How does glycogenin function as a primer in glycogen synthesis?

• A. It cleaves alpha-1,4 bonds to release glucose-1-phosphate.
• B. It forms alpha-1,6 bonds during glycogen synthesis.
• C. It converts glucose-1-phosphate to glucose-6-phosphate.
• D. It initiates glycogen synthesis by attaching initial sugars to tyrosine residues.

Answer: (D)