Lesson 9: Molecular Motors & Actin/Myosin Cycle

motor proteins

translate chemical energy into motion

general motor protein mechanism

use ATP to produce cyclical conformation changes to move along a protein track

common features of molecular motors (2)

1) site for ATP binding & hydrolysis 2) site for binding to protein track

processivity

how long the motor can move along the track before dissociating

duty ratio

proportion of time the motor is bound to the track during 1 ATP cycle

myofibrils contain (2)

thick and thin filaments

sarcomere

repeating contractile unit making up myofibril

myosin (which filament)

thick filament

actin (which filament?)

thin filament

I-band

region with actin only, no myosin

A-band

region containing overlapping actin and myosin

Z-disk

anchors thin filaments; separates each sarcomere unit

titin

elastic protein anchoring myosin to Z-disk

which is motor protein and which is track in muscle contraction

actin = track; myosin = motor protein

myosin subunits

6 subunits; 2 heavy chains, 4 light chains

each terminus of heavy chains

C = extended 𝛼-helix tails, N = globular heads

myosin tails (made of)

2 heavy chains C-termini coil around each other in a coiled coil structure

myosin heads (made of)

2 heavy chains N-termini

thick filaments made of

several hundred myosins' tails bundle together, heads stick out

G-actin

monomer of actin; polymerizes to form F-actin

tropomyosin

covers actin's myosin-binding site

troponin

when Ca2+ binds, causes conformational change in tromomyosin to uncover myosin binding sites

sliding filament model

muscle contraction generated by interaction between actin and myosin

actin/myosin interaction w/ ATP bound

no binding

actin/myosin interaction w/ ADP + Pi bound

loose binding

actin/myosin interaction w/ nothing bound

tight binding

rigor state

myosin head tightly bound to actin in absence of ATP/ADP/Pi

crossbridge cycle (steps)

rigor state; no ATP/ADP/Pi and myosin is tightly bound --> ATP binds and myosin head disassociates from actin --> ATP hydrolyzed; myosin head cocks and weakly attaches to next G-actin molecule --> power stroke; Pi is released and the myosin head pivots, pulling actin along

power stroke

Pi released --> myosin head changes conformation, pulling actin along