Question

Tracing carbons. Consider a cell extract that actively synthesizes palmitate. Suppose that a fatty acid synthase in this preparation forms one molecule of palmitate in about 5 minutes. \(A\) large amount of malonyl CoA labeled with \(^{14} \mathrm{C}\) in each carbon atom of its malonyl unit is suddenly added to this system, and fatty acid synthesis is stopped a minute later by altering the pH. The fatty acids in the supernatant are analyzed for radioactivity. Which carbon atom of the palmitate formed by this system is more radioactive \(-\mathrm{C}-1\) or \(\mathrm{C}-14 ?\)

Short answer

C-14 is more radioactive than C-1.

Step-by-step solution

Understanding Fatty Acid Synthesis

Fatty acid synthesis involves the repetitive addition of two-carbon units from acetyl CoA, with malonyl CoA serving as the two-carbon donor. Each cycle elongates the chain and involves condensation, reduction, dehydration, and a second reduction reaction.

Labeling with Radiolabeled Malonyl CoA

In this scenario, malonyl CoA is labeled with \(^{14}\text{C}\) at each carbon. During each addition cycle, the two carbon atoms from malonyl CoA are incorporated into the fatty acid chain, with the third carbon from the malonyl unit being released as CO₂.

Identifying Radioactive Carbons

Since the labeled malonyl CoA is used in the elongation process, each two-carbon unit incorporated into the growing fatty acid is derived from a radioactive malonyl CoA, except for the first acetyl CoA used at the start.

Locating the Initial Acetyl CoA

The very first two carbon atoms (C-1 and C-2) of palmitate come from the initial acetyl CoA, which is not labeled because the radiolabeling only involves the malonyl CoA. Therefore, C-1 would not be radioactive.

Analyzing Terminal Carbon Radioactivity

All subsequent carbons added (C-3 to C-16) come from radiolabeled malonyl CoA. The terminal carbon of palmitate (C-14) comes from the 6th malonyl CoA incorporation cycle, making it radioactive.

Conclusion

Since the initial acetyl CoA is not labeled and C-14 comes from a radiolabeled malonyl CoA, C-14 will have higher radioactivity than C-1 in this experimental setup.

Key concepts

Radiolabeling in Fatty Acid Synthesis

Radiolabeling is a fascinating process used to trace molecular transformations during biochemical reactions. It involves marking a molecule with a radioactive isotope, such as carbon-14 \((^{14} ext{C})\), to track where that atom ends up in a chemical compound. In the context of fatty acid synthesis, scientists can use radiolabeling to monitor how carbon atoms are incorporated into fatty acids, like palmitate.
Understanding radiolabeling can demystify complex biochemical pathways. By incorporating radioactive isotopes into molecules like malonyl CoA, researchers can identify where each carbon atom goes during synthesis. This is crucial as it provides insights into the biochemical mechanisms at play and allows scientists to understand metabolic pathways in precise detail.
Radiolabels act as molecular beacons. During the synthesis of palmitate, malonyl CoA labeled with \(^{14} ext{C}\) ensures that every malonyl unit added to the growing fatty acid chain contains radioactive carbon. As the palmitate chain grows, you can literally "trace" the incorporation of radioactive carbon atoms, step by step.

Role of Malonyl CoA in Fatty Acid Synthesis

Malonyl CoA plays a pivotal role in the elongation of fatty acid chains. It acts as a building block, providing two-carbon units that are added repeatedly to produce long fatty acid molecules. In every cycle of fatty acid synthesis, malonyl CoA donates two carbon atoms to the growing fatty acid, with a third carbon released as carbon dioxide.
The process begins with an initial acetyl CoA unit, but as the chain extends, malonyl CoA becomes the prime contributor. This critical function underlines why understanding malonyl CoA is essential for comprehending fatty acid synthesis.
In experiments involving radiolabeling, like the one described in the exercise, every malonyl CoA molecule is tagged with \(^{14} ext{C}\). Therefore, the isotopic label provides a clear record of how many cycles of synthesis have occurred and which parts of the resulting molecule are from malonyl CoA, enhancing our understanding of cellular metabolism.

Palmitate Radioactivity and Carbon Tracing

Palmitate is a 16-carbon long fatty acid and serves as a product in this synthesis pathway. When using radiolabeled malonyl CoA, the radiolabeled carbon atoms serve as tracers, providing clarity on which carbons are added during synthesis.
One critical observation in such experiments is radioactivity distribution in palmitate. Since the beginning of the chain, namely C-1 and C-2, is from the original unlabeled acetyl CoA, these carbons won't show radioactivity. All subsequent carbons, from C-3 to C-16, originate from labeled malonyl CoA units, making them radioactive.
Particularly, the terminal carbon \(C-14\) is from the last few cycles involving labeled malonyl CoA, making it notably radioactive. By examining the radioactivity, researchers can confirm the stepwise addition of malonyl CoA to the fatty acid and determine the role and usage of each component in biochemical pathways.