3.2 Biosignaling Flashcards
Ion Channels
proteins that create specific pathways for charged molecules
Facilitated diffusion
- 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)
Ungated channels
have no gates and are therefore unregulated
Voltage-Gated Channels
- 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)
Ligand-Gated Channels
- 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
How does Km relate to ion channels?
refers to the solute concentration at which the transporter is functioning at half of its maximum activity
Km and vmax parameters for ion channels?
Same Km and vmax parameters that apply to enzymes
Enzyme-linked receptors
- have three primary protein domains: a membrane spanning domain, a ligand binding domain, and a catalytic domain
Membrane-spanning domain
- anchors the receptor in
the cell membrane
Ligand-binding domain
- stimulated by the appropriate ligand and induces a conformational change that activates the catalytic domain which initiates a second messenger cascade
Functions of heterotrimeric G proteins
- Gs stimulates.
- Gi inhibits.
- “Mind your p’s and q’s”: Gq activates phospholipase C.
G protein-coupled receptors (GPCR)
- 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
3 main G proteins
- 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
GPCR Activation Steps
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