Conformational change induced by Mg
2+-ATP in the local area containing a specific thiol group, SH
2, in skeletal myosin was regulated by physiological level of free Ca
2+ concentration (pCa ranging from 8 to 4) . The curve relating conformational change to pCa in the presence of Mg
2+-ATP has a characteristic sigmoid shape, showing an inflection point at pCa 5. This result suggests the presence of a Ca
2+-dependent regulatory mechanism in skeletal myosin.
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