Question

1. Specific extracellular signal molecules require a cell surface receptor. What about the chemistry of these signals prevents them from diffusing across the cell membrane?

2. What is the difference between endocrine signaling, paracrine signaling and autocrine signaling?

3. Why do cellular signal molecules bing to a receptor using noncovalent forces rather than a covalent bond?

4. What is the different between kinases and phosphates? How do they impact activity of the target protein. Please give an example that involves kinases and phosphates.

5. How do kinases recognize their substrates and the proteins they act on?

6. What is the difference between the active form and inactive form of GTPase superfamily of proteins?

7. How are second messengers different from first messengers?

Answer 1

1. These signal molecules are impermeable to lipid bilayer (plasma membrane) because of their hydrophilic nature.

2. The major difference between the different types of signalling is the distance that the signal travels to reach the target cell. Endocrine signalling uses the circulatory system to transport ligands, paracrine signalling acts on nearby cells, and autocrine signalling acts on the signalling cell itself.

3. Non-covalent interaction is usually temporary based on the conditions and type of interaction involved. It can be irreversible or reversible being able to drive conformational change in the receptor which transducer the signalling response inside the cell. The binding occurs as a result of an accumulation of weak interactions (non-covalent) such as polar, non-polar, ionic, hydrophobic, hydrophilic, hydrogen bonding. Complementarity of these interactions provides high specificity to the binding. On other hand, covalent bonds are stronger than non-covalent bonds, and their formation dissipates more energy and specificity is limited.

4. Kinase transfers a phosphate group from nucleotide such as ATP to a substrate whereas phosphatase is a type of hydrolase that catalyses the removal of a phosphate group from the substrate. Kinase uses ATP whereas phosphatase uses water. In other words, phosphatases can reverse the effect of kinases. Examples: Protein kinases act on proteins by phosphorylating them on their residues such as serine, threonine, tyrosine, or histidine. Protein phosphatase dephosphorylates a protein by removing a phosphate group from an amino acid residue of its protein substrate.