Molecular Basis of Muscle Contraction Mercedita Macalintal, MD, DPPS

Types of Muscles

Vertebrate Skeletal Muscle n Skeletal Muscle is also called striated muscle n Sarcomere: repeating unit of muscle tissue n Z lines ~ sarcomere border n I band~only actin protein n A band~actin & myosin protein overlap n H zone~central sarcomere; only myosin

Sliding-filament model n Theory of muscle contraction n Sarcomere length reduced n Z line length becomes shorter n Actin and myosin slide past each other (overlap increases)

Actin-myosin interaction n 1- Myosin head hydrolyzes ATP to ADP and inorganic phosphate (Pi); termed the “high energy configuration” n 2- Myosin head binds to actin; termed a “cross bridge” n 3- Releasing ADP and (Pi), myosin relaxes sliding actin; “low energy configuration” n 4- Binding of new ATP releases myosin head n Creatine phosphate~ supplier of phosphate to ADP

Muscle contraction regulation, I n Relaxation: tropomyosin blocks myosin binding sites on actin n Contraction: calcium binds to toponin complex; tropomyosin changes shape, exposing myosin binding sites

Muscle contraction regulation, II n Calcium (Ca+)~ concentration regulated by the…. n Sarcoplasmic reticulum~ a specialized endoplasmic reticulum n Stimulated by action potential in a motor neuron n Ca+ then binds to troponin

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