Final answer:
The question addresses the quaternary structure of the glycogen phosphorylase enzyme and its different forms that contribute to glycogenolysis. The enzyme alternates between the more active R state and the less active T state, and the phosphorylation catalyzed by phosphorylase kinase activates it. This process is essential for the release of glucose during the fight-or-flight response.
Step-by-step explanation:
The glycogen phosphorylase enzyme, which has a quaternary structure, exists in four interconvertible forms and transitions between allosteric and phosphorylated states.
- R state: This is the more active conformation of the enzyme, where the active site is freely accessible.
- T state: This less active conformation of the enzyme reduces its activity.
- 'a' form: Found predominantly in the liver, this form is phosphorylated and more active, favoring the R state.
- 'b' form: Found predominantly in muscles, this form is non-phosphorylated, less active, and favors the T state.
During glycogenolysis, glycogen phosphorylase plays a critical role. Phosphorylase kinase catalyzes the phosphorylation of glycogen phosphorylase (phosphorylation cascade), thus activating it. This active enzyme then catalyzes the hydrolysis of glycogen to glucose-1-phosphate (G-1-P) in the liver, which is converted to glucose-6-phosphate (G-6-P) and subsequently used in glycolysis or gluconeogenesis. The increase in circulating glucose provides energy for the fight-or-flight response, mediated by hormones like epinephrine which trigger this process via the conversion of ATP to cAMP and the activation of protein kinase A (PKA).