Reactions at the a-Carbon in Living Systems

Many reactions that occur in cells involve reactions at the a-carbon—that is, the kinds of reactions you have been studying in this chapter. We will now look at a few examples. 

Glucose, the most abundant sugar found in nature, is synthesized in cells from two molecules of pyruvate. The series of reactions that convert two molecules of pyruvate into glucose is called gluconeogenesis (Section 25.13). The reverse process—the breakdown of glucose into two molecules of pymvate— is called glycolysis (Section 25.6). 

Because glucose has twice as many carbons as pyruvate, you should not be surprised to learn that one of the steps in the biosynthesis of glucose is an aldol addition. An enzyme called aldolase catalyzes an aldol addition between dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. The product is fructose-1,6-bisphosphate, which is subsequently converted to glucose. 

The reaction catalyzed by aldolase is reversible. The reverse reaction—the cleavage of fructose-1,6-bisphophate to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate—is a retro-aldol addition (see page 870). The mechanism for this reaction is discussed in Section 23.12. 

Propose a mechanism for the formation of fructose-1,6-bisphosphate from dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, using hydroxide ion as the catalyst. 

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