Biochemistry : Lipid Catabolism Regulation
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Example Questions
Example Question #1 : Lipid Catabolism Regulation
Why does consuming alcohol (ethanol) promote storage of fatty acids in the form of triglycerides (fatty tissue), especially in the liver?
The 
Ethanol directly inhibits the enzymes which oxidize fatty acids by binding to their active sites
The acetaldehyde produced from ethanol directly inhibits enzymes which oxidize fatty acids
The 
The free radicals produced in this process directly inhibit enzymes which oxidize fatty acids
The
The oxidation of ethanol to acetaldehyde in the liver produces 
Example Question #21 : Lipid Catabolism
In the presence of insulin, cells alter the activity of a key enzyme in fat metabolism. This enzyme is called acetyl-CoA carboxylase. How does insulin signaling affect the activity of this enzyme, and what changes does this have on fat metabolism?
Acetyl-CoA becomes activated and the breakdown of fatty acids is inhibited
Acetyl-CoA becomes inhibited and the breakdown of fatty acids is activated
Acetyl-CoA becomes inhibited and the breakdown of fatty acids is inhibited
Acetyl-CoA becomes activated and the breakdown of fatty acids is activated
Acetyl-CoA becomes activated and the breakdown of fatty acids is inhibited
For this question, we're going to need to know a few things right off the bat.
First, it's important to know what the general "mission" of insulin is. Generally speaking, insulin is a hormone that helps the body out when there is ample energy available. For example, right after eating a meal, blood sugar levels are going to be fairly high. Thus, the body is in a state where it doesn't want to be breaking things down to provide energy. Rather, it wants to store the energy that it has just been given. This storage generally comes in the form of glycogen and fat.
Next, it's important to understand what acetyl-CoA carboxylase (ACC) does. As its name would suggest, it adds a carboxyl group onto acetyl-CoA. By doing so, it generates malonyl-CoA. This is a very important molecule that is used in the synthesis of fatty acids. Thus, when there is a lot of malonyl-CoA in the cell, it's likely that the cell wants to use it up to create fatty acids. Furthermore, in order to ensure that fatty acids aren't being broken down at the same time (which would be wasteful), malonyl-CoA also blocks the breakdown of fatty acids.
Equipped with this information, we can go ahead and piece together the facts as though it were a story. Insulin wants the body to store energy. It activates cellular receptors, which causes a downstream signaling event. Ultimately, this results in the activation of the enzyme ACC. This, in turn, generates a high amount of malonyl-CoA. The presence of high amounts of malonyl-CoA inside the cell gives the signal to make fatty acids and to also block their breakdown. So, all in all, acetyl-CoA becomes activated and the breakdown of fatty acids is inhibited.
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