Glycogenin is a protein with self-glucosylation capacity. To which amino acid of glycogenin, glucose molecules are attached?
**Core Concept**
Glycogenin is an enzyme that plays a crucial role in the initiation of glycogen synthesis. It has the unique ability to self-glucosylate, meaning it can attach glucose molecules to itself. This process is essential for the formation of the glycogen chain.
**Why the Correct Answer is Right**
The correct answer is tyrosine, as glucose molecules are attached to the tyrosine residue of glycogenin. This attachment occurs through an O-glycosidic bond, which is formed between the glucose molecule and the hydroxyl group of the tyrosine amino acid. This self-glucosylation reaction is autocatalytic, allowing glycogenin to serve as a primer for glycogen synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** Asparagine is incorrect because it is involved in N-glycosylation, not the self-glucosylation of glycogenin.
**Option B:** Serine is incorrect as it is not the primary site for glucose attachment in glycogenin.
**Option D:** Lysine is incorrect because it does not participate in the self-glucosylation process of glycogenin.
**Clinical Pearl / High-Yield Fact**
The self-glucosylation capacity of glycogenin is a critical step in glycogen synthesis, and defects in this process can lead to glycogen storage diseases. Remembering the specific amino acid involved can help in understanding the molecular basis of these diseases.
**Correct Answer:** C. Tyrosine.