Cleavage of fructose 1,6.bisphosphate

Figure 29.9 Mechanism of step 4 in Figure 29.7, the cleavage of fructose 1,6-bisphosphate to yield glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

Cleavage of Fructose 1,6-Bisphosphate The enzyme fructose 1,6-bisphosphate aldolase, often called simply aldolase, catalyzes a reversible aldol condensation (p. 485). Fructose 1,6-bisphosphate is cleaved to yield two different triose phosphates, glyceraldehyde 3-phosphate, an aldose, and dihydroxyacetone phosphate, a lcetose ... 

Write a step-by-step sequence showing the chemical mechanisms involved in the action of a type I aldolase that catalyzes cleavage of fructose 1,6-bisphosphate. The enzyme is inactivated by sodium borohydride in the presence of the substrate. Explain this inactivation. 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

Step 4. Cleavage of Fructose-1,6-Bisphosphate into Three-Carbon Phosphates The reaction is... 

Cleavage of fructose 1,6-bisphosphate occurs by a reverse aldol reaction. 

Fructose-1,6-bisphosphate aldolase or, as it is more commonly known, aldolase, is a tetrameric protein catalyzing the cleavage of fructose-1,6-bisphosphate (F1,6BP) to dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde-3-phosphate (G3P). 

Aldolase is named for the mechanism of the forward reaction, which is an aldol cleavage, and the mechanism of the reverse reaction, which is an aldol condensation. The enzyme exists as tissue-specific isoenzymes, which all catalyze the cleavage of fructose 1,6-bisphosphate but differ in their specificities for fructose 1-P. The enzyme uses a lysine residue at the active site to form a covalent bond with the substrate during the course of the reaction. Inability to form this covalent linkage inactivates the enzyme. 

Draw the mechanism for the hydroxide ion-catalyzed cleavage of fructose-1,6-bisphosphate. 

---