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UPTAKE OF NITROGEN BY PLANTS

UPTAKE OF NITROGEN BY PLANTS. THE NITROGEN CYCLE. proteins, nucleic acids electron carriers, alkaloids. Amino Acids. ETC. Higher Plants. Higher Plants. returned to the soil or the atmosphere. NO 3 -. NH 3. Nitrogen Fixing Bacteria. Nitrifying Bacteria

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UPTAKE OF NITROGEN BY PLANTS

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  1. UPTAKE OF NITROGEN BY PLANTS

  2. THE NITROGEN CYCLE proteins, nucleic acids electron carriers, alkaloids Amino Acids ETC Higher Plants Higher Plants returned to the soil or the atmosphere NO3- NH3 Nitrogen Fixing Bacteria Nitrifying Bacteria (Nitrobacter) Nitrifying Bacteria (Nitrosomas) N2 NO2-

  3. In this section we will study the mechanisms by which plants take up nitrogen. First, however, we need to look at a couple of coenzyme systems. NAD+ NADH NADP+ NADPH

  4. THE NAD+ / NADP+ COENZYMES

  5. NICOTINE ADENINE DINUCLEOTIDE NAD+ R = -H membranes NADP+ R = -PO3H cytosol Nature’s Hydride Reducing / Oxidizing Coenzyme (reagent) The two forms differ by a phosphate group which also controls the location in the cell.

  6. NAD+ and NADP+ ARE HYDRIDE ACCEPTORS :B-Enz H-B-Enz ketone or aldehyde -2e- alcohol OXIDATION : REDUCTION +2e- H : -2e- REDUCTION .. OXIDATION +2e- NADH NAD+ hydride transfers NADPH NADP+ NADH and NADPH ARE HYDRIDE DONORS Unlike ordinary chemical reagents, these coenzymes function reversibly. AROMATIC BUT CHARGED UNCHARGED BUT NOT AROMATIC

  7. BIOCHEMICAL REVERSIBILITY H:- H : -2e- REDUCTION .. OXIDATION +2e- NADH hydride acceptor hydride donor NAD+ NADPH NADP+ Unlike ordinary chemical reagents, these coenzymes function reversibly. UNCHARGED BUT NOT AROMATIC AROMATIC BUT CHARGED Many biochemical reagents are designed to reversible - this is the ultimate in conservation

  8. BIOCHEMICAL NOTATION oxidation NAD+ NADH reduction NADH NAD+

  9. OTHER FUNCTIONAL GROUPS CAN BE OXIDIZED / REDUCED .. : reduction imine NADH NAD+ amine Enz-B-H : This process is one of the steps of H : REDUCTIVE AMINATION one of the ways that plants take up nitrogen. ..

  10. REDUCTIVE AMINATION Enz-B-H imine - H2O .. : .. .. aldehyde or ketone H : .. .. NAD+ NADH amine

  11. LEUCKART REACTION REDUCTIVE AMINATION CLASSICAL REACTION + formate ion .. NH4+ HCO2- : .. D - : .. ammonium formate

  12. HOW PLANTS INCORPORATE NITROGEN

  13. FORMATION OF AMINO ACIDS a-ketoglutaric acid GLUCOSE GLYCOLYSIS NADH (NO3-) NH3 PRIMARY METABOLISM NAD+ glutamic acid CITRIC ACIDCYCLE . REDUCTIVE AMINATION Glutamic acid is the only amino acid that can be formed by direct uptake of nitrogen from the soil. a-ketoglutaric acid an intermediate from the citric acid cycle Other amino acids are formed by transamination.

  14. NITROGEN EXCHANGE TRANSAMINATION a-ketoacid PLP + + transaminase enzyme glutamic acid or other a-amino acid Most other amino acids are formed by using the -NH2 groups from glutamic acid in transamination reactions. However, any amino acid can subsequently transfer its amino group to an a-ketoacid to form a new amino acid. Plants have a severe need for nitrogen and they practice as much conservation as possible. Nitrogen is transferred to where it is needed most.

  15. SOME TRANSAMINATIONS phenylpyruvic acid a-ketoglutaric acid glutamic acid phenylalanine pyruvic acid alanine phenyl- alanine phenyl- pyruvic acid a-ketoglutaric acid alanine pyruvic acid glutamic acid

  16. FOR MANY OF THE a-AMINO ACIDS FOUND IN NATURE THERE IS A CORRESPONDING a-KETOACID* a-amino acid a-keto acid alanine pyruvic acid valine a-ketovaleric acid a-ketohexanoic acid leucine phenylalanine phenylpyruvic acid a-ketoglutaric acid glutamic acid tyrosine p-hydroxyphenylpyruvic acid * Some amino acids are made by dimerizations or other non-direct pathways.

  17. CARBAMOYL PHOSPHATE A Second Method of Transamination In some plants the major method of nitrogen uptake utilizes carbamoyl phosphate. CO2 + NH4+ + 2 ATP + H2O + 2 ADP + Pi carbamoyl phosphate :Nu .. + O=C=O + Nu-PO3H

  18. FAD - ANOTHER COENZYME A Return to Modification of Amino Acids Process 4

  19. 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)

  20. 4 FLAVINE ADENINE DINUCLEOTIDE Flavine adenine dinucleotide FAD ( Vitamin B2 ) Cofactor = Mg+2

  21. THIS TOO IS A REVERSIBLE COENZYME 2e- + 2 H+ + FAD FADH2 It is usually associated with the family of enzymes called amineoxidases. imine amine oxidase hydrolysis H2O a-ketoacid FAD FADH2

  22. 4 POSSIBLE CHEMICAL SEQUENCE FOR DEAMINASES imine amine oxidase hydrolysis H2O a-ketoacid FAD FADH2 NADH conversion of an amino acid to an a,b-unsaturated acid NAD+ dehydration - H2O a,b-unsaturated acid a-hydroxyacid

  23. THE MECHANISM FOR FAD OXIDATIONS AND REDUCTIONS IS NOT COMPLETELY KNOWN 1. It may be a radical-like process involving one-electron transfers. 2. A hypothetical ionic mechanism (mine, not proven) can also be formulated: .. .. .. .. .. Enz-BH Enz-BH .. .. + + .. .. Enz-B:

  24. AMINE OXIDASES Monoamine Oxidases (MAO) Diamine Oxidases

  25. AMINE OXIDASES Two types of amine oxidases are common: Monoamine oxidases and Diamine Oxidases Monoamine Diamine ….. we’ll see more of this later

  26. A Table of Coenzyme Function for Future Reference BIOLOGICAL COENZYMES The coenzymes are natures chemical reagents and each has tis own specific reactions that it can carry out. Amazingly, nature uses very few coenzymes to carry out the majority of its chemical transformations.

  27. COENZYMES - 1 MODIFIES AMINO ACIDS Pyridoxyl-5’-phosphate 1) Oxidative deamination to a-ketoacids 2) Transaminations 3) Oxidative deamination-decarboxylation to aldehydes 4) Decarboxylation to amines PLP (P5’P) Flavine adenine dinucleotide 1) oxidation of amines to imines (amine oxidases) 2) oxidation of hydrocarbons ( alkanes to alkenes ) FAD / FADH2 S-Adenosylmethionine 1) Biological methylation reagent for alcohols, phenols, amines SAM (SAdM) Nicotine Adenine Dinucleotide 1) Reduction of C=O 2) Oxidation of -CH-OH NAD / NADP NADH / NADPH

  28. COENZYMES - 2 Adenosine triphosphate 1) Energy storage 2) Modifies alcohols 3) Traps alcohols 4) Coupled reactions ATP 1) Transfers acyl groups Coenzyme A CoA 1) Carries / transfers CO2 in the form of -COOH groups Biotin Lipoic Acid 1) Oxidizes acyl groups 2) Acyl group transfer Thiamine pyrophosphate 1) Formation of acyloins (a-hydroxyketones) 2) Oxidative decarboxylations of a-ketoacids 3) Non-oxidative decarboxylations TPP

  29. COENZYMES - 3 While not strictly speaking coenzymes, these chemicals also bring about changes. Water 1) Hydrolysis 2) Hydration H2O Oxygen 1) Oxidations (hydroxylases) (oxidases) 2) Oxidative couplings (phenols) O2 This list is not exhaustive, but it includes the major coenzymes that we will be discussing.

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