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1. Lipid Metabolism

JS
by Janina S.

Describe the synthesis of fatty acids and membrane lipids in a plant cell. Don’t focus on a detailed description of every single reaction but provide a more general description by making use of the following keywords: plastid/ER, cytosol, Kennedy pathway, DAG-pathway, plasmamembrane.

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Fatty acid synthesis in the plastid. Export to cytosol, incorporated through the Kennedy pathways into glycerolipids in the ER, DAG-pathway: choline or ethanolamine is added, transport to plasmamembrane.


Biosynthesis of saturated fatty acids of normal length (Chloroplast):

  • Acetyl-CoA is produced from pyruvate by pyruvate dehydrogenase in plastids

  • Acetyl-CoA is converted into malonyl-CoA

  • Carrier system exchange: CoA is replaced by ACP (acyl carrier protein)

  • Four reactions that are repeated in a cycle:

    • Condensation: Malonyl-ACP/unfinished fatty acid-ACP and acetyl-CoA (KASIII)

    • Reduction: 3’-keto group is reduced to OH

    • Dehydration: Elimination of OH results in double bond in alpha-position

    • Reduction: double bond is reduced

  • Result: C16 or C18 fatty acid (KAS I until C16, KAS II for C18)

 

Desaturation by a desaturase in the plastid (only 18:0 to 18:1Δ9)


Acyl-ACP can be used within the plastid for the synthesis of plastid membrane lipids OR exported to the cytosol (carrier exchange to CoA)

Synthesis of glycerolipids in the plastid or the ER by transfer of the fatty acid to glycerol-3-phosphate à diacylglycerin (DAG)

 

Headgroup is bound via DAG pathway (activation of head group with phosphate and CDP, Kennedy pathway, major route in eukaryotes) or CDP-DAG pathway (activation of the DAG with CDP)

 

Further desaturation by acyllipid desaturases and elongation by elongases in the ER membrane (only react with esterified fatty acids)

 

Transport of lipids via the ER to the respective organelles/membranes (e.g., plasmamembrane)





Author

Janina S.

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