Contractile Mechanisms & Energy Sources (S1)

Question 1

What is the mechanism of contraction within the smooth muscle?

Answer 1

• Increase in cytosolic Ca²⁺
• Ca²⁺ + Calmodulin
• Activation of MLCK
• Phosphorylation of MLC
• Myosin ATPase activates
• Attachment of myosin head with actin
• Contraction by sliding filament mechanism

Question 2

How is contraction regulated in smooth muscle?

Answer 2

By the thick filament (myosin) - regulated by Ca²⁺ via calmodulin and MLCK

Question 3

How is contraction regulated in skeletal and cardiac muscle?

Answer 3

By thin filaments (actin) - regulated by Ca²⁺ via troponin complex in skeletal and cardiac muscle

Question 4

What is the structure of myosin?

Answer 4

Myosin II

• 2 myosin heavy chains (MHC)
• 4 myosin light chains (MLC)

Question 5

What is the structure of actin in muscle?

Answer 5

Filamentous actin polymer (of globular G-actin) plus troponin/tropomyosin

Question 6

What happens when ATP binds to the myosin head?

Answer 6

Causes detatchment of the myosin head from the actin filament

Question 7

What causes the myosin head to tilt back?

Answer 7

Hydrolysis of ATP to ADP and P_i which makes the myosin head tilt back

Question 8

What happens when the phosphate dissociates from the myosin head?

Answer 8

The myosin head re-attaches to a different actin molecule

Question 9

What happens to the filaments when ADP is released?

Answer 9

A power stroke occurs, the tilting of myosin head to rigor formation, causes actin filament to also slide along the myosin = contraction!

Question 10

What is the calcium regulatory protein that sits on the tropomyosin string (attached to actin)?

Answer 10

Troponin

Question 11

What happens when calcium binds to the troponin subcomplex?

Answer 11

Structural shape is changed & tropomyosin no longer inhibits the actin filament from binding with the myosin head.

Question 12

To which troponin subunit does calcium bind to (TnI, TnT, TnC)?

Answer 12

Troponin C