Calorimetric experiments of Mn 2+-binding to α-lactalbumin

Abstract

We measured by batch microcalorimetry the standard enthalpy change Δ H° of the binding of Mn 2+ to apo-bovine α-lactalbumin; Δ H° = −90 ± 4kJ·mol −1. The binding constants, K Mn 2+ , calculated from the calorimetric and circular dichroism titration curves, are (4.6±1) · 10 5M −1, respectively. Batch calorimetry confirms the competitive binding of Ca 2+, Mn 2+ and Na + to the same site. The relatively small enthalpy change for Mn 2+ binding compared to Ca 2+ binding favours a model of a rigid and almost ideal Ca 2+-complexating site, different from the well-known EF-hand structures. Cation binding to the high-affinity site most probably triggers the movement of an α-helix which is directly connected to the complexating loop.