Lecture 6: Membrane Transport Proteins Flashcards
Transport system
-moving molecules across cell membrane
-cells need to bring in materials and excrete waste
Protein transporter steps
- bind cargo molecule on one side of membrane
- change in protein structure
- Release on opposite side
Pharmacokinetics (PK)
safety and efficacy profiles of drug
Clinical PK studies on drug interactions
-indicate transporters work together with enzymes in drug absorption and elimination
-drug substrate and inhibitor
Drug substrate
translocated across a membrane
drug inhibitor
-impair uptake/efflux of another drug
-not necessarily a substrate
ATP-dependent Transporters
-ATP-binding cassette
-ABC
Important transporters in drug distribution
-ATP-dependent
-Solute carrier (SLC)
P-glycoprotein
-important ABC transporter
-exports many drugs
-responsible for multi-drug resistance
-aka multidrug resistance protein 1
Human Solute Carrier (SLC) gene superfamily
-encodes membrane-bound transporters
-many substrates
-many of membrane transporters belong here
SLC transport substrates
-amino acids, oligopeptides
-glucose and sugars
-cations and anions
-bile salts, carboxylate, organic anions
-acetyl CoA
-essential metals, neurotransmitters, amines, vitamins, fatty acids, lipids, nucleosides, ammonium, choline, thyroid hormone, urea
Classes of Transporters
1st Class: Electrochemical-potential-driven transporters
-direction determined by electrochemical potentials
-move down concentration gradient
-NO energy input needed
-ports
uniport
one molecule one way
antiport
2+ molecules, opposite directions
symport
2+ molecules, same direction
SLC2 (GLUT) Family of transporters
-glucose transported by passive uniport mechanism
Na+/glucose cotransporter (SGLT, SLC5)
-transport of glucose into cells by a Na+ electrochemical gradient
-symport
-Na+ transport is energy source for cotransport of glucose INTO the cell AGAINST a glucose gradient
-secondary active transport
SGLT inhibitors
-treat type II diabetes
-excretes glucose instead of reabsorbing it in the kidney
-lowers blood glucose levels
More Na+ OUTSIDE the cell
-passive, facilitated transport of Na+ INTO the cell
Na+/K+ ATPase
-maintains Na+ gradient
-reason why Na+/glucose cotransporter is secondary active transport
Energy available from concentration gradient
-move from C1 to C2
dG = G2- G1 = RTln(C2/C1) + ZFdeltaPsi
-slide 9 babe
C2<C1
-transport from high to low
-deltaG < 0
-no need for energy input
2nd class: Primary Active Transporters
-AGAINST gradient
-use energy from ATP hydrolysis
-P, V, F-types, and ABC transporters
P-type
-phosphorylated during transport
-Na/K ATPase
V-type
vacuolar
F-type
-mitochondrial
ATP-binding cassette (ABC) transporters
-large family that moves metabolic products, lipids, sterols, drugs
-ATP hydrolysis for energy
-eukaryotic vs prokaryotic
-ABC domain
ABC transporters in prokaryotes
-large number that are both importers (nutrients) and exporters (cell surface components, pathogenesis factors)
ABC transporters in eukaryotes
-exporters/effluxers (NOT importers) that function as pumps out of the cell
-49 genes
-efflux lipids
-some involved in disease
-some involved in multi-drug resistance (anticancer drugs conferring resistance to tumors)
Cystic Fibrosis
-ABC transporter involvement
-abnormal transport of Cl- and Na+ by CFTR
Multidrug Resistance Family (MDR)
-subfamily of ABC transporters
-some extrude xenobiotics
-cause drug resistance
-overexpressed in cancer cells
ABC exporter mechanism (P-glycoprotein)
-alternating access of multidrug transporters
- inward facing = access from cytoplasm and membrane (substrates are hydrophobic)
- substrate binding converts to occuleded state, ATP bound to NBD
- outward facing= change in TM helices reduces affinity and promotes release to outside cell. ATP hydrolysis brives NBDs apart and converts TMDs back to inward facing step 1
Nucleotide binding domain (NBD), or ABC
-located on cytoplasmic side
-highly conserved
-ATP hydrolysis does NOT lead to phosphorylation of transporter
-exact mechanism is unknown
Mdr1
STRUCTURE SLIDE 15
drug resistance
drugs rapidly transported from cell
inhibitors
increase bioavailability of substrate
Efficiency
-transporters move molecules only 10^2-10^4 /s
-ion channels move up to 10^8 ions
What type of compound would be effective for inhibiting p-glycoprotein drug resistance?
-one that binds between TMD1 and TMD2 and competes with drug
-one that competes with ATP binding
