**Question:** Conversion of stearic acid to oleic acid is catalysed by:

A. Stearoyl-CoA desaturase (SCD)
B. Fatty acid synthase (FAS)
C. Lipase
D. Lipoprotein lipase (LPL)

**Core Concept:** Stearoyl-CoA desaturase (SCD) is an enzyme responsible for catalyzing the conversion of saturated fatty acids to their corresponding monounsaturated fatty acids. In this case, the conversion of stearic acid to oleic acid is being discussed.

**Why the Correct Answer is Right:** Stearic acid is a saturated fatty acid with 18 carbon atoms and two hydrogen atoms. Oleic acid is an unsaturated fatty acid with 18 carbon atoms but one double bond between the ninth and tenth carbon atoms, resulting in the presence of one unsaturation. The conversion of stearic acid to oleic acid involves the removal of a hydrogen atom and the formation of a double bond. Stearoyl-CoA desaturase (SCD) catalyzes this process, which is a key enzyme in the biosynthesis of monounsaturated fatty acids.

**Why Each Wrong Option is Incorrect:**

A. Fatty acid synthase (FAS) is involved in the de novo synthesis of fatty acids from acetyl-CoA and malonyl-CoA, but it does not play a role in desaturation or the conversion of saturated fatty acids to unsaturated fatty acids.

B. Lipase is an enzyme involved in the hydrolysis of triglycerides and phospholipids, breaking down fat molecules. It does not catalyze the conversion of fatty acids.

C. Lipoprotein lipase (LPL) is an enzyme involved in the hydrolysis of triglycerides, particularly in the circulation, where it plays a role in the uptake of fatty acids by tissues. However, it does not catalyze the desaturation of fatty acids.

**Core Concept:** Stearoyl-CoA desaturase (SCD) is a crucial enzyme in the biosynthesis of monounsaturated fatty acids. It plays a vital role in converting saturated fatty acids (e.g., stearic acid) into unsaturated fatty acids (e.g., oleic acid) by adding a double bond to the molecule. This process is essential for maintaining proper membrane fluidity, cell signaling, and maintaining membrane fluidity.

**Why Each Wrong Option is Incorrect:**

A. Fatty acid synthase (FAS) is involved in the de novo synthesis of fatty acids from acetyl-CoA and malonyl-CoA, but it does not play a role in desaturation or the conversion of saturated fatty acids to unsaturated fatty acids.

B. Lipase is an enzyme that breaks down triglycerides and phospholipids, not involved in fatty acid synthesis or desaturation.

C. Lipoprotein lipase (LPL) is responsible for the hydrolysis of triglycerides in the circulation, particularly at the tissue level, and does not participate in fatty acid desaturation or the conversion of saturated fatty