Flashcards in Biochemistry - lipid anabolism Deck (23)
What is the name for the synthesis of new fatty acids?
Which tissues can carry out lipogenesis?
Liver, kidney, mammary glands, adipose tissue, brain
When does lipogenesis occur?
During periods of excess energy intake
What happens when excess carbohydrate is taken in?
It is converted to fatty acids and TAG in the liver
How are free fatty acids transported from the liver in the plasma?
bound to alubumin
How is TAG transported from the liver in the plasma?
It is packaged into VLDL
Is lipogenesis a reductive or oxidative process?
What does it therefore require?
It is reductive, therefore requires electrons.
Give a brief overview of lipogenysis from starch.
Starch --> glucose --> pyruvate -->acetyl coA--> Fatty acids --> triglycerides
What is the indirect precursor for the conversion of dietary carbohydrate into fatty acids?
Where does lipogenysis occur?
In the cytoplasm
Where does beta oxidation occur?
How is the acetyl coA transported into the cytoplasm?
In the mitochondrial matrix.
The inner mitochondrial membrane is impermeable to acetyl-coA but citrate can cross it. Acetyl coA is linked to oxaloacetate to form citrate. If a lot of citrate is present in the matrix it can be transported into the cytoplasm.
What is the direct precursor for the synthesis of fatty acids?
Describe fatty acid synthesis, starting from acetyl coA.
Acetyl coA is activated to malonyl coA by acetyl coA carboxylase. This carboxylates acetyl-coA and converts it into a 3 carbon malonyl coA. One ATP is required for each acetyl coA that is converted into malonyl coA.
Malonyl donates the carbons to the new fatty acid.
Which enzyme catalyses the synthesis of fatty acids?
What is the only type of fatty acids it can synthesise?
Fatty acid synthase.
It can only synthesise saturated straight chain fatty acids.
What does fatty acid synthase comprise of?
A dimer of identical polypeptides.
Each of the polypeptides that makes up the dimer has 7 distinct enzyme activities.
It contains an acyl-carrier protein (ACP) which carries the growing fatty acid chain during synthesis.
How does fatty acid synthase synthesise a fatty acid?
Fatty acids are synthesised in a cycle of reactions.
In the 1st step, 1 acyl carrier protein is loaded with an acetyl group from acetyl coA, and the second acyl carrier protein (on the other half of the dimer) is loaded with a malonyl group from malonyl coA.
2 carbons from the malonyl are transferred onto the acetyl group to form a 4 carbon fatty acyl chain.
Then the second acyl carrier protein is loaded with another malonyl group and the cycle repeats itself.
Each cycle of reactions adds another 2 carbon atoms onto the fatty acyl chain.
The reactions are reductive, so NADPH is used as an electron donor.
The cycles repeat until the growing acyl chain has reached 16 carbon length. It is then released from the acyl carrier protein.
a) What is the longest fatty acid chain that fatty acid synthase can make?
b) How are longer, unsaturated, oddly evened or branched fatty acids made?
a) 16 carbons (i.e. palmitic acid)
b) They require additional enzymes.
How is fatty acid synthesis controlled?
By regulating acetyl-coA carboxylase at a systemic level (by hormones) and at the enzymatic level by allosteric regulation.
Which three hormones regulate fatty acid synthesis?
What is their effect?
Insulin: stimulates acetyl coA carboxylase and fatty acid synthesis
Glucagon: inhibits acetyl coA carboxylase and fatty acid synthesis
Epinpherine (adrenaline): Inhibits acetyl coA carboxylase and fatty acid synthesis.
Which molecules regulate acetyl coA carboxylase allosterically?
Citrate: stimulates acetyl coA carboxylase and fatty acid synthesis
Palmitoyl coA: Inhibits acetyl coA carboxylase and fatty acid synthesis (its concentrations are high when there are plenty fatty acids)
As well as fatty acids, what is required for the synthesis of triglycerides?
Where and how is glycerol-3-phosphate synthesised?
In the liver, by phosphorylating glycerol with glycerol kinase
In the adipose tissue: This can produce glycerol-3-phosphate through glycolysis. (there is not glycerol kinase present in the adipose tissue). Therefore this can only happen if there is plenty glucose in the bloodstream.