L25- Fatty Acid Biosynthesis Flashcards
(40 cards)
In which cellular compartment does fatty acid synthesis occur?
Cytoplasm.
What molecule is the major source of carbons for fatty acid chain synthesis?
Acetyl-CoA (from mitochondria).
In order to direct acetyl-CoA into fatty acid synthesis, the TCA cycle must be inhibited. Under high energy conditions, which TCA cycle enzyme is inhibited to favor fatty acid synthesis?
Isocitrate dehydrogenase.
The last two carbons of a fatty acid chain come from which molecule? The remaining carbons come from which molecule?
The last two carbons (the carbons at the omega end) come from acetyl-CoA. The remaining carbons come from malonyl-CoA.
What molecule crosses the mitochondrial membrane into the cytoplasm as a means to generate a cytoplasmic pool of acetyl-CoA?
Citrate.
What is the reaction catalyzed by the enzyme citrate lyase?
It converts citrate (using ATP and CoASH) into acetyl-CoA and oxaloacetate (and ADP + Pi).
What enzyme converts acetyl-CoA to malonyl-CoA in the cytoplasm?
Acetyl CoA carboxylase.
What is the cofactor for acetyl CoA carboxylase?
Biotin (it is a cofactor for every carboxylase).
At which step is fatty acid synthesis regulated?
At acetyl CoA carboxylase (conversion of acetyl-CoA to malonyl-CoA in the cytoplasm).
What is the mechanism by which acetyl CoA carboxylase is deactivated?
In the presence of excess fatty acyl CoA and malonyl-CoA, glucagon and epinephrine stimulate phosphorylation of the enzyme, which in turn leads to depolymerization (i.e. deactivation) of its subunits. The enzyme also undergoes depolymerization when citrate levels are lowered.
On which large polymeric protein does fatty acid synthesis occur?
Fatty acid synthase.
What two residues on fatty acid synthase play a major role in fatty acid synthesis?
An -SH residue on a cysteine and a phosphopantetheine group capable or forming a thioester bond with fatty acids on which fatty acid chains grow in length.
True or False. The chemistry of the steps involved in fatty acid synthesis are the reverse of those involved in fatty acid oxidation.
True.
In mammals, fatty acid synthesis on fatty acid synthase stops when the fatty acid chain reaches how many carbons in length?
16 carbons (the name of this patty acid is palmitate).
What is the enzyme that cleaves palmitoyl-CoA from fatty acid synthase?
Thioesterase.
Which group on fatty acid synthase serves as the acceptor of acetyl-CoA and malonyl-CoA?
The phosphopantetheine group (Pan).
What kind of bond is formed between the malonyl group of malonyl-CoA and the phosphopantetheine group (Pan) of fatty acid synthase?
A thioester bond.
What happens during the condensation step of fatty acid synthesis?
The acetyl group on cysteine is condensed with the malonyl group on phosphopantetheine (Pan), forming the four carbon acetoacetyl-CoA while also liberating a carbon dioxide from the malonyl group.
True or False. The carbon dioxide that is liberated during the condensation reaction of fatty acid synthesis is the same as the carbon dioxide that was added to acetyl-CoA by acetyl CoA carboxylase.
True.
Where on fatty acid synthase do the condensation, reduction and dehydration occur?
On the phosphopantetheine group (Pan).
What is the cofactor required for the reduction reactions that occur on fatty acid synthase in fatty acid synthesis?
NADPH.
What are two sources of the NADPH required for the reduction reactions of fatty acid synthesis?
The pentose phosphate pathway and the reaction catalyzed by NADP+-linked malate dehydrogenase (which converts malate to pyruvate).
How many molecules of acetyl-CoA, ATP and NADPH are required to form one molecule of palmitate?
8 acetyl-CoA’s, 15 ATP’s and 14 NADPH’s.
True or False. Conversion of carbohydrates to fatty acids is 100% efficient.
False. Its efficiency is approximately 81%.
