motility microfilaments

myosins

ATP-dependent motors that interact with and exert force on actin microfilaments, + end directed motors

myosin functions

muscle contraction, cell movement (lamellipodia, filopodia), phagocytosis (rearrangement of the cell cortex), vesicle transport

globular domains

binds actin and uses ATP hydrolysis for energy to move along the actin filament

what is the structure of type 2 myosins

one essential light chain and one regulatory light chain are associated with each heavy chain

type 2 myosins

pull arrays of actin filaments together, resulting in contraction of a cell or group of cells

hundreds of myosin 2 molecules bunch together in a staggered formation to make a

thick filament

what do all motor proteins have in common

they convert chemical energy into motion

sarcomeres

repeating units of myofibrils in each muscle fiber

thin filaments

actin, troponin, and tropomyosin

thick filaments

myosin

sarcomeres contain

bundles of thin and thick filaments

A bands

dark bands

I bands

light bands

what gives myofibrils a pattern of alternating dark and light bands?

the alignment of filaments in skeletal muscle

Z line

in the middle of each I band

what defines a sarcomere

the distance from one Z line to another

what is anchored to the ends of the Z line

+ ends of actin microfilaments

each thick filament consists of

hundreds of molecules of myosin, oriented in opposite directions in the two halves of the filament

how is the myosin arranged

staggered fashion

what do protruding heads of myosin molecules do

contact the adjacent thin filaments, forming cross-bridges

thin filaments __________ with thick filaments

interdigitate

thin filaments contain three proteins

F-actin, intertwined with tropomyosin and troponin

troponin

a calcium-sensitive switch that regulates myosin's access to the actin microfilament

sliding-filament module

muscle contraction is due to thin filaments sliding past thick filaments, with no change in length of either

myosin 2 moves toward the _____ ends so the thick filaments move toward the Z lines during contraction

+

what does the amount of force generated during contraction depend on?

the number of actin-binding domains that make contact with the thin filaments

regions of overlap between thin and thick filaments are always characterized by the presence of these transient

cross-bridges

cross-bridges

formed from links between the F-actin of thin filaments and myosin heads of thick filaments, form + dissociate rapidly

the contraction cycle

(1) cross bridge formation
(2) phosphate released
(3) power stroke; myosin undergoes a conformational change
(4)ADP is released
(5) ATP binds myosin, causing detachment of myosin from actin; cross-bridge dissociates
(6) ATP hydrolysis occurs, cocking myosin

which of the following steps are out of order in the muscle contraction cycle?
(1) ATP binds to myosin
(2) myosin binds to thin filaments and releases P
(3) ATP hydrolysis alters shape of myosin
(4) myosin power stroke is accompanied by release of ADP
(5) myosin is released from thin filaments

step 2 + 3

the regulatory proteins tropomyosin and troponin

regulate the availability of myosin-binding sites on actin filaments in a calcium dependent manner

what blocks myosin binding sites + must be moved for cross-bridges to form

tropomyosin

tropomyosin

physically blocks myosin from the actin

what can troponin do

can pull tropomyosin out of the way and has 2 shapes

troponin shape 1

tropomyosin blocks the myosin + isn't Ca+ bound

troponin shape 2

pulls tropomyosin out of the way + is Ca+ bound to troponin C and muscle contracts

when calcium concentration is loww

tropomyosin blocks the myosin-binding sites on the actin filament, preventing interaction with myosin

at higher concentrations, calcium binds TnC

causing tropomyosin to shift and allowing myosin to bind

if calcium concentration falls

tropomyosin moves back to the blocking position

nerve impulses from _______ activate the appropriate muscle cells

motor neurons

when nerve impulses to the muscle cease

calcium levels decline quickly, and the muscle relaxes

acetylcholine

opens Na+ channels on muscle cell - depolarization

depolarization

opens voltage gated Ca2+ channels

Ca2+

stimulates ryanodine receptors in the sarcoplasmic reticulum

influx of Ca2+

into the sarcoplasm (cytosol in muscle cells)

Ca2+ decline in the sarcoplasm (muscle relaxation)

is due to ATP-dependent Ca2+ pumps in the sarcoplasmic reticulum

smooth muscle

responsible for involuntary contractions in various tissues

instead of Z lines, smooth muscles have

dense bodies, plaque like structures

in smooth muscle, bundles of _____ and ______ filaments are anchored at both ends to the dense bodies

actin, myosin

calmodulin

Ca2+ binds to and activates

myosin light chain kinase (MLCK)

activated by calmodulin and phosphorylates a regulatory light chain of myosin

myosin light chain phosphorylation

leads to a conformational change in myosin, promoting its assembly into filaments and activates the myosin so that It can interact with actin filaments to undergo the cross-bridge cycle

as the calcium levels in the muscle cells fall

MLCK is inactivated

myosin light chain phosphotase

removes the phosphate from the myosin light chain and the muscle cell relaxes

contraction of skeletal muscle and smooth muscle cells both involve Ca2+ ions. how does the use of Ca differ between the two cell types?

the cells have different binding partners for Ca