Chapter 7 - Muscles

Question 1

Organization of Skeletal Muscles

Answer 1

bundle of muscle fibers surrounded by connective tissue

muscle -> fascicle (bundle of muscle fibers) -> indivdual muscle fiber (cell) -> myofibril

Question 2

Muscle cell (myofiber)

Sarcoplasm

Sarcoplasmic reticulum

Sarcosome

Sarcolemma

Answer 2

Other organismal cells

cytoplasm

specialized smooth E.R.

mitochondrion

plasma membrane

Question 3

Myofibrils

Answer 3

bundles of filaments within each muscle cell

Question 4

Sarcoplasmic reticulum

Answer 4

releases Ca2+ after stimulation by a nerve impulse

Question 5

Striations

Answer 5

arise from different bands of filarments: alternating bands

Question 6

Sarcomere

Answer 6

repeating unit along muscle length

Bounded by Z-disks

I-bands - ends of sarcomere

A-bands - contains thick filaments

M-disks - center of sarcomere

Question 7

Thick filaments

Answer 7

150 A consists of myosin motor protein

Question 8

Thin filaments

Answer 8

70A consists of actin-polymeric protein

*filaments linked by cross-bridges where they overlap

Myosin (60-70%) and Actin (20-25%) of muscle tissue

Question 9

Lengthwise Myofibril Anatomy

Answer 9

Thick and think filament overlapping arrangement

during muscle contractions, they move past each other via coordinated ineractions

Question 10

Myofibril Contraction

Answer 10

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

Question 11

Myosin structure

Answer 11

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)

Question 12

Myosin Head and Neck Region

Answer 12

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?)

Question 13

Myosin tail

Answer 13

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

Question 14

Thin filamens: polymer of G-actin to form F-actin

Answer 14

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

Question 15

Microfilament assembly

Answer 15

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

Question 16

Actin Monomer and Polymer

Answer 16

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

Question 17

Actin filaments - Myosin motor protein interaction

Answer 17

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

Question 18

Cycle of myosin “walking” along actin filament monomers

Answer 18

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)

Question 19

Tropomyosin and Troponin

Answer 19

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

Question 20

Intact thin filaments

Answer 20

F-actin, tropomyosin, and troponin C

Question 21

Increased Calcium concentrations

Answer 21

cause conformational changes in tropoyosin-troponin (binds calcium) complex.

Reset when Ca2+ concentrations decrease leads to muscle relaxation

Question 22

Z-disk

Answer 22

alpha actin possibly orients actin filaments

Question 23

Actin Capping

Answer 23

Cap Z and Tropomodulin and Filament Control (Nebulin)

Question 24

Titin

Answer 24

34,350 reidues/300 globular domains!!!

molecular bungee cord that keeps thick filament centered in sarcomere

Question 25

M-disk

Answer 25

mymensin and M-protein bind titin and appear to participate in thick filament assembly with myosin-binding protein C

Question 26

Dystrophin

Answer 26

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

Question 27

Actin filaments in non-muscle cells

Answer 27

forms microfilaments

Active in processes: cell division, endocytosis, organielle transport, cell movement (see tread-milling)