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

Abstract

The apparent binding constant (K_app^E) 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.(K_app^E) values obtained at two different temperatures (5°C and 20°C) were essentially the same when measured at the same pH. However,(K_app^E) depended markedly on pH. In the presence of 0.1 M KCl and 20 m MOPS-KOH buffer (pH 6.8), (K_app^E) was 6.3 × 10⁵ M⁻¹. The absolute binding constant (K) was calculated as 10^10.48 Addition of 10 m MgCl₂ did not change the (K_app^E). 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.