Chapter 5. Ion Channels

Front 1

What are three important properties of ion channels?

Back 1

1) They conduct ions

2) They recognize and select among specific ions

3) They open and close in response to specific electrical, mechanical, or chemical signals

Front 2

What determines the membrane potential at rest?

Back 2

Ion channels that are selectively permeable to K+

Front 3

What does gated refer to with ion channels?

Back 3

They open and close in response to various stimuli

Front 4

How thick is the plasma membrane?

Back 4

6-8 nm

Front 5

What is the polarity of water molecules?

Back 5

Dipolar

Front 6

What are the waters of hydration?

Back 6

Electrostatically bound water that surrounds ions; make it extremely unlikely for an ion to move from solution to the lipid bilayer

Front 7

How can a water-filled channel conduct at high rates and still be selective?

Back 7

By the size of the shell of water surrounding the ion

Front 8

What is the relationship between the size of an ion and its mobility in solution?

Back 8

Inverse

Front 9

What is the carrier model of ion transport?

Back 9

Ions can cross membranes by means of carriers, or transport proteins, such as the Na+-K+ pump

Front 10

What is evidence that some ions are not transported into the neuron by carriers?

Back 10

The rate of ion transfer

Front 11

What is a selectivity filter?

Back 11

A narrow region that acts as a molecular sieve, where an ion sheds most of its waters of hydration and forms a weak chemical bond (electrostatic interaction) with charged or polar amino acid residues that line the walls of the channel

Front 12

Why are ion channels difficult to crystallize?

Back 12

Hydrophobic regions

Front 13

What are two methods used to study ion channels?

Back 13

Single-channel recording and gene cloning

Front 14

Can ion channels open partially or are the all-or-none?

Back 14

All-or-none

Front 15

Which circuit component does the ion channel act like?

Back 15

A simple resistor

Front 16

What is the typical value of the conductance of an ion channel?

Back 16

12 picosiemens

Front 17

Describe the patch-clamp technique.

Back 17

Records current flow from single channels in biological membranes; a glass micropipette containing acetylcholine pressed tightly against the muscle membrane, small unitary current events, representing the opening and closing of single acetylcholine-activated ion channels, were observed in the area of the membrane under the pipette tip

Front 18

What will be required for definitive answers to questions about ion channels?

Back 18

X-ray crystallographic analysis of purified ion channel proteins

Front 19

What do all channels possess at their centers?

Back 19

A central aqueous pore

Front 20

What is a hydrophobicity plot?

Back 20

Used to identify the regions of a protein that are nonpolar; each amino acid residue is assigned a hydrophobicity index based on the nature of its side chain

Front 21

How many alpha-helices are required to form a complete channel?

Back 21

4 to 6

Front 22

Describe the suspected voltage-sensitive part of the ion channel.

Back 22

An alpha-helix membrane-spanning domain that contains positively charged amino acids at every third position

Front 23

How can you test the structure for a channel?

Back 23

Determine whether an antibody binds extra- or intracellularly

Use genetic engineering to compare the properties of a chimeric channel

Use site-directed mutagenesis in which specific amino acid residues are substituded or deleted

Front 24

Are ion channels active or passive?

Back 24

Passive, require no energy

Front 25

What is a rectifier?

Back 25

A channel that conducts ions more readily in one direction than in the other when  the direction of the driving force is reversed

Front 26

How do you characterize the conductance of a rectifying channel?

Back 26

By plotting current versus voltage for the channel over the physiological voltage range

Front 27

Can current flow through a channel saturate?

Back 27

Yes

Front 28

What idea is saturation consistent with?

Back 28

Binding of ions to specific polar sites within the polar sites rather than simple diffusion

Front 29

What defines the dissociation constant for ion binding in the channel?

Back 29

The ionic concentration at which current flow is half-maximal

Front 30

What are allosteric proteins?

Back 30

A protein that has two or more conformational states that are relatively stable

Front 31

What is the transition of a channel between closed and open states?

Back 31

Gating

Front 32

What are three physical models for channel gating?

Back 32

1) A discrete conformational change in one region

2) A generalized conformational change along the length of the channel

3) A blocking particle swings into and out of the channel mouth

Front 33

Is  the binding of a chemical ligand to the ion channel covalent or noncovalent?

Back 33

Noncovalent

Front 34

How does a second messenger act directly on channels?

Back 34

By binding to the channel

Front 35

How does a second messenger act indirectly on channels?

Back 35

By initiating protein phosphorylation that is mediated by protein kinases

Front 36

What are modulatory changes?

Back 36

Relatively long-lasting changes in the functional states of ion channels caused by covalent modification

Front 37

What are the three functional states of ion channels?

Back 37

Closed and activatable (resting), open (active), and nonactivatable (refractory)

Front 38

What is the mechanism of voltage-gating?

Back 38

Changes in the membrane voltage move a charged region of the channel back and forth, driving the channel between closed and open states

Front 39

What is the mechanism of chemical gating?

Back 39

The change in free energy of the ligand bound to its site on the channel leads to channel opening

Front 40

When do ligand-gating channel enter a refractory state?

Back 40

In high concentration of the ligand (desensitization)

Front 41

What is inactivation?

Back 41

The refractory stage of voltage-gated channels

Front 42

How would you block an Ach channel?

Back 42

alpha-bungarotoxin

Front 43

What could cause ion channel variability?

Back 43

Differential expression of two or more homologous genes or by alternative splicing of the mRNA from the same gene

Front 44

Which autoimmune neurological disorders result from antibodies interfering with channel function?

Back 44

Myasthenia gravis and Lambert-Eaton syndrome

Front 45

What is the benefit of patch-clamping?

Back 45

It creates a seal with extremely high resistance, lowering electronic noise