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AMINO ACID METABOLISM

AMINO ACID METABOLISM. AMINO ACID METABOLISM. WHAT A PLANT CAN DO TO AMINO ACIDS. 1. 2. oxidative deamination decarboxylation. oxidative deamination. [O] -NH 3 -CO 2. [O] -NH 3. deamination. decarboxylation. -NH 3. -CO 2. 3. 4.

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AMINO ACID METABOLISM

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  1. AMINO ACID METABOLISM

  2. AMINO ACID METABOLISM WHAT A PLANT CAN DO TO AMINO ACIDS 1 2 oxidative deamination decarboxylation oxidative deamination [O] -NH3 -CO2 [O] -NH3 deamination decarboxylation -NH3 -CO2 3 4 Processes 1 2 3 use PLP, pyridoxyl-5’-phosphate (B6) coenzyme Process 4 uses FAD, flavine adenine dinucleotide (B2) coenzyme and the coenzyme NADH (both discussed later)

  3. COENZYMES NATURE’S CHEMICAL REAGENTS

  4. S Substrate C Coenzyme CRUDE MODEL OF AN ENZYME-MEDIATED REACTION The cofactor may cause an allosteric change when it binds. M+ Polypeptide Chain Cofactor ACTIVE SITE NH3+ -OOC binding forces catalytic groups the coenzyme is the reagent S P -NH2+ proton donor (acid) -COO- proton acceptor (base) reaction takes place here C

  5. COENZYMES AND COFACTORS Coenzyme The biochemical “reagent” that brings about the transformation. These are usually small organic molecules, and they too undergo change upon reaction. Metabolic processes are brought about by a relatively small number of these coenzymes. Nature has a limited repetoire of about 2-3 dozen coenzymes but can nevertheless bring about amazing changes. There is no magic, all changes are chemical reactions that can be understood by the principles of organic or inorganic mechanisms. Cofactor A small ion or molecule that is required for the enzyme to function. The cofactor often brings about a necessary “allosteric” change in the enzyme when it binds.

  6. THE ENZYME-MEDIATED REACTION X All three products, (X, Y, Z) are possible when A and B are mixed together in a flask (in vitro). A + B Y Z The enzyme will selectively enhance (catalyze) one pathway over the other. In addition, it can control the stereochemical outcome. The amount of Z can also be controlled by feedback mechanisms and cofactors.

  7. COENZYMES THAT MODIFY AMINO ACIDS ( see slide #2 ) 1 Pyridoxyl-5’-phosphate PLP ( Vitamin B6 ) 2 3 Cofactor = Co+3 Flavine adenine dinucleotide FAD ( Vitamin B2 ) Cofactor = Mg+2 4

  8. REVIEW OF IMINE CHEMISTRY

  9. Recall : Chapter 16, Organic Text Formation of Simple Imines remove primary amine aldehyde or ketone These reactions do not favor the formation of the imine unless: - the product (imine) is insoluble (crystallizes or precipitates) or - water is removed to drive the equilibrium - the reaction is enzyme-mediated

  10. H H-O H H H-O-H + H-O-H + + H-O-H H-O H H Mechanism of Imine Formation weak base addition - acid catalyzed 2 1 acid-catalyzed addition proton exchanges an imine .. + + loss of water (elimination) deprotonation

  11. Hydrolysis of Simple Imines REVERSAL In an excess of aqueous acid, simple imines hydrolyze back to the aldehyde or ketone and the amine from which they were orginally formed ….. H3O+ Imines that are not soluble, however, are difficult to hydrolyze.

  12. COENZYME MECHANISMS

  13. SIMPLIFIED COENZYMES PLP FAD Reduced to the chemically significant portion - the part that undergoes change.

  14. Pyridoxyl-5’-phosphate (PLP) REACTS WITH AMINO ACIDS TO FORM IMINES .. an amino acid + - H2O pyridoxyl-5’-phosphate ( P5’P ) PLP formation of the imine + imine

  15. THE IMINE CAN LOSE A HYDROGEN AND REPLACE IT ( or …. next page ) Enz-B: : electron flow electron flow -H+ .. + The aromatic ring is restored by adding a proton at another site in the conjugated system. +H+ The positively-charged nitrogen is an “electron sink” attracting the released pair of electrons. It helps to acidify the proton which is removed. Electrons flow downward through the conjugated system to the nitrogen. + A

  16. THE IMINE CAN LOSE A PROTON AND CARBON DIOXIDE AND REPLACE THE PROTON .. :B-Enz .. + H-B-Enz + - CO2 .. -H+ + same imine +H+ +H+ + + B C

  17. HYDROLYSIS + + C + B A H2O H2O H2O

  18. TRANSAMINATION

  19. TRANSAMINATION An amino group is transfered from one amino acid to an a-ketoacid, thereby creating a new amino acid and converting the original amino acid to an a- ketoacid. PLP + + transfer via pyridoxamine +NH2 -NH2 pyridoxyl pyridoxamine pyridoxyl

  20. Pyridoxyl-5’-phosphate (PLP) Converts amino acids to a-ketoacids, and vice versa. Biologically important in transamination reactions. .. + an amino acid pyridoxyl-5’-phosphate - H2O ( PLP ) continued formation of the imine + first imine

  21. :Enz Enz-H converts tautomerism H-Enz + Enz: a-ketoacid first imine H2O + new imine Removing the amino group hydrolysis of the new imine + pyridoxamine

  22. TRANSFERRING THE AMINO GROUP a different a-ketoacid pyridoxamine tautomerism hydrolysis of the imine These steps are the reverse of those on the previous slides. a different amino acid

  23. + + + + SUMMARY TRANSAMINATION PLP PLP We will take a close look at the implications of this process in the next lecture (uptake of nitrrogen).

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