Question

Epinephrine is a protein hormone found in many animals. Epinephrine stimulates a signaling pathway that results in the breakdown of glycogen to glucose in the liver cells. Which of the following describes the initial steps in the process whereby epinephrine stimulates glycogen breakdown?
1. Epinephrine binds to a cell-surface receptor; the activated receptor stimulates production of the second messenger, cAMP
2. Epinephrine binds to a cell-surface receptor; the activated receptor catalyzes the conversion of glycogen to glucose
3 .Epinephrine diffuses through the plasma membrane; the hormone dimerizes in the cytosol
4. Epinephrine is taken into the cell by endocytosis; glycogen is converted to glucose in the endocytotic vesicle

Answer 1

Answer: 1). Epinephrine binds to a cell-surface receptor; the activated receptor stimulates production of the second messenger, cAMP.

Explanation: Epinephrine action begins by binding of the hormone to its specific receptor protein in the plasma membrane known as adrenergic receptor. This protein is a member of a large family of receptors called G-protein coupled receptors. The binding of epinephrine to its receptor triggers a conformational change in the receptor that affects its interaction with a Gs protein. The bound receptor triggers the replacement of GDP bound to Gs by GTP, thereby activating Gs protein. Gs is a heterotriemeric protein having an alpha, beta and gamma subunits. The activation of the Gs protein causes the alpha subunit of the protein to dissociate and move to another membrane protein called Adenylyl cyclase and activate it. The activated of adenylyl cyclase catalyzes the formation of cyclic AMP (cAMP).