Question

HUMAN GROWTH HORMONE

1. Describe the ligand and any external receptors that bind to the ligand. If signaling can occur without an external ligand, explain how this can occur.

2. Describe how the signal is translocated from the exterior of the cell to the interior of the cell. Be sure to describe the specific activity of the receptor in initiating the intracellular signal.

3. Describe the propagation of the signal within the cell from the membrane receptor to the point of cellular response. Provide the details for at least one specific protein interaction that either results in the activation or inactivation of a subsequent protein or signaling molecule to propagate the signal.

4. What is a specific cellular response to the signal. Something will change- what is it?

Answer 1

Please find the answers below:
Answer 1: When an extracellular signal molecule proximates the membrane bound receptor, it gets bound to it by formation of chemical bonds. As soon as the molecule binds to the ligand, it causes a conformational change in the cytoplasmic domain of the receptor. This conformational change then brings about a series of sequential cascade downstreams in the cytoplasm which cause activation of many signalling molecules. However, in the absence of an external ligand, an internal ligand can also induce intracellular signalling. In this case, a signalling molecule is present already in the cytoplasm in inactive form and post-processing in the cytoplasm, it gets converted into a biologically active form. This active form then brings about intracellular signalling by similar method i.e. activation of series of sequential signalling events.

Answer 2: When a receptor binds to the extracellular domain of receptor, it causes a confomrational change in the carboxyl domain in the intracellular part of the cell. This conformational change in the active site of the receptor causes a very spontaneous change in the sequential events which bring about serial activation of many signallingp proteins in ATP dependent manner. Hence, until a final signalling product has been generated, this sequential activation is carried out in the cytoplasm.

Answer 3: When the signal reaches the cytoplasm, it promotes sequential activation of various proteins/enzymes which finally undergo either phosphorylation/dephosphorylation for their activation. A finally active cytoplasmic protein, which might also acts as a receptor for a nuclear receptor, reaches the nuclear membrane and binds to the receptor. For example, an estrogen molecule reaches the nuclear membrane and binds to the estrogen receptor on it. This causes internalization of the estrogen molecule which further causes changes in the pattern of gene expression.

Answer 4: As soon as the signal reaches the nucleus, the protein might act as a regulator of DNA transcription and hence changes the level/trend of gene expression. In this way, an extracellular signal causes change in gene expression.