Glycogen metabolism — Immediate activated precursor: In the glycogen synthase reaction, what is the direct sugar donor to extend glycogen?

Introduction / Context:
Glycogen synthesis requires an activated sugar donor to efficiently form alpha-1,4-glycosidic bonds on a growing glycogen chain. Identifying this donor clarifies the role of nucleotide sugars in biosynthesis.

Given Data / Assumptions:

• Glycogen synthase elongates glycogen from a primer (glycogenin or existing chain).
• Activation of glucose is necessary before bond formation.
• Multiple phosphorylated glucose intermediates exist in the pathway.

Concept / Approach:
Glucose-1-phosphate reacts with UTP to form UDP-glucose and pyrophosphate via UDP-glucose pyrophosphorylase. UDP-glucose is the high-energy donor that glycogen synthase uses to transfer glucose to the nonreducing end of glycogen, releasing UDP.

Step-by-Step Solution:

Verification / Alternative check:
Isotope labeling confirms incorporation from UDP-glucose into glycogen in vivo and in vitro.

Why Other Options Are Wrong:

• Glucose-6-phosphate and glucose-1-phosphate are precursors but not the direct donor to glycogen synthase.
• UTP-glucose is not the correct name of the nucleotide sugar; UDP-glucose is.

Common Pitfalls:
Confusing UDP (uridine diphosphate) with UTP (uridine triphosphate); UTP is consumed to make UDP-glucose, but the donor contains UDP.

Final Answer:
UDP-glucose