Lipogenesis Flashcards
Why does lipogenesis (fatty acid synthesis) occur in the cytosol of the cell where all of the enzymes needed for lipogenesis are located?
Each of the lipogenesis steps is catalyzed by the multi-enzyme complex fatty acid synthase. Fatty acid synthase is located in the cytosol of the cell, so that is where lipogenesis occurs. A metabolic reaction always occurs where the enzyme(s) needed for the reaction are located.
What is the starting input (reactant) for lipogenesis and what is the final output (end products)?
Input: Acetyl CoA (derived from carbohydrate metabolism)
Output = a fatty acid
Three FAs can each bond to a glycerol (via ester bonds) to form a triacylglycerol (TAG) unit, which can be stored as fat in the adipose tissue.
Oxidised coenzymes NADP+ are also produced in lipogenesis.
Where does the input required for lipogenesis come from?
The acetyl CoA required for lipogenesis are derived from carbohydrate metabolism. Specifically, glucose is converted to pyruvate via glycolysis.
The pyruvate is then converted into acetyl CoA at the beginning of aerobic respiration.
Once the energy needs of the cell have been met and the glycogen stores are full, the acetyl CoA (from glucose) are used as the input for lipogenesis.
How is the acetyl CoA moved from the mitochondria into the cytosol in order to begin lipogenesis? How is the acetyl CoA used to begin lipogenesis?
Acetyl CoA is taken from the mitochondria into the cytosol via the citrate malate shuttle, where the CoA component is removed than reattached.
Once in the cytosol, the acetyl CoA is converted into acetyl ACP and malonyl ACP, which are used to make acetoacetyl ACP in the first step on lipogenesis. ACP stands for acyl carrier protein, which is an important component required for lipogenesis.
What is the final output of lipogenesis used for? For what purpose?
Once three fatty acids have been made by lipogenesis they can each bond to a glycerol (via ester bonds) to form a triacylglycerol (TAG) units, which can be stored as fat in the adipose tissue.
The TAG can then be used for energy generation in times where lipid is required to generate energy (fasting state).
Why is it necessary for our cells to perform lipogenesis? Why is it important?
Lipogenesis allows excess carbohydrate to be converted into fatty acids. These fatty acids then combine with glycerol (to make TAG) to be stored as fat in the adipose tissue.
Lipogenesis allows the conversion of carbohydrate to fat, which prevents the excess carbohydrate taken in from the diet being wasted.
The TAG (stored in the adipose tissue) can then be used for energy generation in times where lipid is required to generate energy.
Step 1 of lipogenesis is the condensation step that creates the acetoacetyl ACP unit. What happens in the condensation that allows Acetyl ACP and Malonyl ACP to come together to create Acetoacetyl ACP?
In the condensation step, acetyl ACP detaches from the ACP unit and malonyl ACP detaches from COO- .
As a result, both Acetyl ACP and Malonyl ACP require an extra covalent bond (due to losing atoms). These compounds satisfy their need for an extra covalent bond by attaching to each other, where the carbonyl of Acetyl ACP attaches to the CH2 of Malonyl ACP.
After the new covalent bond has formed the acetoacetyl ACP compound has been created.
A 16 carbon fatty acid is created through lipogenesis. Answer the following questions to explore how multiple cycles of lipogenesis are required to create a 16 carbon fatty acid.
The first lipogenesis cycle produces a 4 carbon fatty acid, and each subsequent cycle adds another two carbons onto the fatty acid. Using the information in the previous sentence, explain how many cycles of lipogenesis are needed to create a 16 carbon fatty acid?
Each cycle of lipogenesis adds 2 carbons onto the growing fatty acid chain, with the product of each previous lipogenesis cycle used to begin the next lipogenesis cycle.
After the first lipogenesis cycle a 4 carbon fatty acid is produced, with the second lipogenesis step adding on an extra two carbon atoms, producing a 6 carbon fatty acid.
To build up to a sixteen carbon fatty acid 7 lipogenesis cycles are required.
Rule to work out how many lipogenesis cycles are required to create a fatty acid:
No. of cycles of lipogenesis needed to create the fatty acid = (no. of carbons in fatty acid/2) – 1
e.g. 16 carbon fatty acid
No. of cycles of lipogenesis needed to create the fatty acid = (16/2)-1
= 7 cycles of lipogenesis required to create a 16 carbon fatty acid
How many NADP+ are created in each lipogenesis cycle?
Each lipogenesis cycle requires two NADPH to facilitate the 2nd and 4th steps of lipogenesis which are redox reactions.
In these redox reactions NADPH is converted to NADP+ . Therefore, in one complete lipogenesis cycle, two NADP+ are generated
How many NADP+ are created in the seven lipogenesis cycles needed to generate a 16 carbon fatty acid?
In one complete lipogenesis cycle, two NADP+ are generated.
In the seven lipogenesis cycles needed to generate a 16 carbon fatty acid there are fourteen (7 cycles times two NADP+ ) are generated.
