Biochemistry Study Set 13

Biology

Quiz 12 :

Lipid Metabolism

Quiz 12 :

Lipid Metabolism

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The system for metabolizing fatty acids in plants is similar to the one in animals. In the formation of erucic acid mentioned in this chapter (in Ancillary Enzymes ), how many condensation steps are there in the fatty acid synthase reaction?
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The erucic acid (
img ) is a monounsaturated fatty acid having
img carbon atoms with as single double bond at 13 th position, so it also notated as 13-docosenoic Erucic 22:1. The fatty acid synthesis occurs by addition of malonyl CoA in the acetyl CoA molecule along with the removal of carbon dioxide on each addition of malonyl CoA (provide 2 carbon atoms). The reaction of a single round is as follows:
img Thus, to obtain 22 carbon atoms one acetyl CoA (2 carbon atoms) is reacted with 10 Malonyl CoA (
img carbons atoms). This will provide erucic acid.
Hence, the number of condensation steps would be 10 in erucic acid.

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Desaturation reactions that produce double bonds following fatty acid synthesis are not linked to energy formation but still utilize oxygen. Why doesn't this compete with mitochondria and potentially impair ATP formation?
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The desaturation (removal of hydrogen atoms) of the fatty acid molecule requires oxygen, which takes up two hydrogen atoms, one from each adjacent carbon atoms and form water. This step ultimately forms water, which is interconnected with other steps that ends at producing NAD + (nicotinamide dinucleotide) (oxidized).
This oxidized moiety is then used up in the mitochondria for ATP (adenosine triphosphate) formation. Also, the fatty acid desaturation occurs in smooth endoplasmic reticulum (SER). Thus, the desaturation does not affect the ATP formation as after using oxygen, the fatty acids provides reducing equivalents that can be used in the ATP formation.

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A separate class of phospholipids is derived from DHAP directly instead of GOLP, and forms a membrane lipid with an ether-linked fatty acid rather than an ester-linked one in the 1-position. What would you predict about the chemical stability of these ether phospholipids?
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The ester linked phospholipids can be formed from the GOLP (glycerol phosphate), which is obtained from DHAP (dihydroxyacetone phosphate). On the other hand, if phospholipids are obtained directly from the DHAP then they are formed as ether linked.
The ether linked phospholipids have been found to have greater stability than the ester linked. This is because the ether linked- phospholipids can be insoluble in the hydrophilic environment and therefore they are more stable than the ester linked one. The ester bonds are hydrolyzed into carboxylic acid and water, which destabilized the membrane. Also, ether linked phospholipids provide stability to the plasma membrane and salt tolerance.

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In the presence of fatty acids, lactate carbon is not oxidized for energy to support gluconeogenesis. Instead, lactate carbon is converted to glucose, in what is called the glucose sparing effect of fatty acids. How do you think fatty acids are able to do that?
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In the fed state, glucose uptake in the liver decreases the oxidation of fatty acids. How do you think glucose is able to do that?
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