Triangular kinetic schemes applied to the stability of a Heme—Globin complex

Abstract

Horse heart apomyglobin traps the heme released from Aplysia california myoglobin. The kinetics fit a triangular mechanism for a biphasic reaction. Laplacian solutions for differential equations appropriate to triangular kinetic schemes involving up to four rate constants are elaborated and confirmed. Two general schemes and two special case are considered. In the first scheme, a rearrangement of the starting material is concurrent with product formation. In the second scheme, the starting material forms two products in equilibrium at two different rates. A general equation for the absorbance-time curve is derived for these triangular schemes, from which rate constants can be estimated. Changes in instantaneous rates versus time are employed to analyze the absorption versus time plots and the curvature of a first-order rate analysis. Aplysia metmyglobin equilibrates between slow donor (pentacoordinate, which lacks the axial water molecule) and fast donor (hexacoordinate). No heme release was observed for deoxy, oxy, carbonyl, or azide derivatives of the Aplysia myoglobin, or when the distal His E7 of the apohemoprotein is replaced by leucine or valine. This suggests a role for hydrophobicity of the active site, and for a trans effect of the axial ligand in determining the stability of the embedded prosthethic heme.