L18 - PLant Lipids Flashcards
How are lipids useful for plants?
How are lipids important for humans?
- Produce more ATP per C than carbohydrates, useful plant energy store
- Enables smaller seeds and proposed responsible for wind dispersal
- Vegetable oils = 25% of dietary calories in developed economies
- Used in lubricants and bulk chemicals
- Potential biodiesel source
Where are most storage lipids found in seeds?
What proportion of seed reserves can lipids make up?
- Endosperm or cotyledon
- Depends on species, can represent majority of reserves, e.g. 75% dry weight in nuts
What is the structure of storage lipids?
What other lipids are they structurally related to?
Sketch the general formula
- Storage lipids are TriAcylGlycerols (TAGs)
- Structurally + chemically related to phospholipids (in membranes) and glycolipids (in thylakoid membranes)
Where in the plant are most TAGs made?
Where does the C needed come from?
Describe the synthesis of the basic components of TAGs.
- Most TAGs made in sink tissues for storage
- Majority of C skeletons from sucrose
- In some seeds light can penetrate in for small amount of photosynthesis + C supply
- Synthesis includes multiple pathways mainly in cytosol, chloroplast + ER
- Glycerol backbone derived from 3C molecules DHAP and GAP generated from sucrose via glycolysis
- FAs generated from Acetyl CoA after decarboxylation of pyruvate
How are FAs classified
- By chain length and no. of double bonds
- E.g. major de novo FA synthesis products in plastids are 16:0 and 18:1
How are TAGs synthesised from FAs and a Glycerol Backbone? Give 5 steps
1) FAs synthesised in plastids are exported to the ER
2) Trans-esterification of 3 FAs onto Glycerol 3-Phosphate backbone catalysed by acyltransferases (AT):
3) GPAT transfers first FA to glycerol-3-phosphate
4) LPAT transfers second FA
5) Phosphate removed and DGAT transfers third FA to make TAG
- ATs determine oil composition due to specificity for FA and position on backbone
Which gene controls FA synthesis to a large extent?
- WRINKLED1 (WR1) gene, impacts function of WR1 TF
- Antisensed WR1 gene = low TAG accumulation
- Overexpressed WR1 = increased seed oil
- Shows power of targeting master regulators
- Identified after screen for Arabidopsis w/ low seed lipid content
How are TAGs stored after synthesis?
- After synthesis in ER, TAGs form oil bodies in cytosol
- Spherical, 1µm diameter
- Surrounded by one layer of phospholipids that form hydrophobic interactions with TAGS
What determines the character and use of a seed oil?
- Composition, particularly of FAs
- > 300 types of FAs
- Novel forms could be introduced. E.g. crops that synthesis fish oils
- Modifications to FAs occur on glycerol backbone
Describe the initial steps of the mobilisation of lipid stores
1) Triglycerides hydrolysed three times by lipase, removing a FA each time: 3 free FAs and glycerol formed
2) Glycerol metabolised via glycolysis
3) FAs enter β-oxidation cycle - converted to Acetyl CoA (2C). 1NADH & 1FADH2 also generated per cycle
What is the problem faced by converting Acetyl CoA to usable energy?
- Acetyl CoA next needs to be converted to sucrose, reverse of respiration
- Large decrease in free energy so cannot be reversed directly
- TCA cycle could be used but two CO2 lost in cycle, not efficient
What cycle has been proposed in the processing of Acetyl CoA to sucrose?
Why is this cycle more efficient?
Describe the crucial parts of the cycle
Where does this cycle occur in the plant?
Sketch a diagram of this cycle
- The Glyoxylate cycle
- C4 acid synthesised from Acetly CoA, no C lost
- Isocitrate lyase catalyses conversion of Isocitrate to Succinate and Glyoxylate (in TCA cycle conversion of Isocitrate to Succinate loses two CO2)
- Malate synthase then catalyses formation of malate from glyoxylate and another Acetly CoA
- Occurs in glyoxysomes, specialised peroxisomes
- See diagram on pg 19
Describe the evidence for the Glyoxylate Cycle being used in lipid mobilisation
- Germinating Castor Bean first identified it
- Two previously unknown enzymes, Isocitrate lyase (ICL) and Malate synthase (MS) found
- Each enzyme present in adequate amounts
- Cycle only seen during conversion of oil to sugar
- C14-acetate fed to endosperm of germinating castor bean appears preferentially in malate, not released as CO2 (as in TCA cycle)
How is the lipid mobilisation pathway regulated?
- ICL and MS not normally expressed - induced during lipid moblisation
- E.g. within 2 days of imbibition of castor beans, ICL and MS increase dramatically and lipid levels fall.
- As seedling transitions from heterotrophic to autotrophic growth, enzymes decline
- Due to gene expression changes (coarse control)
Describe the final steps of lipid mobilisation, after malate has been formed
- Malate converted to PEP via PEP carboxykinase
- PEP undergoes gluconeogenesis
- One CO2 molecule lost in gluconeogenesis per two Acetyl-CoAs converted to sugar (unavoidable C loss)
