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Chapter 34: Problem 768

Mevalonic acid is the precursor of the terpenes. Write its structure. After phosphorylation, it undergoes a decarboxylative elimination to yield 3 -isopentenyl pyrophosphate. Outline the mechanism of this reaction. Where does the driving force for the reaction come from?

Short Answer

Expert verified
Mevalonic acid has the structure \( CH_3-CH_2-C(CH_3)_2-CH_2-CH_2-COOH \). Upon phosphorylation, it converts to mevalonic acid phosphate, forming a reactive intermediate with two phosphate groups. Decarboxylation occurs, releasing CO₂ and forming a double bond between C2 and C3, yielding 3-isopentenyl pyrophosphate. The reaction's driving force comes from the high-energy phosphate bonds in ATP and the release of CO₂ during decarboxylation.

Step by step solution

01

Draw the structure of mevalonic acid

Mevalonic acid is a 6-carbon compound with 2 methyl groups at the 3-carbon position, a carboxylic acid group at one end, and a primary alcohol group at the other end. Its structure is as follows: \[ CH_3-CH_2-C(CH_3)_2-CH_2-CH_2-COOH \] Now that we have the structure of mevalonic acid, let's move on to the mechanism of its conversion to 3-isopentenyl pyrophosphate.
02

Outline the phosphorylation of mevalonic acid

The first step in the reaction is the phosphorylation of mevalonic acid-hydroxyl group. This occurs when a molecule of ATP transfers its terminal phosphate group to the oxygen atom of the hydroxyl group in mevalonic acid, forming a phosphate ester bond and converting ATP to ADP. The resulting intermediate is known as mevalonic acid phosphate.
03

Outline the decarboxylative elimination

In the next stage of the reaction, mevalonic acid phosphate undergoes decarboxylative elimination. A second molecule of ATP transfers a phosphate group to the oxygen atom of the carboxylic acid group in mevalonic acid phosphate, forming another phosphate ester bond and converting ATP to ADP. This creates a highly reactive intermediate with two phosphate groups. The decarboxylation reaction now occurs, with the molecule losing carbon dioxide (CO₂), and the phosphate group from the carboxylic acid end moving to the primary alcohol end. This generates a double bond between C2 and C3, finally yielding 3-isopentenyl pyrophosphate and CO₂ as products.
04

Identify the driving force of the reaction

The formation of the double bond between C2 and C3 is highly favorable, as it ultimately forms the isoprenoid unit, which is the basis for all terpenes. But the driving force for the reaction comes from the high-energy phosphate bonds present in the ATP molecules. The release of energy during the conversion of ATP to ADP and the subsequent attachment of the phosphate groups to mevalonic acid ensures that the reaction is thermodynamically favorable and proceeds in the direction of 3-isopentenyl pyrophosphate formation. Additionally, the release of carbon dioxide during decarboxylation also helps push the reaction forward.

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