BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 12. GLYCOLYSIS

12.4. Formation of Glyceraldehyde 3-Phosphate by Cleavage and Isomerization

The Second Stage of Glycolysis consists of four steps and begins with the Cleavage of

fructose 1,6-bisphosphate to yield glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Subsequent reactions of glycolysis involve three-carbon rather than six-carbon compounds. This conversion is catalyzed by aldolase. The enzyme's name reflects The Nature of the reverse reaction, which is an aldol Condensation.

Glyceraldehyde 3-phosphate is on the direct pathway of glycolysis, whereas dihydroxyacetone phosphate is not. However, dihydroxyacetone phosphate can be readily converted into glyceraldehyde 3-phosphate. These compounds are isomers: dihydroxyacetone phosphate is a ketose, and glyceraldehyde 3-phosphate is an aldose. The isomerization of these three-carbon phosphorylated sugars is catalyzed by triose phosphate isomerase. This reaction is rapid and highly reversible. At equilibrium, 96% of the triose phosphate is dihydroxyacetone phosphate. Nevertheless, The conversion of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate proceeds readily because the latter is efficiently removed.

Thus, two molecules of glyceraldehyde 3-phosphate are formed from one molecule of fructose 1,6-bisphosphate by the sequential action of aldolase and triose phosphate isomerase.

12.5. Energy Storage: Phosphorylation Coupled to the Oxidation of Glyceraldehyde 3-Phosphate

In the preceding steps of glycolysis, one molecule of glucose was converted into two molecules of glyceraldehyde 3-phosphate. No energy has been extracted yet. On the contrary, two molecules of ATP have been consumed. We now turn to a series of steps that harvest the energy contained in glyceraldehyde 3-phosphate.

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The initial reaction in this sequence is the conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate (1,3-BPG), catalyzed by glyceraldehyde 3-phosphate dehydrogenase.

Glyceraldehyde 3-phosphate + NAD ⁺ + Pi ⇄ 1,3-BPG + NADH + H⁺.

This oxidation-reduction reaction generates a high-energy phosphate compound. The aldehyde group at C-1 is converted into an acyl phosphate, which is a mixed anhydride of phosphoric and carboxylic acids.

The energy required for The formation of this anhydride, which has a high phosphate group-transfer potential, is released by The oxidation of the aldehyde group. Note that C-1 in 1,3-BPG is at the oxidation level of a carboxylic acid. The formation of 1,3-BPG is an example of substrate-level phosphorylation. We will consider The Mechanism of this complex reaction, in which oxidation is coupled to phosphorylation, somewhat later (Section 12.14).

12.6. Formation of ATP from 1,3-Bisphosphoglycerate

In the next step of glycolysis, the high phosphoryl group-transfer potential of 1,3-BPG is used to generate ATP. Indeed, this is the first reaction in glycolysis that yields ATP. The transfer of the phosphoryl group from the acyl phosphate group of 1,3-BPG to ADP is catalyzed by phosphoglycerate kinase. The products of the reaction are ATP and 3-phosphoglycerate.

Thus, the Reactions Catalyzed by glyceraldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase accomplish the following processes.

1. Glyceraldehyde 3-phosphate, an aldehyde, is oxidized to 3-phosphoglycerate, a carboxylic acid.

2. NAD⁺ is reduced to NADH.

3. ATP is formed from Pi and ADP.