signaling through enzyme coupled receptors

Question 1

what is a kinase?

Answer 1

enzymes that covalently attach a phosphate group from energy molecules such as ATP onto a substrate molecule. Protein kinases only phosphorylate specific amino acids (consensus sequence)

Question 2

receptor kinases (2 types) vs non-receptor kinases

Answer 2

enzyme linked receptors: transmembrane receptor proteins that may or may not contain integral, intracellular kinase domains
receptor tyrosine kinases: transmembrane receptor proteins that do contain integral, intracellular tyrosine kinase domains
non-receptor kinases: cytosolic tyrosine kinase enzymes that lack transmembrane ligand bind, receptor domain

Question 3

How are receptor tyrosine kinases activated?

Answer 3

1. dimerized ligands bind to RTKs, which also dimerize in response.
2. RTKs cross-phosphorylate their partners tyrosine residues when dimerized.
3. trans-autophosphorylation creates phospho-tyrosines (docking sites) that bind intracellular signaling proteins

Question 4

how do proteins bind tyrosine kinases?

Answer 4

subunits within proteins serve as phospho-tyrosine recognition sites, an example is SH2 domain. SH2 is a modular protein domain so it can be inserted anywhere in a protein without disturbing protein folding or function

Question 5

what role do RTKs play in cancer?

Answer 5

vascular endothelial growth factor (VEGF) stimulates angiogenesis (formation of new blood vessels) which provides tumor cells with vital supplies. Tumors that secrete VEGF grow rapidly. VEGFs signal via RTKs

Question 6

How is Ras activated by an RTK? pathway

Answer 6

Grb-2 (adaptor protein) binds to phosphotyrosines on RTKs (SH2 domains) and to polyproline regions (SH3 domains) on Ras-GEF (Sos). Then Ras-GEF (Sos) induces Ras to become active

Question 7

Describe the cycle of Ras activation (yes, again)

Answer 7

Ras is a molecular switch and is active when GTP bound and inactive when GDP bound. Ras binds GTP and hydrolyzes it with the help of a Ras GAP. Now it is Ras-GDP and must unbind the GDP with the help of Ras GEF to be activated again

Question 8

what keeps Ras inactivated? why is this important?

Answer 8

Ras GAP keeps Ras in the GDP bound form (the inactive form) 95% of the time. This is important to prevent excessive signaling by Ras. hyperactive Ras causes cancer (30% of human tumors express hyperactive Ras)

Question 9

how does the MAP kinase signaling cascade work?

Answer 9

Ras activates the pathway by activating one MAPKKK which then activates numerous MAPKK and each of those activates numerous MAPK. Then each MAPK can activate numerous other target proteins causing changes in protein activity and gene expression