Basis of contractility generation in the muscle

lecture 2

Describe the stages in the cross bridge cycle.

1. myosin head without ATP is bound to actin filament

2. ATP binds to myosin head. 2 things happen next.

3. Myosin head and neck enters “cocked position” - myosin head moves along actin filament. NOT THE POWER STROKE. next binding site further towards plus end of actin filament.

4. Conformational change in myosin reduces affinity for actin, so unbinds from actin filament

5. ATP hydrolysed, but ADP and Pi remain bound to myosin head

6. “Cocked” myosin binds to actin filament weakly. Triggers Pi release.

7. Pi release triggers increased affinity for actin. POWER STROKE OCCURS

8. ADP released and myosin returns to original tightly bound conformation at the start of the cycle.

What are the 3 subunits in skeletal muscle troponin?

Troponin I (inhibitory)

Troponin C (Ca²⁺ binding)

Troponin T (Tropomyosin-binding)

What is the role of Ca²⁺ in skeletal muscle contraction?

Upto 4 Ca²⁺ molecules bind to TnC

Conformational change leads to TnI releasing hold on actin

Tropomyosin displaced deeper into the actin double helix groove

Myosin-binding sites become accessible

What is the role of Ca²⁺ in cardiac muscle contraction?

Very similar overall mechanism to smooth muscle with small differences

Differences: cardiac TnC binds 3 Ca²⁺, and not 4 like in skeletal muscle

What molecule does smooth muscle not have that cardiac and skeletal do?

Troponin

Describe the main activation mechanism in excitation-contraction coupling in smooth muscle.

Every mysoin molecule has 2 myosin regulatory light chains (MRLCs)

Calmodulin (similar to TnC) binds Ca²⁺ and activates myosin light chain kinases (MLCKs)

Phosphorylation of MRLC by MLCK

Conformational change

activation of myosin activity, enhanced actin binding and increased ATPase activity

Describe how contraction is terminated in skeletal and cardiac muscle.

Intracellular calcium levels decrease (see next lecture for more)

Describe how contraction is terminated in smooth muscle.

Intracellular Ca²⁺ levels decrease (not sufficient by iteself)

Myosin light chain phosphatase (MLCP) dephosphorylates MRLC

Latch bridge

Form if MRLC dephosphorylated while myosin is bound to an actin filament - myosin remains bound with high affinity

Allows maintenance of tension without crossbridge cycling or ATP consumption

Describe another level of contractility regulation in smooth muscle.

Regulation of contraction at the actin filament level

mediated by caldesmon and calponin

bind actin and tropomyosin

inhibit interaction between actin and myosin

Clinical implications of muscle dystrophies

inherited conditions

disorganisation of the myofilaments and eventual paralysis

some associated with titin and dystrophin

Dystophin

protein that connects actin thin filaments to the extracellular matrix

network of elastic fibres that surrounds the muscle fibres.

Duchenne muscular dystrophy (DMD)

X-linked recessive condition

mutation disrupts dystrophin

muscles easily damaged - damage accumulates and general muscle weakening over time

satellite cells cannot sufficiently repair damage & quickly become depleted

Clinical implications of hypertrophic cardiomyopathies (HCM)

thickening of ventricular walls and small ventricular chamber

single mutation in cardiac beta myosin heavy chain, cardiac myosin light chain, tropomyosin or troponin

many lead to hyper-contractile cardiac beta myosin

Distribution of myosin in the heart

Alpha cardiac myosin has a faster twitch and is found in the atria

beta cardiac myosin gives longer lasting and stronger contraction and is found in the ventricles