Active Transport

Question 1

Why is the active transport system called the sodium pump?

Answer 1

Since the extrusion of Na+ takes place against a steep electrochemical gradient (concentration and electrical)

Question 2

Structure of sodium pump:

Answer 2

ATP binding sites on intracellular surface

Cardiac glycoside binding sites on extracellular surface

2 alpha and 2 beta subunits

Question 3

What do the subunits in the sodium pump form?

Answer 3

Tetratmer

Question 4

Where do ions bind on sodium pump?

Answer 4

Na+ bind internally (3 per alpha subunit)

K+ bind externally (2 per alpha subunit)

Question 5

What is the sodium pump equation?

Answer 5

3Na+ (inside) + 2K+ (outside)

Against gradient (backwards reaction) = requires energy from breakdown of ATP

Reversible reaction

Question 6

In sodium pump what is ATP hydrolysed into?

Answer 6

Into ADP by enzyme (Na + K)ATPase

Question 7

What’s the Km for Na+ in sodium pump?

Answer 7

20nM

Question 8

What’s the Km for K+ in sodium pump?

Answer 8

1mM

Question 9

How fast does sodium pump reaction occur?

Answer 9

100 times per second

Question 10

What can sodium pump reaction be inhibited by?

Answer 10

Removing K+ concentration on outside

Question 11

What can sodium pump reaction be stimulated by?

Answer 11

By increasing NA+ concentration on inside

Question 12

What can sodium pump be inhibited by?

Answer 12

Cardiac glycosides (digoxin)

Question 13

What does digoxin do to the sodium pump?

Answer 13

Competes with K+ on outside of pump

Question 14

What happens if there is a decrease in K+ on outside?

Answer 14

There’s an increased affinity between cardiac glycosides and sodium pump

Question 15

What’s a therapeutic index?

Answer 15

Dose producing toxicity in 50% of population
————————————————————————
Minimum effective dose for 50% of population

E.g. 2:1 is a narrow therapeutic index = toxic effect close to therapeutic effect

Question 16

What’s hypokalaemia?

Answer 16

Low potassium blood concentration

Leads to an increase in digoxin binding

Result in abnormal heart rhythms, muscle weakness, spasms

Question 17

What’s normokalaemia?

Answer 17

Normal blood potassium levels

Question 18

What’s hyperkalaemia?

Answer 18

High blood potassium levels

Can result in nausea, palpitations, muscle weakness

Question 19

How may a patient develop digoxin toxicity?

Answer 19

May become hypokalaemic if on digoxin and diuretics

This reduces competition between K+ and digoxin

This results in increased digoxin binding to sodium pump

Digoxin has very narrow therapeutic index

Question 20

Emergency treatment for digoxin toxicity:

Answer 20

Administration of digitoxin binding antibody e.g. Digiband

Reverses toxicity by causing digoxin to dissociate from sodium pump

Question 21

What are primary active transport systems?

Answer 21

They directly couple the hydrolysis of ATP to molecular movement

Question 22

What are secondary active transport systems?

Answer 22

They use the energy stored in Na+ gradient to drive molecular transport against the electrochemical gradient