14.2 Myosin Motors

Question 1

() is a molecular motor - a protein that converts chemical energy (ATP) to mechanical energy → generates force and movement

Answer 1

myosin

Question 2

what does a molecular motor do

Answer 2

a protein that converts chemical energy (ATP) to mechanical energy → generates force and movement

Question 3

skeletal muscles are bundles of () - single large cells formed by the fusion of many individual cells during development

Answer 3

muscle fibers

Question 4

most of the cytoplasm of muscle fibers consists of () - cylindrical bundles of thick myosin filaments and thin actin filaments

Answer 4

myofibrils

Question 5

myofibrils are cylindrical bundles of thick (1) and thin (2)

Answer 5

1. myosin filaments
2. actin filaments

Question 6

each myofibril is organized as a chain of contractile units called () → responsible for the striated appearance of skeletal and cardiac muscle

Answer 6

sarcomeres

Question 7

ends of sarcomeres are defined by ()

Answer 7

Z discs

Question 8

actin filaments are attached to Z discs at their ()

Answer 8

plus ends

Question 9

myosin filaments are present in the () band of sarcomeres

Answer 9

dark A

Question 10

on the other hand, myosin is absent in the () band of sarcomeres

Answer 10

light I

Question 11

actin filaments overlap with myosin at their ()

Answer 11

minus ends

Question 12

in the () of the sarcomere, ONLY myosin is present

Answer 12

H zone

Question 13

sarcomeres are symmetrical about the (), where the myosin filaments are anchored at the center of the sarcomere

Answer 13

M-line

Question 14

the sliding filament model states that ()

Answer 14

muscle contraction results from actin and myosin filaments sliding past one another

Question 15

which bands in the sarcomere shorten during muscle contraction? which bands stay the same width?

Answer 15

1. I bands and H zones become thin (almost completely disappear)
2. A band stays the same width

Question 16

molecular basis of actin-myosin sliding interaction is the ()

Answer 16

binding of myosin to actin filaments → myosin functions as a motor that drives filament sliding

Question 17

the myosin type in muscle

Answer 17

myosin II

Question 18

myosin II (myosin type in muscle) has ()

Answer 18

2 heavy chains and 2 pairs of light chains

Question 19

each myosin protein consists of a (1) region and a (2) region

Answer 19

1. globular head
2. long ⍺-helical tail

Question 20

how are cross-bridges between thick and thin filaments in muscle formed

Answer 20

globular heads of myosin bind to actin

Question 21

activity of myosin as a molecular motor is powered by ()

Answer 21

binding and hydrolyzation of ATP at its head group

Question 22

overview of movement cycle of actin-myosin

Answer 22

1. myosin (no ATP) is tightly bound to actin
2. ATP binding at myosin head groups dissociates actin-myosin complex
3. hydrolysis of ATP induces a conformational change in myosin → affects neck region of myosin that binds light chains
4. light chains on myosin act as a lever arm to displace myosin head by about 5 nm
5. products of hydrolysis (ATP and Pi) remain bound to myosin head currently in a “cocked” position
6. myosin head rebinds at a new postion on actin filament → results in release of Pi
7. release of Pi triggers a “power stroke” in which ADP is released and myosin head returns to initial conformation → actin slides towards M line

Question 23

muscle contraction is triggered by nerve impulses that stimulate the release of Ca2+ from the ()

Answer 23

sarcoplasmic reticulum

Question 24

how does the release of Ca2+ contribute to muscle contraction

Answer 24

high Ca2+ concentrations cause a shift in the troponin-tropomyosin complex position, allowing myosin heads to interact with actin filaments

Question 25

the troponin complex is composed of:

Answer 25

• troponin I - inhibitory
• troponin C - Ca2+ binding
• troponin T - tropomyosin binding

Question 26

contractile assemblies (contractile rings) in nonmuscle cells function primarily in ()

Answer 26

cell division (cytokinesis)

Question 27

actin-myosin filaments in nonmuscle cells aren’t ()

Answer 27

organized into sarcomeres

Question 28

give 2 examples of contractile assemblies in nonmuscle cells

Answer 28

• stress fibers - contraction produces tension across the cell → cell is able to pull on a substratum
• adhesion belts - contraction alters the shape of epithelial cell sheets

Question 29

toward the end of mitosis (in yeast and animal cells), a () consisting of actin and myosin II filaments is assembled by membrane-bound myosin beneath the plasma membrane

Answer 29

contractile ring

Question 30

contraction of actin-myosin in nonmuscle cells is regulated primarily by the ()

Answer 30

phosphorylation of the regulatory light chain on myosin II

Question 31

phosphorylation of regulatory light chain on myosin II is catalyzed by ()

Answer 31

myosin light-chain kinase

Question 32

myosin light-chain kinase is regulated by the Ca2+ binding protein ()

Answer 32

calmodulin

Question 33

how does phosphorylation contribute to contraction of contractile assemblies on nonmuscle cells

Answer 33

promotes assemply of myosin into filaments and increases myosin catalytic activity → enables contraction to proceed

Question 34

how does calmodulin regulate MLCK?

Answer 34

increased cytosolic Ca2+ promotes binding of calmodulin to kinase → responsible for activating myosin in smooth muscle and nonmuscle cells by phosphorylating the regulatory light chain on myosin II

Question 35

() are nonmuscle myosins that don’t form filaments and are not involved in contraction

Answer 35

unconventional myosins

Question 36

2 examples of unconventional myosins

Answer 36

myosin I and myosin V

Question 37

unconventional myosin involved in vesicle transport; its movement along actin can move cargo

Answer 37

myosin I

Question 38

unconventional myosin important for vesicle/cargo transport in neurons

Answer 38

myosin V