3.2 Biosignaling

Question 1

Ion Channels

Answer 1

proteins that create specific pathways for charged molecules

Question 2

Facilitated diffusion

Answer 2

• type of passive transport, is the diffusion of molecules down a concentration gradient through a pore in the membrane created by this transmembrane protein
• used for molecules that are
  impermeable to the membrane (large, polar, or charged)

Question 3

Ungated channels

Answer 3

have no gates and are therefore unregulated

Question 4

Voltage-Gated Channels

Answer 4

• the gate is regulated by the membrane potential change near the channel
  *Voltage-gated nonspecific sodium–potassium channels are found in cells of the sinoatrial node of the heart (serve as the pacemaker current; as the voltage drops, these channels open to bring the cell back to threshold and fire another action potential)

Question 5

Ligand-Gated Channels

Answer 5

• the binding of a specific substance or ligand to the channel causes it to open or close
  *neurotransmitters act at
  ligand-gated channels at the postsynaptic membrane
  *Membrane receptors may also display catalytic activity in response to ligand binding

Question 6

How does Km relate to ion channels?

Answer 6

refers to the solute concentration at which the transporter is functioning at half of its maximum activity

Question 7

Km and vmax parameters for ion channels?

Answer 7

Same Km and vmax parameters that apply to enzymes

Question 8

Enzyme-linked receptors

Answer 8

• have three primary protein domains: a membrane spanning domain, a ligand binding domain, and a catalytic domain

Question 9

Membrane-spanning domain

Answer 9

• anchors the receptor in
  the cell membrane

Question 10

Ligand-binding domain

Answer 10

• stimulated by the appropriate ligand and induces a conformational change that activates the catalytic domain which initiates a second messenger cascade

Question 11

Functions of heterotrimeric G proteins

Answer 11

• Gs stimulates.
• Gi inhibits.
• “Mind your p’s and q’s”: Gq activates phospholipase C.

Question 12

G protein-coupled receptors (GPCR)

Answer 12

• large family of integral membrane proteins involved in signal transduction
• characterized by their seven membrane-spanning α-helices
• receptors differ in specificity of the ligand-binding area found on the extracellular surface of the cell.
• G proteins are named for their
  intracellular link to guanine nucleotides (GDP and GTP)
• The binding of a ligand increases the affinity of the receptor for the G protein

Question 13

3 main G proteins

Answer 13

• Gs stimulates adenylate cyclase, which increases levels of cAMP in the cell
• Gi inhibits adenylate cyclase, which decreases levels of cAMP in the cell
• Gq activates phospholipase C, which cleaves a phospholipid from the
  membrane to form PIP2. PIP2 is then cleaved into DAG and IP3; IP3 can open calcium channels in the endoplasmic reticulum, increasing calcium levels in the cell

Question 14

GPCR Activation Steps

Answer 14

In its inactive
form, the α subunit binds GDP and is in a complex with the β and γ
subunits. When a ligand binds to the GPCR, the receptor becomes activated
and, in turn, engages the corresponding G protein, as shown in Step 1 of Once GDP is replaced with GTP, the α subunit is able to
dissociate from the β and γ subunits (Step 2). The activated α subunit alters
the activity of adenylate cyclase. If the α subunit is αs, then the enzyme is
activated; if the α subunit is αi, then the enzyme is inhibited. Once GTP on
the activated α subunit is dephosphorylated to GDP (Step 3), the α subunit
will rebind to the β and γ subunits (Step 4), rendering the G protein inactive