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27.4 Synthesis of Amino Acids

27.4 Synthesis of Amino Acids. O. O. CH 3 CHCOH. CH 3 CHCO. Br. N H 3. From a -Halo Carboxylic Acids. H 2 O. –. +. +. 2 N H 3. N H 4 Br. +. (65-70%). O. CH 3 CH. CH 3 CHC. N. N H 2. 1. H 2 O, HCl, heat. 2. HO –. O. –. CH 3 CHCO. +. N H 3. Strecker Synthesis.

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27.4 Synthesis of Amino Acids

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  1. 27.4Synthesis of Amino Acids

  2. O O CH3CHCOH CH3CHCO Br NH3 From a-Halo Carboxylic Acids H2O – + + 2NH3 NH4Br + (65-70%)

  3. O CH3CH CH3CHC N NH2 1. H2O, HCl, heat 2. HO– O – CH3CHCO + NH3 Strecker Synthesis NH4Cl NaCN (52-60%)

  4. O O C C C OCH2CH3 CH3CH2O CH3CNH H O Using Diethyl Acetamidomalonate • Can be used in the same manner as diethyl malonate (Section 21.7).

  5. O O CH3CH2OCCCOCH2CH3 H CH3CNH O O O CH3CH2OCCCOCH2CH3 CH2C6H5 CH3CNH O Example 1. NaOCH2CH3 2. C6H5CH2Cl (90%)

  6. O O CH3CH2OCCCOCH2CH3 CH2C6H5 CH3CNH O O O Example HOCCCOH CH2C6H5 H3N + HBr, H2O, heat

  7. O O CH3CH2OCCCOCH2CH3 CH2C6H5 CH3CNH O O Example (65%) HCCOH CH2C6H5 H3N + HBr, H2O, heat

  8. 27.5Reactions of Amino Acids

  9. O O O + – H3NCH2CO CH3COCCH3 O O CH3CNHCH2COH Acylation of Amino Group • The amino nitrogen of an amino acid can be converted to an amide with the customary acylating agents. + (89-92%)

  10. O + – H3NCHCO CH3 O – Cl Esterification of Carboxyl Group • The carboxyl group of an amino acid can be converted to an ester. The following illustrates Fischer esterification of alanine. + CH3CH2OH HCl + H3NCHCOCH2CH3 (90-95%) CH3

  11. O O + – OH H3NCHCO OH R O – O O O N RCH O O Ninhydrin Test • Amino acids are detected by the formation of a purple color on treatment with ninhydrin. + + + + CO2 H2O

  12. 27.6Some Biochemical Reactionsof Amino Acids

  13. O HO2CCH2CH2CCO2H – HO2CCH2CH2CHCO2 + NH3 Biosynthesis of L-Glutamic Acid • This reaction is the biochemical analog of reductive amination (Section 22.11). + NH3 enzymes and reducing coenzymes

  14. O – + HO2CCH2CH2CHCO2 CH3CCO2H + NH3 Transamination viaL-Glutamic Acid L-Glutamic acid acts as a source of the amine group in the biochemical conversion of a-ketoacids to other amino acids. In the exampleshown, pyruvic acid is converted to L-alanine.

  15. O – + HO2CCH2CH2CHCO2 CH3CCO2H + NH3 O – CH3CHCO2 HO2CCH2CH2CCO2H + NH3 Transamination viaL-Glutamic Acid enzymes +

  16. O – + HO2CCH2CH2CHCO2 CH3CCO2H + NH3 Mechanism The first step is imine formation between theamino group of L-glutamic acid and pyruvicacid.

  17. O – + HO2CCH2CH2CHCO2 CH3CCO2H + NH3 Mechanism – HO2CCH2CH2CHCO2 N – CH3CCO2

  18. – HO2CCH2CH2CCO2 N – CH3CCO2 • Formation of the imine is followed by proton removal at one carbon and protonation of another carbon. H

  19. HO2CCH2CH2CCO2 N – CH3CCO2 H – HO2CCH2CH2CCO2 N – CH3CCO2 H

  20. HO2CCH2CH2CCO2 N – CH3CCO2 H – • Hydrolysis of the imine function givesa-keto glutarate and L-alanine.

  21. HO2CCH2CH2CCO2 N – CH3CCO2 H + NH3 – – HO2CCH2CH2CCO2 CH3CCO2 O H – H2O +

  22. – CH2CHCO2 + NH3 Biosynthesis of L-Tyrosine L-Tyrosine is biosynthesized from L-phenylalanine. A key step is epoxidation of the aromatic ring to give an arene oxide intermediate.

  23. – CH2CHCO2 O + NH3 – CH2CHCO2 + NH3 Biosynthesis of L-Tyrosine O2, enzyme

  24. – CH2CHCO2 O + NH3 – HO CH2CHCO2 + NH3 Biosynthesis of L-Tyrosine enzyme

  25. Biosynthesis of L-Tyrosine • Conversion to L-tyrosine is one of the major metabolic pathways of L-phenylalanine. • Individuals who lack the enzymes necessary to convert L-phenylalanine to L-tyrosine can suffer from PKU disease. In PKU disease, L-phenylalanine is diverted to a pathway leading to phenylpyruvic acid, which is toxic. • Newborns are routinely tested for PKU disease. Treatment consists of reducing their dietary intake of phenylalanine-rich proteins.

  26. N – CH2CHCO2 NH + NH3 Decarboxylation • Decarboxylation is a common reaction of a-amino acids. An example is the conversion of L-histidine to histamine. Antihistamines act by blocking the action of histamine.

  27. N CH2CH2 NH2 NH N – CH2CHCO2 NH + NH3 Decarboxylation –CO2, enzymes

  28. – CO2 + H3N H H H OH Neurotransmitters • The chemistry of the brain and central nervous system is affected by neurotransmitters. • Several important neurotransmitters are biosynthesized from L-tyrosine. L-Tyrosine

  29. Neurotransmitters – CO2 + • The common name of this compound is L-DOPA. It occurs naturally in the brain. It is widely prescribed to reduce the symptoms of Parkinsonism. H3N H H H HO OH L-3,4-Dihydroxyphenylalanine

  30. Neurotransmitters H • Dopamine is formed by decarboxylation of L-DOPA. H2N H H H HO OH Dopamine

  31. Neurotransmitters H H2N H OH H HO OH Norepinephrine

  32. Neurotransmitters H CH3NH H OH H HO OH Epinephrine

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