Question

How does Calcium in the cytosol influence muscle contraction? Describe the ATPase cycle of myosin II.

Answer 1

Answer

The contraction of skeltal muscle is under the voluntary control of the organism .When the impluse from the brain reaches the neuro-muscular junction the axonal endings release Acetylcholine neurotranismitter.This neurotransmitter gets attached to its receptors in the sarcolemma. This results in the transduction of signal inside the cell, the cell responds by generating Inositol trisphosphate ( IP3 ). This IP3 molecule has its receptor, a calcium ion channel in ENDOPLASMIC RETICULUM. when this IP3 molecule binds its receptor in endoplasmic reticulum the calcium ion channel opens and the Ca2+ moves into the cytoplasm ( sarcoplasm ).The released Ca ions bind to a subunit of troponin. The troponin keeps on masking the binding sites on actin and thus prevent meromyosin subunits of myosin to bind them. But when Ca2+ bind the troponin it displaces it and exposes the binding sites on the actin filaments. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. The energy of ATP hydrolysis pulls the attached actin filaments causing shortening of the sarcomere, i.e., contraction, as explained under:

ATPase Cycle of Myosin II : Myosin II in addition of the actin binding domain has another domain which binds the ATP ansd this domain has ATPase activity.

when the myosin binding sites on actin are exposed due to removal of troponin Myosin head forms cross bridge with actin and does not need ATP. Now ATP binds to the myosin II, ATP binding causes a little conformation change in the myosin head that leads to hydrolysis of ATP into ADP and Pi which remain bound to myosin. this hydrolysis again causes a liitle conformational change in myosin head and Pi is released which reinforces the contact between actin and myosin The release of inorganic phosphate reinforces the binding interaction between myosin and actin and at the same time triggers the power stroke. The power stroke is the key force-generating step used by myosin motor proteins. Forces are generated on the actin filament as the myosin protein reverts back to its original conformation.

As myosin regains its original conformation, the ADP is released, but the myosin head remains tightly bound to the filament and the muscle remains contracted. To break this actin-myosin interaction ATP is needed which is hydrolysed to seperate the two. If ATP is not present the muscle will remain in contraction for ever as after death. This condition is called RIGOR MORTIS.