Download Actin

Skeletal Muscle
~40-45% of BW
400+ in body
Responsible for all movement and
support
Composition
75% water
20% protein
5% other: fats, CHO, high energy
phosphates, urea, lactic acid,
enzymes, Na+, K+, Cl-, and
minerals: Ca2+, Mg, and
phosphorous
Striated in Apperance
Structure is fascia and connective
tissue sheaths which separate
individual muscles and hold muscles
in place
Under fascia: epimysium, CT
surrounding muscle
Under epimysium: fascicles, bundles
of muscle fibers enclosed by
perimysium
– There are ~150 fibers/fasciculus
Fasciulus
Bundle of muscle cells (fibers)
surrounded by perimysium
Each fiber in fasciulus is surrounded
by endomsysium
Under endomysium is sarcolemma,
cell membrane
Connective tissue surrounds muscle
and forms a network that extends
throughout the muscle
Sarcoplasm
Cytoplasm of muscle cell
Under sarcolemma
Contains contractile proteins,
enzymes, fat, and glycogen particles,
nuclei, and specialized cellular
organelles
Proteins
Greatest amounts:
Myosin
Actin
Tropomysin
Also, large amounts of myglobin
Muscle cells
Multi-nucleated
Striated appearance
Sarcoplasmic Reticulum (SR)
Embedded in sarcoplasm
Network of channels and vessicles
Lies in parallel to the myofibrils
Lateral end: sac-like cistern which
stores Ca2+
Transverse Tubules (T-Tubules)
Perpendicular to myofibril
T-tubule and two terminal cisternae are
the region of the Z-line
Called a triad
Two triads/sarcomere
One sarcomere: Z line – Z line
T-tubules open to the outside of each
muscle fiber
Functioning as a network, spreading the
AP from outer to inner portion of muscle
fiber
Muscle Blood supply
With exercise, increase in blood flow
to muscle
Myosin molecule
composed of 6 polypeptide chains
2 heavy
4 light
One myosin filament is made up of 200 or
more myosin molecules
Exactly 1.6 micrometers in length
Tails are in center
Heads coming out from the center (0.2
micrometers)
hinges at two points
– arm separates from the filament
– where the head attaches to the arm
filament is twisted, cross-bridges are displaced
from previous set by 120°
Insures that cross-bridges extend in all
directions from the filament
Actin
Primary protein in thin filament
Forms the backbone of filament
Approximately 3000 actin/myofibril
Troponin and Tropomyosin
Two other proteins in filament
Two shapes of actin
1. Globular (G-actin) polymerizes and
unfolds into
2. Fibrous (F-actin) double stranded in helix,
complete revolution every 70 nanometers
13 G-actin molecules in each revolution of
the helical strand
Each G-actin has an ADP molecule
attached to it
ADP are purported to be the active sites
on the actin for cross-bridge interaction
Active sites are staggered, one site on
total filament every 2.7 nanometers
each actin is 1 micrometer long
Filament bases
Inserted into Z discs, other end protrudes
into sarcomere
In spaces between myosin molecules
Z line
Actin filaments extending from either side
Into neighboring sarcomeres
Passes from myofibril to myofibril
Attaching the myofilaments to each other
across the muscle fiber
Sarcomere
Basic contractile unit, from Z line to Z line
Tropomyosin
Associated with the actin filament
Loosely connected to the F-actin strands
Wrap themselves spirally around the sides
of the F-actin helix
At rest, tropomyosin molecules lie on top
of the actin active sites, inhibiting
interaction between actin and myosin
each tropomyosin covers about seven
active sites
Troponin
Protein attached near one end of each
tropomyosin molecule
Complex of 3 protein subunits
1. Troponin I (TnI) strong affinity for actin
2. Troponin T (TnT) strong affinity for
tropomyosin
3. Troponin C (TnC) strong affinity for
calcium
Troponin complex may attach the
tropomyosin to the actin
Each subunit plays a role in the contractile
process
Sarcomere Ultrastructure
alternating light and dark bands along the
length of the muscle fiber give it its
characteristic striated appearance
Lighter area: I band
Darker zone: A band
I band (isotropic): when light passes through this
band, its velocity is the same
A band (anisotropic): light does not scatter
equally
Z line bisects the I band and adheres to the
sarcolemma, adding stability to the sarcomere
Z line has alpha actin (maintains spacing of
actin) and desmin (connects z lines of different
myofibrils together)
Position of the actin and myosin results in an overlap of
the filaments in the sarcomere
Center of the A band is the H zone, a lighter area due to
the absence of actin filaments in this region
Central part of the H zone is bisected by the M line,
which delineates the sarcomere’s center
M line is protein structures that support the arrangement
of the myosin filaments
also has myomesin which provides an anchor for titin
(elastic filament
Helps maintain centering between Z lines) and M-CK,
provides ATP from CP