Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.09.14.296582v1?rss=1 Authors: Chen, Y., Matveev, V. Abstract: We examine closed-form approximations for the equilibrium Ca2+ concentration near a point Ca2+ source representing a Ca2+ channel, in the presence of a mobile Ca2+ buffer with 2:1 Ca2+ binding stoichiometry. We consider buffers with two Ca2+ binding sites activated in tandem and possessing distinct binding affinities and kinetics. This allows to model the impact on Ca2+ nanodomains of realistic endogenous Ca2+ buffers characterized by cooperative Ca2+ binding, such as calretinin. The approximations we present involve a combination or rational and exponential functions, whose parameters are constrained using the series interpolation method that we recently introduced for the case of 1:1 Ca2+ buffers. We conduct extensive parameter sensitivity analysis and show that the obtained closed-form approximations achieve reasonable qualitative accuracy for a wide range of buffer's Ca2+ binding properties and other relevant model parameters. In particular, the accuracy of the newly derived approximants exceeds that of the rapid buffering approximation in large portions of the relevant parameter space. Copy rights belong to original authors. Visit the link for more info