Chapter 7 - Muscles Flashcards
Organization of Skeletal Muscles
bundle of muscle fibers surrounded by connective tissue
muscle -> fascicle (bundle of muscle fibers) -> indivdual muscle fiber (cell) -> myofibril
Muscle cell (myofiber)
Sarcoplasm
Sarcoplasmic reticulum
Sarcosome
Sarcolemma
Other organismal cells
cytoplasm
specialized smooth E.R.
mitochondrion
plasma membrane
Myofibrils
bundles of filaments within each muscle cell
Sarcoplasmic reticulum
releases Ca2+ after stimulation by a nerve impulse
Striations
arise from different bands of filarments: alternating bands
Sarcomere
repeating unit along muscle length
Bounded by Z-disks
I-bands - ends of sarcomere
A-bands - contains thick filaments
M-disks - center of sarcomere
Thick filaments
150 A consists of myosin motor protein
Thin filaments
70A consists of actin-polymeric protein
*filaments linked by cross-bridges where they overlap
Myosin (60-70%) and Actin (20-25%) of muscle tissue
Lengthwise Myofibril Anatomy
Thick and think filament overlapping arrangement
during muscle contractions, they move past each other via coordinated ineractions
Myofibril Contraction
the sliding filament model
Thick and thin filaments do not change
shorten along H-zone, volume doesnt change, so it gets thicker
Sarcomere changes:
H-zone (only thick) decreases
I-band (only thin) decreases
Myosin structure
dimer, has ampiphathic helices thing
Myson - 6 polypeptides: 540 kDa
2 identical heavy chains (220 kDa each)
2 pairs light chains: essential and regulatory (15-20kDa)
Myosin Head and Neck Region
N-terminal half of heavy chaing: globular structure
ATPase (binds and hydrolyzes ATP)
Neck forms lever between head and tail
Features: Light chain in neck region
Actin binding site
ATP binding pocket (occupied by sulfate ions?)
Myosin tail
designed for interaction with each other
sequence supports filament formation and ultrastructrue
7-,28-,and 196- residue repeats repsonsible for organizing thick filaments
coiled-coil tails -> thich filamet (several hudred myosin molecules)
Residues (1 and 4) a and d in 7-residue repeat -> hydrophobic
Residues 2,3, and 6 (b, c, and f) -> alternating ionic (-/+) in 28-residue pattern for packing in filament
Additional 196-residue pattern for stability of filaments around each other
Thin filamens: polymer of G-actin to form F-actin
very abundant filamentous protein in muscle and cytoskeleton
Functions: support, overall movement
Organelle movement along tracks: motor proteins along cytoskeleton
Monomer: globular, 375 aa, ATP binding (G-actin), 4 domains
Polymer: F-actin, polaryit (+ end bines to Z disk), and consistent orienation, double-chained helix
Microfilament assembly
Filaments grow at either end (not necessarily equal)
+ end - rapid addition
Dynamic: polymerization is reversible
Treadmilling: same rate of growth and dissociation -> in cell movement, etc
Actin Monomer and Polymer
ATP bound to monomer (not essential for polymerization)
Polymer subunits contains mostly ADP (ATP hydrolyzed)
One subunit contacts 4 others w/ head to tail orientation
One myosin binding site per actin subunit
No covalent bonds - ion pairing and hydrophobic interactions
Actin filaments - Myosin motor protein interaction
Myosin heads: independently binds to 1 actin monomer at a time. Myosin progresses along microfilament in steps due to movement of myosin head
ATP hydrolysis triggers conformational changes in myosin neck region: leads to mechanical forces
1) Protein conformation change
2) ATP hydrolysis
3) Myosin release & re-binding to new subunit in one direction
Cycle of myosin “walking” along actin filament monomers
Powered by ATP hydrolysis
1) ATP binds to myosin head; actin binding site opens; myosin head releases actin ->(ATP binds) 2) Active site closure followed by ATP hydrolysis causing cocking of myosin head -> (H2O binds) 3) weak binding of myosin head to actin ->4) Pi release resulting in strong binding of myosin head to actin ->(Pi released) 5) Power stroke 6)ADP release -> (ADP released, back to 1)
Tropomyosin and Troponin
associated with thin filaments
complex regulates myosin head access to acti filament
Ca2+ binding alters complex conformation exposing myosin binding sites on actin
Neural impulses stimulate muscle contraction through Ca2+ release from the sarcoplasmic reticulum
[Ca2+] increases approxiamely 100x
Intact thin filaments
F-actin, tropomyosin, and troponin C
Increased Calcium concentrations
cause conformational changes in tropoyosin-troponin (binds calcium) complex.
Reset when Ca2+ concentrations decrease leads to muscle relaxation
Z-disk
alpha actin possibly orients actin filaments
Actin Capping
Cap Z and Tropomodulin and Filament Control (Nebulin)
Titin
34,350 reidues/300 globular domains!!!
molecular bungee cord that keeps thick filament centered in sarcomere
M-disk
mymensin and M-protein bind titin and appear to participate in thick filament assembly with myosin-binding protein C
Dystrophin
protects sarcolemma from forces by anchoring actin filaments to extracellular matrix
Flexible, rod-shaped actin binding protein
Absent or low function in muscular dystrophy patients
Actin filaments in non-muscle cells
forms microfilaments
Active in processes: cell division, endocytosis, organielle transport, cell movement (see tread-milling)
