General properties of ion channels

Question 1

Ion channels

Answer 1

• proteins that serve as channels for ions
• uniport, symport, antiport
• direction in which ion goes depends on ions concentration gradient and general electric gradient
• conc. gradient is more important

Question 2

Patch clamp technique

Answer 2

• used for understanding the structure and way of function of ion channels
• a micro electrode clamps a piece of membrane and measures the current across it
• Permeability of membrane for ions is a result of opening of discrete pores- each pore can be studied individually
• Purification from native tissues and reconstitution in liposomes of presumed ion channel protein results in appearance of unitary ion currents
• are proteins heterologous expression of cloned ion channel genes- many unitary ionic currents

Question 3

Open probability

Answer 3

the time the channel stays in open conformation

Po= total time open /total observation time

Question 4

Voltage gated channels

Answer 4

• has several open/closed conformations, rate constant determine probabilities of transition among these states
• voltage + ligand affect gating by altering rate constants + open probability is function of latter
• changes in open probability of single ion channels underlie time-dependent changes in membrane ion permeability

Question 5

Throughput rate

Answer 5

• is the amount of ions transported through channel at given time
• is almost same as theoretical max diffusion limit
• ion channel a bit slower- binds the ion, allowing for selective transport only for that ion
• limit how fast ion channel can transport - gets saturated

Question 6

Selectivity

Answer 6

• throughput rate = upper limit, still perfect selectivity
• pore contains binding sites for permeate ions - lower energy barrier, saturable conductance
• some pores accommodate more than 1 ion
• conserved P-loop sequenses detrmine selectivity of K+ and voltage gated Na+ and Ca2+ channels

Question 7

Structure of K+ channel

Answer 7

• 4 protein units, each has 6 transmembrane helices, 2 of which separated by P-loop
• P-loop lines pore + control specificity
• gate on intracellular side
• selectivity filter

Question 8

Selectivity filter

Answer 8

• bottle neck for ions on extracellular side
• K+ present only in 4 positions, in single file
• only 2 ions present the same time, separated by water
• K+ coordinated by main chain carbonyl oxygen atoms of TXGYG sequence
• AA side chains anchored in interior or protein - rigid d
• in binding site, K+ coordinated by 8 carbonyl oxygens arranged to mimic hydration shell of K+ in solution
• in transition state - coordinated by 6 oxygens - low energy barrier
• diameter of rigid oxygen rings are too big for Na+