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what is the difference between the specificity of pump carrier and channel?
- pumps are absolute
- carriers are intermediate
- channels are only 10-20X
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what is the rate ion/s of pumps,carrier, and channel?
pumps carry 100, <1000, 10^6
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what does it mean when carriers uses an ion gradient to provide energy to transport a substrate?
it catalyzes a secondary reaction
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which membrane transport has many conformational changes?
channels
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which membrane transport protein requires energy to transport molecules against their concentration gradient?
pumps
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what is CFTR distributed and responsible for?
distributed in respiratory, pancreas PM, substrates are ATP and Cl, functions as Cl secretion
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what are MDR1, MDR2, and CFTR, TAP1,2 transporters under?
ABC transporters; largest family of transporters, can function as transporters or channels
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what are vitamin B12 transporters?
BtuC and BtuD
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what are the distribution, substrate and function of TAP1, 2
distribution in the ER, antigenic peptides, functions as ER/cytoplasm transport
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what is the function and distribution of MDR1, 2?
distribution in the plasma/liver membrane, drugs and organics (1), phosphatidylcholin (2), functions in drug secretion and flippase
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ABC transporters need what?
ATP for conformational change
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what causes the multiple drug resistance of human tumors?
overexpression of MDR ABC transporter
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what are sources of energy for active transporters?
light, ATP hydrolysis, ion gradients
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what maintains steep Na gradient across vertebrate membranes?
Na-K ATPase
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what is the purpose of selective permeable membranes?
maintain osmotic balance and electrical neutrality of cell despite large differences in solute concentrations across cell membranes; permeability of lipid bilayers to ions and hydrophilic molecules is very low
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what catalyze the diffusion of molecules across membranes?
ion channels and carrier proteins
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what do enzymes demonstrate in binding proteins?
substrate specificity, speed and saturability, theoretically capable of catalyzing transfer in either direction
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what comprises 1/3 of carriers?
MFS (major faciliator superfamily)
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what are some examples of uniporters?
Glucose and amino acid, GLUT
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what is the difference between a symporter and an antiporter?
symporters: use energy of concentration gradient of Na to transport another molecule through, antiporters use the conformational change from binding of one molecule to move another molecule up the gradient
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what is unique about uniporters?
- rate of diffusion is far higher than simple diffusion, transported molecules never enter the hydrophobic core of the lipid bilayer
- limited number of uniporter molecules,
- transport is specific--each transporter only transports a single species of molecule
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what are two major MFS carrier proteins from E. Coli?
LacY symporter (Lactose and H) and GlpT antiporter (G3P and Pi)
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what are three types of carriers?
- uniporters (C+S-->CS), antiporters, (C+(S1 or S2-->CS) symporters (C+S-->CS+Na-->CSNA)
- very similar in structure
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what is co-transport?
transport of one solute can be coupled to transport of a second solute
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symporter?
binding of first solute facilitates binding of second solute (ex. epithelial Na glucose transporter, Na amino acid transporter)
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antiporte
- solutes bind to different faces of membrane
- ex Na-H+ antiporter (pH control); Na-K ATPase
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what does glucose transporter in intestinal brush border use to concentrate glucose?
sodium gradient
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what results in vectorial transport of glucose across epithelial cells?
placement of transporters in different domains
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what establishes the existence of membrane carriers?
transport of radioactive glucose into red blood cells
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what does the rate of transport depend on?
- rate of substrate binding to enzyme
- at high substrate concentrations, the substrate binding sites on carrier are saturated, rate plateaus at max velocity owing to rate limiting conformational changes
- enzymatic like qualities
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what does the rate of substrate movement depend on?
rate of formation of substrate carrier complex on two sides of membrane (conformational change is reversible)
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what does the rate of secondary order reactions depend on?
substrate conformation
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how do we minimize leaks of one substrate across the membrane of symporters?
transmembrane reorientation of substrate binding site is more favorable for free carrier and carriers with 2 bound substrates than for carriers with one bound substrate
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what are some signals that channels respond to?
ligands, physiological or environmental stimuli, changes in membrane potential, mechanical forces
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what is the function of ion channels?
regulate electrical potential across membrane; coordinated openings or closing of channels change membrane potential and produce an electrical current.
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what causes nerve cells to be excitable?
opening and closing of channels
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what is the effects of kurtoxin from the South african scorpion?
messes with calcium channels
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what is the effects of pufferfish?
tetrodotoxin messes voltage gated Na channels
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what is affects of indian krait?
alpha bungarotoxin messes with AchR
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what are the effects of green mamba?
dendrotoxin, which blocks voltage gated K+ channels,nerve cells respond to these
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what determines the selectivity of the ion channels?
amino acids lining the pores
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what are channels made up of?
polypeptide chains (many of them are single polypeptide chain)
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how fast do ions pass ?
ions pass single file at a very high rate; 100 million/sec
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what is membrane potential?
sum of all charges
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what does it mean that prokaryotes have similar structures of ion channels
origins from prokaryotes, same structures, though with variation, elaboration due to gene duplication
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how does scorpion toxin affect cells?
block K entry, carbonyl reacts with K on inside
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who won the nobel prize in 1991 in physiological or medicine for discoveries concerning the function of single ion channels
erwin neher and bert sakmann
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how do we study membrane channels?
- using patch clampling (ripping off membrane, expressing channel to different solutions)
- can measure current produced by movement of ions through channels in patch
- currents measured in picoamp range
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who won the nobel prize in 1993 in chemistry for structural and mechanistic studies of ion channels?
roderick MacKinnon
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what can of gates are in ion channels?
ligands, mechanical, or voltage
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what are the functions of voltage gated channels?
produce action potential in excitable cells, convert electrical signal to chemical signals when they admit Ca, which it acts as a second messenger to stimulate secretion, activate protein kinases, etc
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what can trigger channels to open intracellularly?
cAMP
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what are some cyclic nucleotide-gated channels?
Na/Ca channels gated by cAMP or cGMP binding to a cytoplasmic domain
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what are glutamate receptors?
K+/Na+ channels gated by glutamate binding to the extracellular domain
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what does it mean when the different types of glutamate receptors show similarity in sequences?
- the polypeptides show gene duplication, evolutionarily conserved
- ATP kinase: b/c it has ATP binding domain (same binding domain and structure)
- domains bind to specific ligands
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how do we know something is an ATPase or a kinase?
has ATP binding pockets; kinases hydrolyze ATP, put phosphate on substrate
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electrocytes in torpedo electric organ are densely packe with what?
Acetylcholine receptors (can deliver 200 volt shock for 1 sec)
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describe Ache gated receptors
- two of the five subunits have binding sites for Ache
- ring of negatively charged amino acids is selectivity filter
- both K and Na pass through, less selective than K channel
- bound Ache increases channel opening time
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what helps water molecules move through aquaporin?
ASN 192 and ASN 76
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describe the process of hearing and how it is related to mechanical stress that open ion channels
hair cells have stretch activated membrane channels; hair cells have stereocillia on top and nerve cells at basal surface, embedded in supporting cells between two layers of membrane (tectorial and basilar) sound causes basilar membrane to vibrate, causing stereocila to touch tectorial; stereocilia tilt and trigger electrical response in hair cell which activates nerve cells below
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how does the stereocilia trigger nerve cells?
- each group of cilia responds to diff frequency of sound
- by moving stereocilia, they open mechanicaly gated cells, which depolarizes the cell and transmits to nerve to brain
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inactivation with ball and stick is what dependent?
voltage dependent, since it rarely occurs unless channel is open
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which side does ball on a chain block the channel?
cytoplasmic side
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why does opening channels change membrane potential but not cytoplasmic ion composition?
b/c only a few ions need to pass membrane to produce a large change in membrane potential--this conserves energy so pumps won't be used
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