BMSC230 Mod. 12

• Humans can only synthesize some amino acids while Bacteria and yeast can synthesize all 20
• Vertebrates can't fix nitrogen in a useable form
• Nitrogen assimilation


  The forming of organic N molecules from inorganic N compounds in the environment

  
• Nitrogen fixation


  Bacteria in soil and plant roots fix N2 gas into usable forms like NH4+, nitrites and nitrates

  
• NH4+ is the end point of the Nitrogen Cycle
• Overall nitrogen fixation reaction

  N2 + 10H+ + 16ATP → 2NH4+ + H2 + 16ADP + 16Pi

  
• Incorporation of NH4+ into glutamate and glutamine

  Enzyme glutamate dehydrogenase uses NADPH or NADH

  2. Enzyme glutamine synthetase uses glutamate, NH4+, ATP to produce glutamine

  3. Enzyme glutamate synthase uses glutamine and α-ketoglutarate to produce 2 glutamates

  
• Amino group transfer


  Glutamate/glutamine donate amino groups to α-ketoacids to form most amino acids, catalyzed by aminotransferases

  
• Glutamine synthetase


  Major enzyme in nitrogen assimilation, present in all organisms, catalyzes reaction: Glutamate + NH4+ + ATP → Glutamine + ADP + Pi

  
• Glutamine synthetase regulation


  • Highly regulated by covalent modification and allosteric inhibition by 8 N-containing molecules

  Adenylylation of a tyrosine residue near the active site sensitizes the enzyme to inhibitors
• One-carbon transfer reactions


  Required to synthesize some amino acids and nucleotides

  1. S-adenosylmethionine (SAM) donates methyl groups

  2. Tetrahydrofolate carries various one-carbon units

  
• One-carbon transfer reactions

  • Methylation of norepinephrine to epinephrine

  Synthesis of methionine from homocysteine

  Serine synthesis from glycine

  
• Amino group transfer


  Glutamine/glutamate donate amino groups, e.g. Aspartate to Asparagine

  
• Folate


  Vitamin B9
• Folate reduction


  1. Dietary folate is reduced by NADPH to dihydrofolate

  2. Then reduced to tetrahydrofolate (again by NADPH)

  3. Tetrahydrofolate is the active form in the cell

  4. Both catalyzed by dihydrofolate reductase
• 1 carbon units carried by tetrahydrofolate

  • N5/N10 indicate where they attach

  
• Sources of 1 carbon units

  • Serine
  • Formate
• Oxygen is provided by formate, nitrogen is provided by NH3+
• Requires either ATP or NADPH
• Synthesis of methionine from homocysteine

  • Requires methylation catalyzed by methionine synthase
  • Uses N5-methyltetrahydrofolate for methyl donor
  • N5,N10-methylene-tetrahydrofolate used in serine synthesis from glycine

  
• Amino group transfer


  1. Glutamine/glutamate donor

  2. Example: Aspartate -> Asparagine

  
• Histidine synthesis in plants

  1. ATP + PRPP -> Phosphoribosyltransferase (ATP-PRT)

  2. IGP -> Imidazolecetol-P aminotransferase

  3. Phosphoribosyl-AMP cyclohydrolase

  4. Histidinol-P phosphatase

  5. Histidinol Dehydrogenase

  
• Biosynthetic pathways grouped by 6 metabolic precursors


  • α-Ketoglutarate
  • Pyruvate
  • 3-phosphoglycerate
  • Oxaloacetate
  • Ribose 5
  • Erythrose 4

  
• Bacterial and plants can synthesize all amino acids, mammals can only synthesize some - the rest are required from diet
• Types of amino acids

  • Nonessential (we can synthesize)
  • Essential (required through diet)
  • Conditionally essential (only essential at certain times)
• Lysine synthesis


  1. From aspartate through many complex reactions

  2. Glutamate used as amino group donor

  
• Product inhibition


  An end product acts as an allosteric inhibitor of the first committed step to prevent unnecessary production
• Types of product inhibition

  • Sequential inhibition (one or more branch points with 2+ products)
  • Concerted inhibition (both end products inhibit the same enzyme)
• Enzyme multiplicity


  • First committed step done by 2 isoenzymes (coded by different genes)
  • Isoenzymes inhibited by 1 of 2 end products
• Aspartate as a precursor

  • Aspartyl-β-phosphate has 3 isoenzymes
  • A1 inhibited by lysine + isoleucine
  • A2 not subject to feedback inhibition
  • A3 inhibited by threonine

  
• Ensures flux exists through the pathway