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14.3 Aldol Reaction and Claisen Condensation in Biology

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1 14.3 Aldol Reaction and Claisen Condensation in Biology
Main Menu Enol – Keto Tautomerization Aldol Reaction Claisen Condensation

2 Enol – Keto Tautomerization
a-H is required to have keto-enol tautomerization. In acidic condition a Enol Keto + H+ - H+ In basic condition Enolate

3 Examples of Enol – Keto Tautomerization

4 Enol – Keto Tautomerization in Carbohydrates
Keto-enol tautomerization can lead to structural isomerization between aldose and ketose. Keto Enol Keto D-glucose (an aldose) D-fructose (a ketose)

5 Enol – Keto Tautomerization in Glycolysis
Keto-enol tautomerization catalyzed by phosphoglucoisomerase phosphoglucoisomerase Reaction 2 Fructose-6-phosphate Glucose-6-phosphate (a ketose) (an aldose)

6 Isomerization of Glucose-6-phosphate
1 1 1 1 2 2 Glucose-6-phosphate Opened form fructose-6-phosphate Opened form glucose-6-phosphate Enol Cyclic hemiacetal form Keto Fructose-6-phosphate 1 2 Cyclic hemiketal form

7 Enol – Keto Tautomerization in Glycolysis
Keto-enol tautomerization catalyzed by Triosephosphate isomerase Triosephosphate isomerase Keto Keto Reaction 5 Glyceraldehyde-3-phosphate Dihydroxyacetone phosphate (an aldose) (a ketose) Enol

8 Learning Check 1. What represent the correct enol-keto tautomerization for the following ketone or aldehyde? I, II (b) I, II, III (c) II, III, IV (d) II, IV (e) None of the above I II III IV

9 Formation of new C-C bond by combining keto and enol (enolate)
Aldol Reaction Formation of new C-C bond by combining keto and enol (enolate) b In acidic condition + Enol Keto The product contains an aldehyde and an alcohol functional groups: Aldol reaction. The product may have two new stereocenters. The product contains a b-hydroxycarbonyl motif. Aldol reaction can be reversible.

10 Electron Flow in Aldol Reaction
Aldol reaction: Formation of new C-C bond In acidic condition b a + Enol Keto Retro-Aldol reaction: Breaking of C-C bond between a and b carbons + b a Enol Keto

11 Electron Flow in Aldol Reaction
Aldol reaction: Formation of new C-C bond In basic condition H+ b + a Keto Enolate Retro-Aldol reaction: Breaking of C-C bond between a and b carbons + b a Enolate Keto

12 Retro-Aldol Reaction in Glycolysis
Retro-Aldol catalyzed by fructose-1,6-bisphosphate aldolase Fructose-1,6-bisphosphate aldolase + Reaction 4 Fructose-1,6-bisphosphate Glyceraldehyde-3-phosphate Dihydroxyacetone phosphate

13 How does Retro-Aldol Reaction Generate Two 3-Carbon Sugars?
Equilibrium between cyclic hemiketal and opened form 1 + H+ b 2 Fructose-1,6-bisphosphate Opened form Cyclic hemiketal form 1 : Retro-aldol reaction 2 : Keto-enol tautomerization

14 Aldol Reaction in Citric Acid Cycle
Citrate synthesis catalyzed by citrate synthase + Citrate synthase Reaction 1 acetyl CoA Citrate oxaloacetate 2 HS-CoA 1 oxaloacetate H2O 1 : Aldol reaction 2 : Hyrolysis of thioester

15 Learning Check 1. What represent the correct aldol reactions?
(a) I, III, IV (b) I, II, III (c) I, II, IV (d) II, IV (e) None of the above I II III IV

16 Formation of new C-C bond by combining carboxyl and enol (enolate)
Claisen Condensation Formation of new C-C bond by combining carboxyl and enol (enolate) + b Enol (Acetyl CoA) Carboxyl (Ester) + ROH Forging two carboxyl functional groups and removal of a small molecule: Claisen condensation. The electron flow is similar to that of an aldol raction. The product contains a b-ketocarbonyl motif that can undergo decarboxylation spontaneously. Claisen condensation can be reversible.

17 Electron Flow in Claisen Condensation
Claisen condensation: Formation of new C-C bond + b Enol (Acetyl CoA) Keto (Acetyl CoA) a HSCoA b-ketocarbonyl Reaction in ketogenesis!

18 Electron Flow in Retro-Claisen Condensation
Retro-Claisen condensation: Breaking of a C-C bond HSCoA b 1 a + 2 1 : Similar to the formation of hemiketal 2 : Keto-enol tautomerization The 4th reaction in the repeated cycle of b-oxidation.

19 Carboxylation and Claisen Condensation
Synthesis of malonyl CoA catalyzed by acetyl CoA carboxylase Acetyl CoA Carboxylase + Malonyl CoA Bicarbonate ATP ADP + Pi (for lipogenesis) Bicarbonate: water soluble form of carbon dioxide + H+ + H+ - H+ - H+ Carbonate Bicarbonate Carbonic acid CO2 + H2O

20 Mechanism in the Synthesis of Malonyl CoA
Acetyl CoA carboxylase contains biotin as the carboxyl group carrier. 1 1 ATP ADP Pi + 2 H+ Biotin-enzyme 2 Acetyl CoA Biotin-enzyme Malonyl CoA 1 : Similar to transacylation 2 : Claisen condensation

21 Decarboxylation Is Similar to a Retro-Claisen Reaction
a-ketoglutarate synthesis catalyzed by isocitrate dehydrogenase isocitrate dehydrogenase + CO2 Reaction 3 a-ketoglutarate isocitrate 2 1 NAD+ NADH 1 : Oxidation 2 : Decarboxylation intermediate

22 Electron Flow in Decarboxylation
The molecule contains b-ketocarbonyl motif can undergo decarboxylation spontaneously. CO2 H+ b a-ketoglutarate enolate

23 Claisen Condensation in Lipogenesis
Reaction catalyzed by b-ketoacyl ACP synthase b-ketoacyl ACP synthase + CO2 + ACP-SH CO2 2 1 + H+ ACP-S- ACP-SH 2 1 : Decarboxylation 2 : Claisen condensation

24 Carboxylation in Calvin Cycle and Claisen Condensation
Synthesis of 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) Ribulose-1,5-bisphosphate carboxylase + Carbamate 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate ribulose-1,5-bisphosphate - H+ CO2

25 Mechanism in Carbon Dioxide Fixation
Synthesis of 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate catalyzed by Rubisco 1 2 ribulose-1,5-bisphosphate Enol intermediate 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate 1 : Keto-enol tautomerization 2 : Claisen condensation

26 Retro-Claisen Condensation in Calvin Cycle
Synthesis of two molecules of 3-phosphoglycerate catalyzed by Rubisco H2O H+ 1 2 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate hydrated (ketal) intermediate two molecules of 3-phosphoglycerate 1 : Hydration of ketone 2 : Retro-Claisen condensation

27 Electron Flow in the Synthesis of 3-Phosphoglycerate
+ H+ hydrated (ketal) intermediate two molecules of 3-phosphoglycerate

28 Learning Check 1. What represent the correct Claisen condensations?
Main Menu 1. What represent the correct Claisen condensations? (a) I, III, IV (b) I, II, III (c) I, II, IV (d) II, IV (e) None of the above I II III IV

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