Enzymes myosin light chain kinase

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

Many of the biochemical and molecular events that are responsible for uterine smooth muscle contraction are the same as those that control other smooth muscle tissues (Fig. 62.1). Once uterine smooth muscle sensitivity has been augmented, actin and myosin must interact for contraction to occur. This interaction depends on the phosphorylation of the contractile proteins by the enzyme myosin light chain kinase (MLCK). This enzyme requires Ca++ and is active only when associated with calmodulin. Activation of the entire muscle contraction... 

This enzyme [EC 2.7.1.123], also referred to as calcium/ calmodulin-dependent protein kinase type II, and micro-tubule-associated protein MAP2 kinase, catalyzes the reaction of ATP with a protein to produce ADP and an 0-phosphoprotein. The enzyme requires calcium ions and calmodulin. Proteins that can serve as substrates include vimentin, synapsin, glycogen synthase, the myosin light-chains, and the microtubule-associated tau protein. This enzyme is distinct from myosin light-chain kinase [EC 2.7.1.117], caldesmon kinase [EC 2.7.1.120], and tau-protein kinase [EC 2.7.1.135]. 

NO released by GTN activates soluble, cytosolic form of guanylyl cyclase in vascular smooth muscles by interacting with haem group in the enzyme. This converts GTP to cGMP. cGMP dephosphorylates myosin light chain kinase and prevent myosin interaction with actin leading to relaxation. 

Pires, E. Perry, S.V. Thomas, M.A.W. Myosin light-chain kinase, a new enzyme from striated muscle. FEES Lett, 41, 292-296 (1974)... 

Walsh, M.P. Hinkins, S. Flink, I.L. Hartshorne, D.J. Bovine stomach myosin light chain kinase purification, characterization, and comparison with the turkey gizzard enzyme. Biochemistry, 21, 6890-6896 (1982)... 

Q6 Salbutamol is a selective beta-2-adrenoceptor (/ -adrenoceptor) agonist which is effective in relieving mild to moderate bronchoconstriction. Inhalation of salbutamol induces bronchodilation by acting on /J2-receptors on bronchial smooth muscle this lasts for approximately three to five hours. It also inhibits mediator release and improves the clearance of mucus from the lung. Stimulation of the /02-receptor increases the cellular concentration of cyclic adenosine monophosphate cAMP and activates a protein kinase. This kinase in turn inactivates myosin-light-chain kinase, an enzyme necessary for contraction in smooth muscle, and so relaxes bronchial smooth muscle. 

Myosin light chain kinase is a Ser/Thr-type protein kinase involved in regulation of smooth muscle. The enzyme is also found in smooth muscle and platelets. This assay uses a synthetic substrate that is not radioactively labeled. 

The value of X (Eqn. 2) appears to be variable depending on the number of binding subunits for a given enzyme. For example, the cyclic nucleotide phosphodiesterase is frequently described as being a dimeric molecule with each of the two monomers capable of binding one molecule of calmodulin. Similarly, the myosin light chain kinase is reported to bind one calmodulin molecule per subunit (51). Multiple calmodulin binding sites per monomer have not as yet been described for any enzyme with the possible exception of phosphorylase Id kinase. 

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