In fatty acid synthesis, CO2 loss occurs in which step
**Core Concept**
Fatty acid synthesis is a crucial metabolic pathway that involves the creation of fatty acids from acetyl-CoA and malonyl-CoA molecules. This process occurs in the cytosol and is essential for the production of triglycerides and other lipids. The pathway involves a series of enzyme-catalyzed reactions that result in the elongation of the fatty acid chain.
**Why the Correct Answer is Right**
CO2 loss occurs in the step catalyzed by the enzyme **β-ketothiolase** (also known as **β-ketacyl-CoA thiolase**). This enzyme is responsible for the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA, which is then converted into β-ketohexanoyl-CoA. However, the correct step for CO2 loss is actually the conversion of acetyl-CoA to malonyl-CoA, where the enzyme **acetyl-CoA carboxylase** uses ATP and CO2 to convert acetyl-CoA into malonyl-CoA. This CO2 loss is crucial for the synthesis of fatty acids.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a particular step in the fatty acid synthesis pathway where CO2 loss occurs.
* **Option B:** This option is incorrect because it does not accurately describe a step in the fatty acid synthesis pathway.
* **Option C:** This option is incorrect because it does not correctly identify the step where CO2 loss occurs.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that fatty acid synthesis is an energy-intensive process that requires ATP, NADPH, and CO2. The enzyme acetyl-CoA carboxylase is a key regulator of fatty acid synthesis and is often targeted in the treatment of metabolic disorders such as obesity.
**Correct Answer:** D.