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32 Cards in this Set

  • Front
  • Back

How does cotransport by symporters and anitporters work?

-Couple the movement of one type of ion/molecule against its concentration gradient


-Symporter = same direction


-Antiporter = different direction

Give examples of symporters(2) and antiporters(1)

-mammalian cells express many types of Na+ coupled symporters


-Na+ linked symporters (2 Na+/one glucose symporter)


-Na+ linked anti porters (3 Na+/one Ca2+ anti porter)

Describe how Na+ linked symporters work

-Enable animal cells to import glucose and AAs against high concentration gradients


-Most cells move glucose down a concentration gradient (GLUT1)


-Some cells need to move glucose up a concentration gradient from the extracellular


-Such cells use a two Na+/one-glucose symporter (this is a protein the couples import 1 glucose molecule to the import of 2 Na+ ions)

Describe how two Na+/one glucose symporters work

2Na+(out) + glucose(out) ⇌ 2Na+(in) + glucose(in)

Describe how Na+ linked antiporters work

-regulates the strength of cardiac muscle contractions


-in all muscle cells a rise in the cytosolic Ca2+ triggers contraction

Describe how three Na+/one Ca2+ antiporters work

-in cardiac muscle cells


maintains the low concentration of Ca2+ in the cytosol


3Na+(out) + Ca2+(in) ⇌ 3Na+(in) + Ca2+(out)


-10,000 fold higher on the outside

Cotransporters

-regulate cytosolic pH


-anaerobic metabolism yields lactic acid


-aerobic metabolism produces CO2 combining with water to make carbonic acid (H2CO3)


-2 types of cotransport proteins remove excess H+


1) Na+HCO3-/Cl- anti porter imports 1 Na+ and 1 HCO3- and exports 1 Cl- ion


2) Na+/H+ anti porter couples entry of 1 Na+ down its concentration gradient to the export of 1 H+


-if cell pH gets to high, the cell copes with excess OH- using an anion antiporter (catalyzes exchange of HCO3- and Cl- across the plasma membrane)


-at high pH Cl-/HCO3- anitporter exports one molecule o HCO3- for one molecule of Cl-, lowering cytosolic pH


-all three antiporters are regulated by the cytosolic pH

Red blood cells use an anion antiporter to _________ ___________

transport CO2

Transcellular transport steps

-ATPase pump

G-actin vs F-actin

-addition of a cation to a solution of G actin causes polymerization to F actin


-F actin will depolymerize into G actin when ionic strength of solution is low

G-actin

-globular monomer


-two lobes by deep cleft


-base of cleft is ATPase fold, where ATP and Mg2+ are bound

F-actin

-a chain of G actin


-(+) end favored for addition of subunits


-(-) end favored for dissociation & exposed binding site for ATP


-dissociation is equal on both sides


-association rate is different ( + end has higher association rate (this is why actin grows faster on one side))


-fx: mediate phagocytosis and serve as tracks for myosin motor proteins

Actin

-basic building block of microfilaments, is a major protein of eukaryotic cells and is highly conserved


-subunits in a filament are all oriented in the same direction, with nucleotide-binding site exposed on the (-) end

Steps of actin polymerization (3)

1) Nucleation


-longest wait time


2) Elongation


3) Steady state phase


-rate limiting step: the formation of a short actin oligomer (nucleus) that can then be elongated into filaments

Critical concentration

-above which ATP G actin will polymerize into filaments


-at monomer concentrations below CC no polymerization takes place


-when G actin is above CC it will grow, when it is below CC it will shrink


-ATP G actin adds much faster at the (+) end than at the (-), resulting in a lower CC at the (+) end

Accessory proteins used in actin polymerization (4)

-Profilin: enhances exchange of ADP for ATP on G actin. Block disassembly of G actin from the (+) of the filament


-Cofilin: enhances rate of loss of ADP-actin from the filament (-) end.


-Thymosin-B4: Sequesters G actin for polymerization


-Capping proteins: bind to filament ends, blocking assembly and disassembly


-CapZ caps (+) end


-Tropomodulin caps (-) end


Formins vs Arp2/3 complex

-Formin: assemble long actin filaments (of stress fibers and contractile rings)


-found in virtually all eukaryotic cells


-Arp2/3 complex: assemble branched actin filaments around the leading edge of motile cells

How do formin nucleate actin?

1) FH1: rich in proline residues and binds profilin ATP actin


-Increases the local concentration of profilin ATP G actin complexes


2) The actin is then fed into the FH2 domain to add G actin to the (+) end

How is formin regulated?

-many formins exist in a folded, inactive conformation


-this is due to an interaction between the first part of the protein and the C-terminus


-When Rho is activated by GTP it can bind and activate the formin

How does Arp2/3 nucleate actin?

-Many different NPFs, but major family is characterized by a region called WCA (WH2, connector, acidic)


1) actin subunit binds to W domain


2) The A domain binds the Arp2/3 complex


3) The Arp2/3 complex binds to the side of an actin filament and the actin bound to the W domain binds to the complex


4) This nucleates filament assembly at the (+) end.


5) The angle between the old filament and the new one is 70°

Explain how Listeria uses actin to move in the cell.

-The Listeria bacteria are being pushed through the cytoplasm by actin "tails"


-Listeria motility can be reconstituted in vitro with bacteria and four proteins


-ATP-G-actin


-Arp2/3 complex


-CapZ


-Cofilin


-Listeria hijacks cellular machinery to move


-ActA mimics NPF, activating the Arp2/3 complex


-Arp2/3 nucleates new filament assembly


-Filaments grow at their (+) end until capped by CapZ


-Free actin quickly regenerated by cofilin disassembling the (-) end

What are the three types of myosin?

Myosin 1


-one head domain


-fx: membrane association, endocytosis


Myosin 2


-can assemble into bipolar filaments


-fx: contraction


Myosin 5


-bind specific receptors on organelles, which they transport


-fx: organelle transport

How does myosin work?

-All move towards the (+) end


-Use ATP hydrolysis to move along the actin filament


-In the absence of ATP, the myosin head is attached to the actin filament


-State is short lived in living muscles, but is responsible for stiffness after death


-Myosin head pulls along the actin filament

How does Myosin 2 move?

1) The myosin head releases the actin filament when it binds ATP


2) The head hydrolyzes the ATP to ADP and Pi. "Cocked state" stores energy released by ATP hydrolysis as elastic energy.


3) Myosin in the "cocked" state binds actin


4) "Power stroke" moves actin filament with respect to the myosin neck domain


5) Head will remain bound to the filament when ADP is released and before ATP binds

Difference between processive and non-processive movement and what duty ratios are.

-Duty ratio: percent of contact to F actin during ATPase cycle


-Processive: >50% duty ration


-Non-processive: <50% duty ratio


-Myosin 2 has duty ration of 10% (non-processive)


Myosin 5 has duty ratio of >70% (processive)

How muscles contract, the accessory proteins, and how it is regulated

The arrival of action potential causes:


-the opening of voltage gated Ca2+ channels in the sarcoplasmic reticulum


-induces a change in troponin and tropomyosin


-the triggering of an action potential in the sarcolemma


accessory proteins:


-CapZ, Titin, Nebulin, Tropomodulin


Regulated:


-phosphorylation and dephospho rylation of the myosin 2 regulatory light chain


-contraction = myosin binding site exposed and +Ca2+

Pathway of cell movement

1) Extension


-One or more lamellipodia extending from the leading edge (Arp2/3 pushing it forward)


2) Adhesion


-Some lamellipodia adhere to the substrate by focal adhesion


3) Translocation


-The bulk of the cytoplasm flows forward


4) De-adhesion and endocytotic recycling


-The tail eventually detaches and retracts in the cell body

Morphology of microtubules

-Protofilaments (13 longitudinal repeating units)


-alpha and beta tubulin


-Major protein = tubulin


-associated proteins = MAPs (Microtubule associated proteins)


-alpha beta tubulin presented in all eukaryotes


-y tubulin = involved in microtubule assembly


-alpha tubulin = never be hydrolyzed because it is stuck between a & b interface


-beta subunit = can be hydrolyzed when GTP is bound

MTOCs

-mictotubule-organizing centers


-spontaneous nucleation unfavorable so all microtubules are nucleated from MTOCs


-(+) extends


-centrosome is the main MTOC in animal cells


-In a neuron, microtubules in both axons and dendrites are assembled to form a MTOC


-The microtubules that make up the shaft of a cilium or flagellum are assembled from an MTOC known as a basal body

Dynamic instability

-alphabeta tubulin concentrations need to be above critical concentration


-greater CC dimers add faster to one end then the other


-(+) end preferred for assembly, with beta tubulin exposed


-individuals can grow and the experience a catastrophe to a shrinking phase


-sometimes a depolymerizing microtubule end could go through a rescue and start polymerizing again


-dynamic of microtubule end determined by rate of growth, frequency of catastrophes, the rate of depolymerization and frequency of rescues (known as dynamic instability).


dynamic instability generally occurs at the (+) end


-growing = blunt


-shrinking = rams horns


-need GTP and beta subunit, if not it will depolymerize


-GTP = assembly


-GDP = disassembly

Anterograde vs retrograde

-anterograde: transport proceeds from the cell body to the synaptic terminal


-associated with axon growth and delivery of synaptic vesicles


-retrograde: opposite direction


-old membranes from the synaptic terminals move rapidly toward the cell body, may be degraded by lysozymes

Organelles in axons are transported along microtubules in both directions

-movement of vesicles along microtubules requires ATP


-rate is similar to that of fast axon transport in intact cells and it can proceed in both anterograde and retrograde directions