Using your knowledge of fatty acid biosynthesis, provide an...

Using your knowledge of fatty acid biosynthesis, provide an explanation for the following experimental observations. (a) Addition of uniformly labeled $[^{14} \mathrm{C}]$ acetyl-CoA to a soluble liver fraction yields. palmitate uniformly labeled with$ ^{14} \mathrm{C}$. (b) However, addition of a trace of uniformly labeled $[^{14} \mathrm{C}]$ acetyl-CoA in the presence. of an excess of unlabeled malonyl-CoA to a soluble liver fraction yields palmitate labeled with $^{14} \mathrm{C}$ only in $\mathrm{C}-15$ and $\mathrm{C}-16$.

Using your knowledge of fatty acid biosynthesis, provide an explanation for the following experimental observations.
(a) Addition of uniformly labeled $[^{14} \mathrm{C}]$ acetyl-CoA to a soluble liver fraction yields. palmitate uniformly labeled with$ ^{14} \mathrm{C}$.
(b) However, addition of a trace of uniformly labeled $[^{14} \mathrm{C}]$ acetyl-CoA in the presence. of an excess of unlabeled malonyl-CoA to a soluble liver fraction yields palmitate labeled with $^{14} \mathrm{C}$ only in $\mathrm{C}-15$ and $\mathrm{C}-16$.

Key Concepts

Fatty Acid Biosynthesis

Fatty acid biosynthesis is a metabolic pathway in which acetyl-CoA and malonyl-CoA are assembled into long-chain fatty acids through a series of enzyme-catalyzed condensation, reduction, dehydration, and reduction reactions. This process is performed by the fatty acid synthase complex and involves the iterative addition of two-carbon units derived from malonyl-CoA to a priming acetyl-CoA molecule, ultimately producing fatty acids such as palmitate.

Roles of Acetyl-CoA and Malonyl-CoA

Within the fatty acid biosynthetic pathway, acetyl-CoA serves as the initial primer, providing the first two carbons of the fatty acid, while malonyl-CoA supplies the additional two?carbon units required for elongation. The primer remains at one end of the molecule (typically found at the methyl terminus after synthesis), whereas the successive malonyl-CoA units are used to extend the carbon chain through repeated cycles of addition.

Decarboxylative Condensation and Label Loss

A key aspect of fatty acid synthesis is the decarboxylative condensation reaction during which the carboxyl group of the malonyl-CoA is lost as CO2. This reaction allows for the coupling of two-carbon units while ensuring that the carbon from malonyl-CoA that would have been labeled is removed. Consequently, when trace labeled acetyl-CoA is used in the presence of an excess of unlabeled malonyl-CoA, only the carbons originating from the acetyl-CoA (which become the terminal carbons of the final fatty acid) retain the label.

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