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

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

Examples of Enol – Keto Tautomerization

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)

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

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

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

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

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.

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

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

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

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

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

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

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.

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!

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.

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

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

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

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

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

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

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

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

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

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