The Apparent Binding Constants of Ca2+ to EGTA and Heavy Meromyosin1

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Abstract
The apparent binding constant (KappE) of EGTA to Ca2+ was determined using a color indicator, murexide, or a Ca2+-selective electrode in the pH range between 6.5 and 7.5, at 5°C and at 20°C.(KappE) values obtained at two different temperatures (5°C and 20°C) were essentially the same when measured at the same pH. However,(KappE) depended markedly on pH. In the presence of 0.1 M KCl and 20 m MOPS-KOH buffer (pH 6.8), (KappE) was 6.3 × 105 M−1. The absolute binding constant (K) was calculated as 1010.48 Addition of 10 m MgCl2 did not change the (KappE). The binding of Ca2+ to heavy meromyosin was studied in 0.1 M KCl and 20 m MOPS-KOH buffer (pH 7.2), at 5°C. The binding constant and maximum binding number (mol/mol) were obtained as 1.8 × 106 M− and 1.4, respectively.