3. Sulfur Assimilation

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3. Sulfur Assimilation
2011-11-12 19:59:43
PMB 135 exam5

plant physiology and biochemistry exam 5
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  1. overall view of sulfur assimilation
    • 1. sulfate to APS
    • 2. APS to sulfide
    • 3. sulfide to cysteine

    • overall process is sulfate reduced to cysteine
    • cysteine forms many sulfur compounds - glucosinolates, glutathione, methionine
    • cys and met are incorporated into proteins
    • site of sulfur assimilation:
    • all enzymes present in plastids
    • chloroplasts are primary site for cys synthesis
    • root plastids also active
    • part X - met from cys
    • SAM - met forms S-adenyl-L-methionine, important methyl donor
  2. ATP sulfurylase
    enzyme for sulfate activation - catalyzes the conversion of sulfate and magnesium-ATP into high-energy molecule, APS and magnesium pyrophosphate

    APS and pyrophosphate are removed from the reaction - achieved through the action of inorganic pyrophosphate which hydrolyzes pyrophosphate pulling the reaction to the right

    • regulation of sulfate assimilation:
    • increase in activity in response to sulfate starvation
    • decrease in activity by feeding reduced sulfur compounds, gene expression regulated by the demand for pathway end products
  3. How many forms of ATP sulfurylase are there?
    • chloroplast isoform - accounts for most of total
    • cytoplasmic isoform
    • root plastid isoform
  4. reduction of APS to sulfite
    • APS sulfotransferase: APS + GSH -> S-Sulfoglutathione + 5’-AMP
    • APS reductase: S-Sulfoglutathione + GSH -> SO32- + GSSG

    • the reduction of glutathione in plants is controlled by NADPH-dependent glutathione reductase
    • source of electrons traced to NADPH generated by photosynthetic light reactions or oxidative phosphate pathway

    regulation - concentration of APS reductase mRNA is increased by S starvation and repressed by treating plants with reduced forms of sulfur
  5. reduction of sulfite to sulfide
    • leaves: sulfite reductase adds six electrons from reduced ferredoxin to free sulfite, forming sulfide
    • ferredoxin is reduced directly by the photosynthetic light reactions in leaves

    roots: sulfite reductase is found in the plastids where reductant NADPH is generated by the oxidative pentose pathway

    sulfite is a toxic anion - would cause cell damage if it accumulated, maintained in excess of preceding enzyme, APS reductase
  6. sulfide to cysteine
    Serine -> O-acetylserine -> cysteine

    Serine acetyltransferase: Serine + acetyl-CoA -> O-acetylserine + CoA

    pyruvate dehydrogenase: in plastids, can be used to generate acetyl-CoA

    O-acetylserine(thiol)lyase: O-acetylserine + S2- -> cysteine + acetate
  7. cysteine to methionine
    • Cysteine -> Cystathionine -> Homocysteine -> Methionine
    • transsulfuration pathway: the thiol group of cys is transferred to homoserine to produce homocysteine through a cystathione intermediate
    • cystathione gamma synthase: enzyme combines cys with homoserine-4-phosphate to form cystathione
    • cystathione-beta-lyase: converts cystathione into homocysteine, cleavage makes the reaction irreversible
    • methionine synthase: methylates homocysteine with tetrahydrofolate to form methione, functions in recycling S-adenosyl-L-homocysteine
  8. What happens to methione?
    • transmethylation reactions - met incorporated into lipids, pectins, chlorophyll, and nucleic acids
    • production of S-adenosylmethione (SAM), a very important methyl donor
    • - energy-demanding process
  9. What is SAM, a methyl donor, used in?
    • nucleic acids - DNA and RNA modification
    • structural components - pectin, lignin precursors, choline
    • ethylene - carbon skeleton of the met moeity of SAM is used as a precursor to plant hormone ethylene and polyamines