The calcium and magnesium binding sites on troponin and their role in the regulation of myofibrillar adenosine triphosphatase.

摘要: Purified troponin (Tn), the complex of Ca-2+ binding subunit (TnC), inhibitory (TnI), and tropomyosin (TnT) binds 4 mol per mol. Two sites bind with a constant 5 times 10-8 M- minus 1, two 10-6 1. In presence 2 mM MgCl2 to four can be characterized single affinity TnC also mol; have 10-7 1 one 10-5 higher is reduced while lower unaffected. Assuming competition between Mg-2+ for high on Tn, changes in accounted KMg values 10-3 10-4 respectively. Tn absence Cs-2+. The fact that at [Ca-2+] similar 10- M only are bound further supports view there direct Tn. These results then suggest contain six divalent cation sites: competitively (Ca-2+-Mg-2+ sites); do not (Ca-2+-specific but (Mg-2+-specific sites). TnI (1:1 molar ratio) has same properties as suggesting conformational change upon interaction TnI. Studies myofibrillar ATPase activity function free concentration different concentrations full activation by occurs Ca-2+-specific does require Ca-2+-Mg-2+ sites.