Glycogen metabolism regulation in human biochemistry: Which enzyme has its active form generated by dephosphorylation, given that glycogen phosphorylase is activated by phosphorylation?

Introduction / Context:
Efficient control of glycogen metabolism relies on reversible phosphorylation. In classic hormonal regulation, glycogen phosphorylase (glycogen breakdown) and glycogen synthase (glycogen formation) respond in opposite ways to phosphorylation states to avoid futile cycling. Understanding which enzyme is activated by dephosphorylation tests foundational knowledge of metabolic regulation.

Given Data / Assumptions:

• Glycogen phosphorylase is activated by phosphorylation (the a form).
• We must identify which enzyme is conversely activated when dephosphorylated.
• Context is mammalian liver and muscle regulation by hormones such as adrenaline and glucagon via cAMP/PKA and by insulin via phosphatases.

Concept / Approach:
Hormonal signaling through protein kinase A (PKA) phosphorylates many targets. Phosphorylation converts glycogen phosphorylase b to the active a form (via phosphorylase kinase), promoting glycogenolysis. In contrast, insulin signaling activates protein phosphatase-1 (PP1), which dephosphorylates glycogen synthase, converting it to the more active a form and thereby promoting glycogenesis. Thus, opposite regulatory patterns coordinate storage versus mobilization of glucose units.

Step-by-Step Solution:
Identify antagonistic enzyme pair: glycogen phosphorylase (breakdown) vs glycogen synthase (synthesis).Recall rule: phosphorylation activates phosphorylase; dephosphorylation activates synthase.Map hormonal signals: glucagon/adrenaline → PKA → phosphorylation; insulin → PP1 → dephosphorylation.Therefore the enzyme activated by dephosphorylation is glycogen synthase.

Verification / Alternative check:
Biochemical assays show high synthase activity in the dephosphorylated state; pharmacologic activation of PP1 or insulin treatment increases glycogen synthesis rates in hepatocytes and myocytes.

Why Other Options Are Wrong:

• Glycogen semisynthase / hydrolase / dehydrogenase: Non-standard or incorrect enzyme names in glycogen metabolism; they do not represent the regulated synthase.
• Phosphorylase kinase: A regulatory kinase activating phosphorylase; not the synthase regulated by dephosphorylation.

Common Pitfalls:
Assuming phosphorylation always activates enzymes. In carbohydrate metabolism, synthase is a classic exception, being more active when dephosphorylated.

Final Answer:
Glycogen synthase.