Because myosin undergoes a conformational change, the muscle will stay contracted even if calcium and activated MLC kinase concentrations are brought to normal levels.

The muscle will remain in this relaxed position until myosin is phosphorylated by MLC kinase and undergoes a conformational change.

However, when it bonds to the second subunit of myosin phosphatase, MYPT1 (MW ~130 kDa), this catalytic cleft changes configuration.

[2] The mechanism of removing the phosphate from Ser-19 is very similar to other dephosphorylation reactions in the cell, such as the activation of glycogen synthase.

[1][8] Once in the proper configuration, both the phyosphorylated serine and a free water molecule are stabilized by the hydrogen-bonding residues in the active site, as well as the positively charged ions (which interact strongly with the negative phosphate group).

After shuffling protons to stabilize (which happens rapidly compared to the attack on phosphorus), the phosphate and alcohol are formed, and both leave the active site.

[4] Two competing theories are that either arachidonic acid acts as a co-messenger in the rho-kinase cascade mentioned above, or that it binds to the c-terminal of MYPT1.

[4] When the regulatory systems of myosin phosphatase begin to fail, there can be major health consequences.

Since smooth muscle is found in the respiratory, circulatory, and reproductive systems of humans (as well as other places), if the smooth muscle can no longer relax because of faulty regulation, then a wide number of problems ranging from asthma, hypertension, and erectile dysfunction can result.