Fatty acid biosynthesis Flashcards
Give an example of specialised signalling molecules
phosphatidyl inositol
What is the biosynthesis of fatty acids
- Synthesis requires energy and reducing equivalents- opposite of breakdown (beta-oxidation)
- Breakdown (oxidation) of fatty acids- releases energy and reducing equivalents
- The body is a dynamic system in which fatty acids are constantly being broken down and synthesised
How much fatty acids are generally turned over per day
- 200g of fatty acids are turned over per 24 hrs
2. This turnover accounts for 8% of the body’s basal 02 consumption
What is the major biosynthetic product of fatty acid synthesis
- Palmitic acid: C16:0
2. CH3(CH2)14COOH
What do most naturally occurring fatty acids have
- Most naturally occurring fatty acids have an even number of carbon atoms
- Synthesised in way that adds 2 carbon units at a time to the growing fatty acid chain
Where does breakdown occur and where does synthesis occur
- Breakdown- mitochondria
2. Synthesis- cytoplasmic
What else does breakdown need
- NAD+/NADH
2. No CO2
What does synthesis need
- NADP+/NADPH
2. CO2 required as HCO3-
Describe the first two steps in the experiment which showed that synthesis occurs in the cytosol
- Homogenise tissue- make into a mixture that looks all the same- by blending tissue
- release internal contents
- Centrifuge at 100000 x g
- Centrifuge separates contents-Lumpy and heavy things sink quickly
- Forms a soluble fraction and a pellet
Describe what is found in the soluble fraction and pellet
- Soluble fraction-
a) No membrane fractions
b) Only soluble proteins - Pellet-
a) Nuclear material
b) Mitochondrial fragments
c) All membranous debris
What happens once the soluble fraction and pellet have formed
- Can then assay pellet and soluble fraction to see where activity is- which will synthesis fatty acid
- Soluble fraction- synthesis fatty acids
- Treat with a salt (proteins have a hydration cell with lots of water molecules in the outside and hold each other in solution)
- Take away water with salt and so doesn’t have a hydration cell around it and so comes out of solution
- Makes solution more concentrated effectively
- See which protein precipitate first and eventually have all the proteins without the salt or water
- Fractionate the original and test each fraction to see if it has the reaction were looking for
What was the result of the experiment
- none of fractions give fatty acid biosynthesis on its own
- Only need to combine 2 fractions (unusual because lots of enzymes involved in breakdown)
- One of these fractions contain biotin
What are the two steps of fatty acid biosynthesis
- The ATP-dependent carboxylation of acetyl-CoA by acetyl-CoA carboxylase forming malonyl-CoA
- Exergonic decarboxylation of malonyl group in the condensation reaction catalysed by fatty acid synthase
Describe the basics of the first step of fatty acid biosynthesis
- Activation step Acetyl-CoA is carboxylated to form malonyl-CoA
- This step does not occur without biotin
- Catalysed by acetyl CoA carboxylase
- This stage is irreversible
- Is a two-step process very similar to that of pyruvate carboxylase
Describe the mechanism of the first step of fatty acid biosynthesis
- First carbon part from bicarbonate ion attaches to biotin which is covalently linked to biotin to form biotin-CO2 complex
- This is transcarboxylated to move CO2 on to acetyl part of CoA
- Acetyl-CoA carboxylase is not a single protein- made up of 3 protein parts each carry out different thing
Give the equations for the first step of fatty acid biosynthesis
- Overall: Acetyl-CoA + bicarbonate + ATP –> malonyl-CoA + water + ADP + Pi
- First part catalysed by biotin carboxylase:
Biotin + HCO3- + ATP –> Biotin-CO2- + ADP + Pi - Second part catalysed by transcarboxylase:
Biotin-CO2- + acetyl-CoA –> biotin + malonyl-CoA
Describe the basics of the second step of fatty acid biosynthesis
- Chain elongation
- Long chain fatty acids are formed from acetyl-CoA and malonyl-CoA by the successive additions to a growing chain
- Catalysed by fatty acid synthase/synthetase
Describe the mechanism of the second step of fatty acid biosynthesis
- Enzyme takes single acetyl-CoA and multiple malonyl-CoA (7) and 14 NADPH (2 for each malonate) and releases CO2 and CoA
- 1 acetyl CoA - primes the reaction and becomes the methyl terminus of the fatty acid chain
- 7 malonyl CoA - each provides a 2 carbon unit to extend the chain by 7x2 = 14 carbons
- Total length is 14 + 2 = 16
What is the overall reaction for the second step in the biosynthesis of palmitate
- acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H+ –> palmitate + 7CO2 + 14NADP+ + 8CoA + 6H2O
What is the overall equation for the biosynthesis of pamlitate
- 8 acetyl-CoA + 7ATP + 14NADPH + 14H+ –> palmitate + 8CoA + 6H2O + 7ADP + 7Pi + 14NADP+
Describe fatty acid synthase
- Catalyses seven separate reactions each of which has to have an active site
- is found in the cytosolic fraction of cells
- requires malonyl-CoA & acetyl-CoA
- requires NADPH
- relatively insoluble proteins
- no free, unbound intermediates accumulate-
- 4 phosphopantetheine required
What is the first reaction of fatty acid synthase reactions to form palmitate
- Acetyl CoA is first attached to the protein ACP (ACYL CARRIER PROTEIN)- by acetyl transacylase.
- The Acetyl is then transferred to the beta-ketoacyl ACP synthase protein/domain
What is the second reaction of fatty acid synthase reactions to form palmitate
- Malonyl transacylase then transfers a malonyl group to ACP from malonyl CoA
What is the third reaction of fatty acid synthase reactions to form palmitate
- Condensation and decarboxylation reaction catalysed by beta-ketoacyl-ACP synthase
- acetyl-synthase + malonyl-ACP form beta-ketoacyl-ACP and CO2 is released
- Energy comes from cleavage of CO2
What is the fourth reaction of fatty acid synthase reactions
to form palmitate
- Reduction catalysed by beta-ketoacyl ACP reductase
- beta-ketoacyl-ACP reduced to beta-hydroxyacyl-ACP
- NADPH + H+ –> NADP+
What is the fifth reaction of fatty acid synthase reactions to form palmitate
- DEHYDRATION catalysed by beta-hydroxyacyl-ACP dehydrase
- removes water –> double bond
- beta-hydroxyacyl-ACP forms trans enoyl-ACP
What is the sixth reaction of fatty acid synthase reactions to form palmitate
- REDUCTION catalysed by Enoyl-ACP reductase
- This acyl group is then transferred to the β-ketoacyl synthase domain of the protein- this was where the original acetyl group was attached
- Freeing the ACP to bind another malonyl group
- trans enoyl-ACP forms acyl-ACP
What is the seventh reaction of fatty acid synthase reactions to form palmitate
- palmitoyl-ACP –> palmitate + ACP-SH
2. Catalysed by palmitoyl thioesterase
At what point are the steps repeated to build up the fatty acid chain
- After step 6
- With a C4 acyl group attached to the β-ketoacyl synthase domain and Malonyl attached to ACP the steps can begin again to produce a C6 acyl.
- This is repeated until there is a C16 acyl group attached to ACP- palmitoyl-ACP
Where is palmitate mainly produced
- Major product in mammalian liver
2. Other tissues and species produce different products – in those a secondary thioesterase is likely active
