Unit 7

Question 1

1st class extracellular signaling molecules

Answer 1

• Larger than 2nd class
• Extracellular signaling molecules are too large & too hydrophillic to cross plasma membrane
• Receptors are on outer surface of plasma membrane of target cell

Question 2

2nd class extracellular signaling molecules

Answer 2

• Smaller than 1st class
• Sufficiently small & hydrophobic to diffuse across plasma membrane
• Receptors are in interior of target cell, either in cytosol or in nucleus

Question 3

G-protein coupled receptors

Answer 3

• Found in virtually all cells
  1.) Binds to its ligand: Induces an interaction of receptor with the inactive heterotrimeric GTP-binding (G) protein
  2.) Binding to the receptor activates G protein
  3.) α subunit of activated G protein leaves receptor & binds the inactive effector enzyme
  4.) Binding to activated G protein activates the effector enzyme that generates a specific 2nd messanger
• Some stimulate production of cAMP (Gs), some inhibit production of cAMP (Gi)
• Some stimulate production of diaglycerol & inostitol-1, 4, 5-triphosphate by activating phospholipase C & are referred to as Gq

Question 4

Ion-channel-coupled receptors

Answer 4

• Ligand binding changes conformation of ion-channel-linked receptor & opens channel for ion flow

Question 5

Enzyme-coupled receptors

Answer 5

• Binds its extracellular ligand switching on an enzyme activity on the opposite side of the plasma membrane. Enzyme may be part of cytoplasmic domain of the receptor or it may be an enzyme that associates with cytoplasmic domain of the receptor

Question 6

NO (small, diffusable signaling molecule)

Answer 6

• Nitric oxide
• Produced by breakdown of arginine
• Short-lived
• Local-acting (paracrine signaling)
• Important in cardiovascular system
• Vasodilator
• Produced through action of acetylcholine
• Activates guanylyl cyclase; an enzyme that produces cGMP, a 2nd messenger that activates a protein kinase that phosphorylates specific substances leading to smooth muscle cell relaxation & vasodilation

Question 7

Cortisol

Answer 7

• Produced by adrenal gland when:
  a.) Under stress
  b.) Low glucose
• Causes body to produce glucose & suppress immune system
  1.) Diffuses across plasma membrane & binds to its receptor protein, located in cytosol
  2.) Binding of cortisol induces a conformational change to its receptor are transported into the nucleus through nuclear pores
  3.) In nucleus, receptor binds to specific DNA regulatory sequences & induces expression of downstream genes

Question 8

Thyroxine

Answer 8

• Thyroxine receptor in nucleus & is bound to DNA either in presence or absence of thyroxine
• In absence, receptor binds to repressor to prevent gene transcription
• Produced in thyroid gland & increases metabolic rate & protein synthesis
• Can diffuse across plasma membrane, then transported from cytosol to nucleus through nuclear pores
• Binding of thyroxine to its receptor includes conformational change such that receptor now binds to transcriptional activator, in turn recruits RNA polymerase. Gene then transcribed

Question 9

Nitroglycerin

Answer 9

• Used to treat angina (inadequate blood flow to the heart)
• Converted to NO, leading to vasodilation