Chapter Text 35.2

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Chapter Text 35.2
2014-10-31 15:31:40
Test Three
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  1. ·         Myosins, kinesins, and dyneins move by what?.
    o   Myosin moves along __
    cycling between states with different affinities for the long, polymeric macromolecules that serve as tracks along which they move

  2. Actin structure: __
    each actin monomer comprises four domains, which come together to surround a bound nucleotide, either ATP or ADP; these actin monomers (G-actin) come together to form actin filaments (F-actin)
  3. o   F-actin is __and resembles a __
    §  Because each actin monomer is oriented in what way, the structure is __
    ·         One end: __
    ·         Other end: __
    • helical 
    • two stranded cable

    • in the same direction along the F-actin filament
    • polar

    • barbed (plus) end
    • pointed (minus) end
  4. ·         Actin filaments do what even though what? To overcome this unfavorability, specialized protein structures, such as Arp2/3, serve as __ for __. Once such a filament exists, __.
    self assemble although the aggregation of the first two or three monomers to form a filament is unfavorable

    nuclei for actin assembly in cells

    adding subunits is favorable
  5. ·         Ex of polymerization reaction with actin filament (containing n subunits) An, which can bind an additional monomer to form An+1.
    o   The dissociation constant, __, defines the __, because the concentration of polymers of length n+1 will be equal to that for polymers of length n.

    monomer concentrations at which the polymerization reaction will take place
  6. §  In other words, the polymerization reaction will proceed until __.
    ·         If below Kd, what?; existing filaments will __until the __
    o   Therefore, Kd is referred to as the __for the polymer
    • the monomer concentration is reduced to the value of Kd
    • the polymerization reaction will not proceed at all
    • depolymerize 
    • monomer concentration reaches the value of Kd
    • critical concentration
  7. ·         Actin filaments inside cells are __ structures that are continually __ and __.
    o   __by complexes such as Arp2/3 can initiate __.
    o   Hydrolysis of bound ATP to ADP favors __
    ·         This reaction acts as a __ to make __
    o   Proteins that __ or __ also play roles
    o   Polymerization reactions can __ pushing or pulling on cell membranes
    • highly dynamic
    • gaining and losing monomers.
    • Nucleation 
    • the polymerization of actin-ATP
    • actin depolymerization
    • timer 
    • actin filaments kinetically unstable
    • bind actin monomers
    • promote the severing of actin filaments
    • exert force
  8. ·         __ is central to the changes in cell shape associated with __ in amebae as well as in human cells such as macrophages
    ·         Treatment of actin filaments with myosin S1 fragments in the absence of ATP results in a complex called a __
    o   The myosin head domain is in a conformation what?
    o   The structure also reveals the __
    o   The modeling suggests that the __
    §  These conformational changes result in a __
    • Regulated actin polymerization
    • cell motility
    • decorated actin
    • close to that observed for the nucleotide-free form
    • interaction surfaces between myosin and actin
    • myosin head-domain conformation changes somewhat to increase its interaction with the actin filament
    • slight opening of the nucleotide-binding site in myosin
  9. ·         To study mechanisms of muscle contraction and other complex processes, an __is used. Explain the process
    • optical trap, which relies on highly focused laser beams and small beads
    • o   Each bead could be caught in an optical trap (one at each end of the filament) and the actin filament could be pulled taught over a microscope slide containing other beads that had been coated with fragments of myosin such as the heavy meromyosin fragment
  10. §  On the addition of ATP, what was observed in the optical trap.
    ·         The size of the displacement steps was fairly uniform with an average size of 11 nm
    ·         Individual myosin heads __ and undergo a conformational change that does what?

    o   After a period of time, the myosin head does what?
    transient displacements of the actin filament were observed along its long axis

    bind the actin filament

    pulls the actin filament, leading to the displacement of the beads.

    releases the actin, which then snaps back into place
  11. ·         Phosphate release triggers the __
    o   A key observation is that the addition of ATP to a complex of myosin and actin results in __. Thus, ATP binding and hydrolysis cannot be directly responsible for the __.
    o   The binding of ATP to actin results in __. With ATP bound and free of actin, the myosin domain __.
    §  This conformational change results in the __. 
    • myosin power stroke
    • the dissociation of the complex
    • power stroke
    • dissociation of myosin from actin
    • undergoes the conformational change associated with the formation of the TS for ATP hydrolysis
    • reorientation of the lever arm
  12. o   In this form, the myosin head can __; phosphate is __.
    §  This conformation change represents the __ and moves the body of the myosin molecule relative to the __ by approximately 110 angstroms. 
    • dock onto the actin filament
    • released with concomitant motion of the lever arm
    • power stroke
    • actin filament
  13. o   The __ completes the cycle
    ·         The mechanism of moving a single myosin molecule relative to an actin filament explains how muscles contract
    o   Voluntary muscle is __, consisting of __cells bounded by an electrically excitable plasma membrane.
    • release of ADP
    • striated
    • multinucleated
  14. §  The muscle cell has __; and, the functional unit, called a __, repeats every 2.3 micrometers along the fibril axis in relaxed muscle
    ·         A __ and __ alternate regularly
    o   The central region of the __, termed the __, is less dense than the rest of the band.
    o   The __ is bisected by a dense, narrow __
    • myofibrils
    • sarcomere
    • dark A band and a light I band
    • A band
    • H zone
    • I band
    • Z line
  15. ·         In a sarcomere, there are two kinds of interacting protein filaments
    o   The thick filaments have diameters of about __ and consist primarily of __
    o   The thin filaments have diameters of approximately __ and consist of __
    §  Muscle contraction is achieved through the __
    • 15 nm
    • myosin

    • 8 nm
    • actin as well as tropomyosn and the troponin complex

    sliding of the thin filaments along the length of the thck filaments, driven by hydrolysis of ATP
  16. o   To form the thick filaments, myosin molecuels do what, with the __ doing what?
    §  About __ line the surface of each thick filament. Each head-rich region associates with two __, one on each side of the myosin molecules
    ·         The interaction of __with __units causes __and thus __
    • self-assemble into thick bipolar structures
    • myosin heads
    • protruding at both ends of a bare region int eh center
    • 500 head domains

    • actin filaments
    • individual myosin heads 
    • actin 
    • sliding 
    • muscle contraction
  17. ·         __ and __ regulate the __in response to __.
    o   Under resting conditions, tropomyosin does what?
    §  A nerve impulse leads to an __
    A component of the __ senses the __ and, in response, __
    • Tropomyosin and the troponin complex
    • sliding 
    • nerve impulses

    • blocks the intimate interaction between myosin and actin
    • increase in calcium ion concentration within the muscle cell
    • troponin complex
    • increase in Ca2+
    • relieves the inhibition of myosin-actin interations by tropomyosin
  18. ·         How does the myosin reaction cycle apply to muscle contraction?
    o   The head domains are paired in myosin dimers, but the two heads within each dimer act independently. Actin filaments associate with each head-rich region, with the barbed ends of actin toward the Z line.
  19. o   In the presence of normal levels of ATP, most of the myosin heads are __
    §  Each head can do what? Because few other heads are attached, the actin filament is __
    ·         Each head cycles five times per second with a movement of 110 angstroms per cycle.
    detached from actin

    independently hydrolyze ATP, bind to actin, release Pi, and undergo its power stroke

    relatively free to slide
  20. ·         However, when hundreds of heads are interacting with the same actin filament, the overall rate of movement of myosin relative to the actin filament may reach __, allowing what?
    o   Having many myosin heads briefly and independently attaching and moving an actin filament allows for __
    • 80,000 agnstroms per second
    • a sacromere[1] to contract from its fully relaxed to its fully contracted form rapidly.
    • much greater speed than could be achieved by a single motor protein
  21. ·         The length of the lever arm determines __, which acts as an __ by __to achieve the 110-Angstrom movement along the actin filament that takes place in each __
    o   Prediction: the length traveled per cycle should depend on the __, which should influence the __
    • motor veolicty
    • amplifier
    • amplifying small structural changes at the nucleotide-binding site 
    • ATP hydrolysis cycle
    • length of the lever arm
    • overall rate at which actin moves relative to a collection of myosin heads
  22. §  This was tested with the use of mutated forms of myosin with lever arms of different lengths
    ·         The lever arm in muscle myosin has __; so, investigators did what by deleting the sequences that correspond to one or both of these binding sites, then examining the rates at which __
    o   The rate __as the lever arm was shortened
    • binding sites for two light chains
    • shortened the lever arm
    • actin filaments were transported along collections of these mutated myosins
    • decreased
  23. ·         A mutated form of myosin with an unusually __ was generated by inserting 23 amino acids corresponding to the binding site for an additional regulatory light chain.
    o   This form was found to __ 
    §  These results strongly support the proposed role of the lever arm in contributing to myosin motor activity
    • long lever arm
    • support actin movement that was faster than that of the wild-type protein.