Organic Chemistry

826 CHAPTER 19 Carbonyl Compounds III

19.21 Reactions at the -Carbon

in Biological Systems

Many reactions that occur in biological systems involve reactions at the —

the kinds of reactions you have studied in this chapter. We will now look at a few

examples.

A Biological Aldol Addition

D-Glucose, the most abundant sugar found in nature, is synthesized by biological systems

from two molecules of pyruvate. The series of reactions that convert two molecules of

pyruvate into D-glucose is called gluconeogenesis. The reverse process—the breakdown

of D-glucose into two molecules of pyruvate—is called glycolysis(Section 25.1).

BecauseD-glucose has twice as many carbons as pyruvate, it should not be surprising

that one of the steps in the biosynthesis of D-glucose is an aldol addition. An enzyme

called aldolase catalyzes an aldol addition between dihydroxyacetone phosphate and

D-glyceraldehyde-3-phosphate. The product is D-fructose-1,6-diphosphate, which is sub-

sequently converted to D-glucose. The mechanism of this reaction is discussed in

Section 24.9.

PROBLEM 40

Propose a mechanism for the formation of D-fructose-1,6-diphosphate from dihydroxyace-

tone phosphate and D-glyceraldehyde-3-phosphate, using as the catalyst.

A Biological Aldol Condensation

Collagen is the most abundant protein in mammals, amounting to about one-fourth of

the total protein. It is the major fibrous component of bone, teeth, skin, cartilage, and

tendons. It also is responsible for holding groups of cells together in discrete units. In-

dividual collagen molecules—called tropocollagen—can be isolated only from tissues

of young animals. As animals age, the individual molecules become cross-linked,

which is why meat from older animals is tougher than meat from younger ones. Colla-

gen cross-linking is an example of an aldol condensation.

HO-

aldolase

CH 2 OPO 32 −

CO

HO H

HOH

HOH

CH 2 OPO 32 −

CO

CH 2 OH

CH 2 OPO 32 −

CH

CH 2 OPO 32 −

O

H OH

D-fructose-1,6-diphosphate

dihydroxyacetone phosphate

D-glyceraldehyde-3-phosphate

OO

2 CH 3 CCO−

CH 2 OH

HOH

HO H

HOH

HOH

HC O

several steps

D-glucose

pyruvate

gluconeogenesis

glycolysis

a-carbon

A

3-D Molecule:

Aldolase