These enzymes are important in the mechanism of contraction in muscle. Once there is an influx of calcium cations (Ca2+) into the muscle, either from the sarcoplasmic reticulum or from the extracellular space, contraction of smooth muscle fibres may begin. First, the calcium will bind to calmodulin. This binding will activate MLCK, which will go on to phosphorylate the myosin light chain at serine residue 19. This will enable the myosin crossbridge to bind to the actin filament and allow contraction to begin (through the crossbridge cycle). Since smooth muscle does not contain a troponin complex, as striated muscle does, this mechanism is the main pathway for regulating smooth muscle contraction. Reducing intracellular calcium concentration inactivates MLCK but does not stop smooth muscle contraction since the myosin light chain has been physically modified through phosphorylation. To stop smooth muscle contraction this change needs to be reversed. Dephosphorylation of the myosin light chain (and subsequent termination of muscle contraction) occurs through activity of a second enzyme known as myosin light-chain phosphatase (MLCP).
^Gao Y, Ye LH, Kishi H, Okagaki T, Samizo K, Nakamura A, Kohama K (June 2001). "Myosin light chain kinase as a multifunctional regulatory protein of smooth muscle contraction". IUBMB Life51 (6): 337–44. doi:10.1080/152165401753366087. PMID11758800.
^Manning G, Whyte DB, Martinez R, Hunter T, Sudarsanam S (December 2002). "The protein kinase complement of the human genome". Science298 (5600): 1912–34. doi:10.1126/science.1075762. PMID12471243.
Kim MT, Kim BJ, Lee JH, et al. (2006). "Involvement of calmodulin and myosin light chain kinase in activation of mTRPC5 expressed in HEK cells.". Am. J. Physiol., Cell Physiol.290 (4): C1031–40. doi:10.1152/ajpcell.00602.2004. PMID16306123.