Lipid Synthesis - General Flashcards
Triacylglycerol and phospholipids both have a _____ backbone, to which several _____ are linked. What is the difference between the connections?
Glycerol, fatty acids.
- Triacylglycerol: three fatty acids are linked to each of the three carbon atoms of glycerol.
- Phospholipids: two fatty acids linked to carbons 1 and 2 of glycerol, while a phosphate group and one of several different head groups are linked to carbon 3.
_____ is a common precursor for both classes of lipids. What can it be derived from?
Glycerol-3-P. It can be derived either from glycerol released from adipose when triacylglycerol is degraded (in liver only), or from the conversion of dihydroxyacetone phosphate (in liver and other tissues).
Where in the body does triacylglycerol synthesis occur? Where does phospholipid synthesis occur?
Triacylglycerol synthesis only occurs in liver and adipose, whereas phospholipid synthesis occurs in all cells since they are major components of membranes.
A molecule called _____ is the precursor for both triacylglycerol and phospholipids, and is synthesized from glycerol-3-P.
Phosphatidate.
How is phosphatidate synthesized?
The synthesis of phosphatidate from glycerol-3-P consists of two consecutive additions of a fatty acid (using an acyl-CoA derivative) to carbon 1 and then to carbon 2, via ester linkages.
- A saturated fatty acid is usually linked to carbon 1.
- An unsaturated fatty acid is usually linked to carbon 2.
- Carbon 3 has the phosphate group attached to it, which will facilitate the addition of different head groups to form different phospholipids
Synthesis of triacylglycerol from phosphatidate is a two-step process. What enzyme catalyzes this reaction? Where is it located in the cell? What are the two steps?
Triacylglycerol synthetase complex that is located on the ER membrane.
- In the first step, the phosphate group is cleaved off to form diacylglycerol,
- In a second step the third fatty acid is linked to carbon 3 to form triacylglycerol.
Under normal condtions, where is most of the triacylglycerol that is synthesized in the liver sent to (2)? What purpose does it serve in these tissues?
Either the muscle to be used as fuel or the adipose to be stored for future needs.
Common membrane phospholipids, including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol, are all very similar in structure, but differ from each other in what way?
Differ in the head group attached to the phosphate group on carbon 3 of the glycerol backbone.
Phospholipids are _____ molecules, having a very non-polar region due to the two fatty acid chains and a highly polar head group.
Amphiphilic
Phospholipid synthesis takes place in the _____ and uses _____ as a precursor to which an alcohol is linked (ethanolamine, choline, or serine).
Endoplasmic reticulum, phosphatidate.
During lipid synthesis, one of the components is _____.
Activated
In the case of lipid synthesis, the activated precursor is a derivative of what?
Cytidine diphosphate (CDP).
In the case of lipid synthesis, either the _____ or the _____ may be linked to CDP to activate it, depending on the reactant.
Diacylglycerol or the alcohol group.
Phospholipid synthesis from an activated diacylglycerol is accomplished by linking what two things? Describe this reaction (3 steps).
A cytidine diphosphate (CDP) molecule to a phosphatidate, forming CDP-phosphatidate
Reaction:
- Phosphatidate reacts with CTP to form CDP-diacylglycerol.
- The pyrophosphate (PPi) released is quickly hydrolyzed in the cell to two Pi; the energy released from this hydrolysis drives this reaction strongly forward.
- CDP-diacylglycerol then reacts with an alcohol such as choline to form the phospholipid with the release of CMP
In what organisms does synthesis using an activated diacylglycerol occur?
Primarily in yeast and bacteria.
In what organisms does synthesis from an activated alcohol occur?
Mammals.
How an activated alcohol formed? The synthesis of what lipid is done by this pathway?
The alcohol is activated by linking it to CDP. The synthesis of phosphatidylethanolamine is done by this pathway.
Both synthesis from an activated diacylglycerol and activated alcohol rely on sufficient dietary _____ being available. Describe the reactions that happen if it isn’t to form phosphatidylcholine. What enzyme is used?
Choline. The liver possesses an alternative route for the synthesis of phosphatidylcholine.
Phosphatidylcholine is synthesized from phosphatidylethanolamine by the enzyme phosphatidylethanolamine methyltransferase.
The amino group of ethanolamine is methylated three times using S-adenosylmethionine (SAM) as the methyl donor. This alternative pathway ensures that phosphatidylcholine needs will be met even if choline itself is in limited supply.
What are the 4 stages of cholesterol synthesis? Where do the first two occur in the cell? Where do the last two occur?
- Regulated step, which is the synthesis of mevalonate from three acetyl CoA molecules.
- Two molecules of acetyl CoA condense to form acetoacetyl CoA in the first step, and then a third acetyl CoA is condensed to form hydroxymethylglutaryl-CoA (HMG-CoA).
- This molecule is then reduced, using NADPH, to mevalonate by the enzyme HMG-CoA reductase.
- Multi-step conversion of mevalonate to an activated isoprene molecule.
- Synthesis of squalene from six isoprene units.
- Completed synthesis of cholesterol from squalene that includes several cyclization steps to form the rings.
