed keto-enol tautomerisms.

Short answer

Question: Describe the steps involved in the conversion of D-glucose 6-phosphate to D-fructose 6-phosphate through enzyme-catalyzed keto-enol tautomerisms, and identify the intermediate compound formed. Answer: The conversion of D-glucose 6-phosphate to D-fructose 6-phosphate occurs through two enzyme-catalyzed keto-enol tautomerisms. In the first tautomerism, a hydrogen atom is transferred from carbon 2 to the oxygen in the aldehyde group, resulting in a carbon-carbon double bond between carbons 1 and 2 and an alcohol group at carbon 1, forming an intermediate enol form called D-glucose-6-phosphate-enol. In the second tautomerism, a hydrogen atom is transferred from the oxygen in the alcohol group at carbon 1 to carbon 2, forming a keto group at carbon 2 and resulting in the formation of D-fructose 6-phosphate.

Step-by-step solution

Understand the given molecules

First, we need to know the molecular structures of D-glucose 6-phosphate and D-fructose 6-phosphate. D-glucose 6-phosphate has an aldehyde group (CHO) at carbon 1 and a phosphate group at carbon 6. D-fructose 6-phosphate has a keto group (C=O) at carbon 2 and a phosphate group at carbon 6.

Recognize the enzyme-catalyzed keto-enol tautomerism

Keto-enol tautomerism is a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol form (an alcohol group adjacent to a carbon-carbon double bond). In our case, the conversion of D-glucose 6-phosphate to D-fructose 6-phosphate involves the transfer of the aldehyde group at carbon 1 to a keto group at carbon 2 through two enzyme-catalyzed reactions.

First enzyme-catalyzed keto-enol tautomerism

The first enzyme-catalyzed keto-enol tautomerism consists of the transfer of a hydrogen atom from carbon 2 to oxygen in the aldehyde group, resulting in a carbon-carbon double bond (C=C) between carbons 1 and 2 and an alcohol group (OH) at carbon 1. This intermediate compound is an enol form called D-glucose-6-phosphate-enol.

Second enzyme-catalyzed keto-enol tautomerism

The second enzyme-catalyzed keto-enol tautomerism consists of the transfer of a hydrogen atom from the oxygen in the alcohol group at carbon 1 to carbon 2, restoring the original hydrogen atom at carbon 2, and forming a keto group (C=O) at carbon 2. This results in the formation of D-fructose 6-phosphate. In conclusion, the conversion of D-glucose 6-phosphate to D-fructose 6-phosphate is accomplished through two enzyme-catalyzed keto-enol tautomerisms, with an intermediate enol form (D-glucose-6-phosphate-enol) formed during the process.