BB Lecture 15: Carrier Proteins

Question 1

What is primary active transport?

Answer 1

when a solute that is moved against its gradient has a binding site on a pump which directly carries it through the membrane

Question 2

What is secondary active transport?

Answer 2

where a pump establishes a gradient for another solute (often Na+) across the membrane that can be harnessed by transporters to move a solute

Question 3

What kinds of pumps have ATPase activity?

Answer 3

all pumps

Question 4

List the different classes of pumps.

Answer 4

P-class
V/F class
ABC (ATP binding cassett) class

Question 5

What are examples of P-class pumps?

Answer 5

Na+/K+ ATPase

SERCA

Question 6

Describe the mechanism of a Na+/K+ ATPase pump.

Answer 6

three NA+ ions bind the high affinity sites in the cytoplasm (two low-affinity sites for K+ are unoccupied)
ATP binds the pump and is hydrolyzed by its ATPase activity
aspartate residue on the cytoplasmic side is phosphorylated
pump undergoes conformational change
Na+ ions move to low affinity sites exposed to the extracellular space and release
3 Na+ ions diffuse away
two K+ ions bind to the now high affinity sites on the intracellular side
hydrolysis of the aspartyl-phosphate bond
conformational change back to original state
release of K+ ions into extracellular space

Question 7

What are P-class pumps?

Answer 7

move only ions across the membrane

all operate similarly to the Na+/K+ pump

Question 8

What is a SERCA pump?

Answer 8

sarcoplasmic/endoplasmic reticulum Ca2+/ATPase
removes Ca2+ from the cytoplasm by sequestering it within intracellular storage organelles (the ER for most cells- SR in muscles)

Question 9

Describe the mechanism of action for a SERCA pump.

Answer 9

just like a K+/Na+ pump but with Ca2+ and H+

Question 10

What are V/F class pumps?

Answer 10

pump only protons
V-class pumps contribute to acidification of organelles (eg. lysosome) by pumping protons from the cytoplasm to the lumen of the organelle
F-class pumps use the movement of protons down their gradients to synthesize ATP

Question 11

What are ABC class pumps?

Answer 11

ATP binding cassette pumps
transport uncharged and some hydrophobic molecules
diverse structurally and functionally
bind ATP through conserved regions (ATP binding cassettes)
undergo conformational change to the active site which hydrolyzes ATP to ADP

Question 12

Provide examples of ABC-class pumps.

Answer 12

Multi-drug resistance (MDR) proteins

cystic fibrosis transmembrane regulator (CFTR)

Question 13

What are MDR proteins?

Answer 13

ABC class pumps
expressed highly in epithelial cells
transport small, polar molecules (including some products of normal metabolism)
also can pump a wide variety of drugs out of cells

Question 14

What are CFTR proteins?

Answer 14

ABC class pumps
expressed in lungs and other organs
DOES NOT HAVE a known pumping function
incorporates a channel permeable to Cl- ions and is regulated by protein kinase A
operates in cycles to open and close the Cl- channels

Question 15

What are unitransporters?

Answer 15

conduct a single species of molecule down its gradient

use facilitated diffusion

Question 16

What is facilitated diffusion?

Answer 16

a thermodynamically favored process that circumvents the hydrophobic barrier of the lipid bilayer and transport molecules

Question 17

What is a co-transporter?

Answer 17

couple thermodynamically favorable movement of one type of molecule (down its gradient) to the unfavorable movement of another (secondary active transport)

Question 18

What are the kinds of co-transporters?

Answer 18

symporter (move different solutes in the same direction across the membrane)
exchanger/antiporter (move the solutes in opposite directions)

Question 19

What is a GLUT?

Answer 19

glucose uniporter
binds single molecule of glucose at a time
conformational change exposes the glucose-binding site alternately to the extracellular and intracellular sides
rate of cycle determined by occupation of binding site in a given conformation
direction of cycle determined by concentration gradient of glucose

Question 20

What are SGLTs?

Answer 20

Na+/glucose transporters

couple Na+ influx to glucose uptake

Question 21

Describe the use of SGLTs in the kidney.

Answer 21

SGLT-2 is expressed in the tubule near the glomerulus (glucose concentration high) and exchanges glucose at a 1xNa+ : 1xglucose ratio
SGLT-1 is expressed at the distal end (the glucose concentration is significantly lower –> glucose gradient not able to be used effectively) and exchanges 2xNa+ : 1xglucose to establish a greater gradient
essentially no glucose should be excreted into the urine

Question 22

What is the Na+/Ca2+ exchanger (NCX)?

Answer 22

antiporter that couples Na+ entry to Ca2+ efflux (ratio of 3:1 respectively)
complementary to SERCA pump
contributes to Ca2+ clearance in cardiac cells

Question 23

Which ion is most commonly used as a gradient for co-transporters?

Answer 23

Na+
because it has a resting membrane potential driving force that is much greater than ions like K+ and Ca2+ is highly regulated because of its use as an important second messenger