Question

In muscle cells, adrenaline binds to the β-adrenergic receptor to initiate a signaling cascade that leads to the breakdown of glycogen. At what points in this pathway is the signal amplified?

1) Adrenaline binding to GPCR
2) GTP-Gα mediated activation of AC
3) Production of cAMP by AC

Answer 1

Signal amplification in the adrenaline-mediated breakdown of glycogen in muscle cells occurs at the GTP-Gα mediated activation of adenylyl cyclase and the subsequent production of cAMP by AC.

Step-by-step explanation:

In muscle cells, when adrenaline binds to the β-adrenergic receptor, it initiates a signaling cascade leading to the breakdown of glycogen. There are several points at which this signal is amplified. The first step of signal amplification occurs when adrenaline binds to the G-protein coupled receptor (GPCR), activating it. Following this, the GPCR activates a G protein by causing a GDP to GTP exchange on the α subunit of the G protein (GTP-Gα mediated activation). This activated α subunit then engages with adenylyl cyclase (AC), which in turn converts ATP into cyclic AMP (cAMP).

The production of cAMP by AC represents a significant amplification step, as each activated adenylyl cyclase enzyme can catalyze the formation of multiple cAMP molecules, spreading the signal further. Subsequently, cAMP activates protein kinase A (PKA), leading to downstream effects such as glycogen breakdown. Hence, both the GTP-Gα mediated activation of AC and the subsequent production of cAMP by AC are points where the signal is amplified. The correct options that indicate points of signal amplification in this pathway are 2) GTP-Gα mediated activation of AC and 3) Production of cAMP by AC.