Glycolysis aldehyde oxidation

Figure 17-3. Mechanism of oxidation of giyceraldehyde 3-phosphate. (Enz, glycer-aldehyde-3-phosphate dehydrogenase.) The enzyme is inhibited by the— 5H poison iodoacetate, which is thus abie to inhibit glycolysis. The NADH produced on the enzyme is not as firmly bound to the enzyme as is NAD. Consequently, NADH is easily displaced by another molecule of NAD". ...

The idea that enzymes whose normal substrate is orthophosphate can use arsenate in its place, and that the esters of arsenate formed are rapidly hydrolyzed, was given by Braunstein (16) in 1931, to explain the effects of arsenate on glycolysis. This idea was formulated more precisely for glyceraldehyde-phosphate dehydrogenase in 1939. This enzyme, E—SH, normally oxidizes its aldehyde substrate with phosphate uptake to give an acyl phosphate, as follows ... 

Glyceraldehyde-3-phosphate dehydrogenase is an example of an enzyme that uses NAD as an oxidizing coenzyme. The enzyme catalyzes the oxidation of the aldehyde group of glyceraldehyde-3-phosphate (GAP) to an anhydride of a carboxylic acid and phosphoric acid. This is a reaction that occurs in glycolysis (Figure 25.3). 

The preceding reactions yield two molecules of NADPH and one molecule of ribose 5-phosphate for each molecule of glucose 6-phosphate oxidized. However, many cells need NADPH for reductive biosyntheses much more than they need ribose 5-phosphate for incorporation into nucleotides and nucleic acids. In these cases, ribose 5-phosphate is converted into glycer-aldehyde 3-phosphate and fructose 6-phosphate by transketolase and transaldolase. These enzymes create a reversible link between the pentose phosphate pathway and glycolysis by catalyzing these three successive reactions. 

---