Modeling the Response of Cardiac Troponin C to Calcium on the Thin Filament: Effects of Disease-Related and Post-Translational Modifications

Abstract

Calcium binding to and dissociation from cardiac troponin C (TnC) are essential to cardiac muscle contraction/relaxation. It is well documented that TnC's calcium-binding properties are not constant, but are sensitive to complex interactions between the additional thin and thick filament proteins. There is a growing body of evidence that protein modifications/mutations within different subunits of the Tn complex (TnT, TnI) exert their effects by altering the apparent TnC calcium sensitivity/exchange kinetics. There are a number of potential mechanisms that could alter TnC's calcium-binding, potentially the most significant being the ability of the regulatory domain of TnC to bind the switch peptide region of TnI. We have developed a mathematical model that can simulate the steady-state and kinetic calcium-binding properties of several disease-related and post-translational protein modifications in the isolated troponin complex and reconstituted thin filament. We propose that roughly half of the studied modifications do not alter any of the intrinsic calcium or TnI-binding constants of TnC, but rather the ability of TnC to “find” TnI in the presence of calcium. Considering TnI is essentially tethered to TnC and cannot diffuse away in the absence of calcium and that TnI also binds to actin, we suggest that the apparent calcium-binding properties of TnC are highly dependent upon an “effective concentration” of TnI available to bind TnC, which is modulated by Tn modifications. Futhermore, studies with TnC proteins and TnI peptides support the conclusions that TnI concentration strongly influences TnC's calcium-binding properties. Additionally, TnC-TnI chimeras display calcium-binding properties similar to that of TnC in the presence of a saturating TnI concentration. These studies greatly demonstrate the role of TnI concentration in modulating TnC's calcium-binding properties.