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Adrenaline raises cAMP inside muscle cells, which activates PKA. PKA then phosphorylates two enzymes involved in glycogen metabolism. What is the combined effect on those two enzymes?
AIt inhibits the enzyme that breaks down glycogen and activates the enzyme that builds glycogen
BIt activates both the glycogen-degrading enzyme and the glycogen-building enzyme at once
CIt has no effect on either enzyme and instead acts only on the nucleus
DIt activates the enzyme that breaks down glycogen and inhibits the enzyme that builds glycogen
Answer & Solution
Correct answer: D. It activates the enzyme that breaks down glycogen and inhibits the enzyme that builds glycogen
1. Activation of beta-adrenergic receptors by adrenaline raises cyclic AMP inside muscle cells, which activates PKA.
2. PKA phosphorylates glycogen phosphorylase kinase, activating it, which in turn activates glycogen phosphorylase to break down glycogen into glucose.
3. PKA also phosphorylates glycogen synthase, but this phosphorylation inhibits its ability to form glycogen from glucose.
4. The combined effect is that glucose becomes available while its reconversion back into glycogen is prevented, avoiding a futile cycle.
5. The option that swaps the two effects, inhibiting glycogen breakdown and activating glycogen synthesis, reverses the actual outcome described.
6. The option activating both enzymes simultaneously ignores that one enzyme is activated while the other is inhibited.
7. Adrenaline's effect is on cytoplasmic enzymes through PKA, not directly on the nucleus, so that option misdescribes the target.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 9 "Cell Communication", section 9.3 | Response to the Signal_
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