LG 1.7 - Biochemistry of Muscle (Actin/Myosin)

Question 1

define sarcomere

Answer 1

Smallest contractile unit of skeletal muscle. A bundle of overlaped myosin and actin

Question 2

define Myofibril

Answer 2

sarcomeres linked end-to-end for full length of a muscle <30 cm

Question 3

define muscle fiber, aka muscle cell

Answer 3

myofibrils bundled together within a cell for full length of a muscle

Question 4

define muscle Fascicle

Answer 4

A bundle of muscle fibers

Question 5

what’s another name for actin? what’s actin?

Answer 5

1. thin filament 2. formed by polymerization of
   G-actin (globular-actin) to form
   F-actin filament (fibrous actin)
2. then two f-actin filament twist around one-another to form actin

Question 6

what’s another name for myosin? what’s myosin?

Answer 6

1. thick filament. 2.formed by bundling together many extremely thin filaments that are made from coiled-coils of a-helices - and it has a large head group. 3 called thick filament because lots and lots of these extremely thin filaments combine to form one myosin filament

Question 7

what’s tropomyosin?

Answer 7

1. is a coiled-coil of a-helices (similar to myosin) but wraps around F-actin
2. initially tropomyosin blocks myosin head binding site (until Ca2+ binds to Troponin C)

Question 8

what’s troponin?

Answer 8

1. complex composed of I, C, and T subunits binds to tropomyosin
2. troponin C binds to Ca2+
3. troponin I binds to Actin (in doing so, Troponin I ‘I’nhibits binding to myosin)
4. Troponin T tethers troponin complex to tropomyosin

Question 9

Explain how myosin preserves the energy released by ATP hydrolysis and how that energy is used to in the power stroke.

Answer 9

• energy preserved in form of ADP + Pi

- Pi release supplies energy for powerstroke

Question 10

Explain mechanistically how the binding of calcium to troponin signals muscle contraction.

Answer 10

1. The sarcoplasmic reticulum releases Ca2+, which
2. binds to troponin, and induces a conformational change that moves tropononin away from the myosin binding site which
3. allows myosin to bind and initiates the crossbridge cycle

Question 11

Based on the cross-bridge cycle, explain the mechanism that underlies rigor mortis.

Answer 11

After ADP is released from myosin, the myosin head is attached to actin (and will NOT let go) until another ATP signals myosin head release from actin. If you’re dead, no ATP will signal that release - causing rigor mortis in muscle.

Question 12

Why is the myosin in the coiled-coiled alpha-helices configuration?

Answer 12

hydrophobic parts are pointing in and the hydrophilic parts are pointing out. these interactions help hold helices together.

Question 13

What are the subunits that make up troponin, what do each do?

Answer 13

• Toponin I: Inhibits binding of myosin.
• Troponin C: binds to Calcium.
• Toponin T: Tethers troponin complex to tropomyosin.