Question

1. What physical and chemical properties distinguish small signaling mediators from larger signaling proteins? What kinds of functional differences might result from these properties?

2. For Ca2+ signaling, where the concentration of cytoplasmic Ca2+ is used to transmit information, which molecules play the roles of writers and erasers? What is the source of the energy driving Ca2+ signaling?

Answer 1

Answer-1:

In a biological system, cell signalling molecules can be amino acids, phopholipids, lipids, carbohydrates, proteins, glycoproteins or even gases. If a signalling protein is larger in size then it emphasizes on the fact that these molecules are bound to the surface of the cell receptors. Larger signalling moleculaes are usually hydrophilic. They are anchored to the receptor ad the cell membrane with the help of their trans-membrane domains (made of alpha-helices).These type of signalling proteins transmits the extracellular signal towards the cell interior. Larger signalling proteins are usually specific to cell types. Examples of large signalling molecules includes Acetylcholine, Vasopressin, etc.

However, smaller signalling molecules are usually destined to enter inside the cell, which usually bind to the receptors present inside the cell and they are hydrophobic in nature. Once these small signalling proteins are inside the cell, they brings about the transcription regulation to form mRNA. Internal receptors are capable to influence the gene expression without any need of by-passing the signal to other signalling molecules.

Answer-2:

This can be explained by quating an example of muscle cells during contraction.

During the onset of contraction signal, Sarcoplasmic Reticulum (or ER) act as a source of Ca+ (or like a role of writer, as stated in question). However, when the contraction process is over, excess of the Ca+ are sequestered to a protein called Calsequestrin (or like a role of an eraser).

Energy for driving Ca+ signalling comes from ATPase pump.