A Biophysical Model of the Mitochondrial ATP-Mg/Pi Carrier

TL;DR

This paper develops a thermodynamically balanced biophysical model of the mitochondrial ATP-Mg/Pi carrier, accurately reproducing experimental data and elucidating the roles of pH, Mg2+, and Ca2+ in its function.

Contribution

It introduces a novel kinetic model of the APC that incorporates multiple exchange mechanisms and regulatory factors, validated against extensive experimental data.

Findings

pH gradient is the main driving force for AdN transport

Extra-matrix Ca2+ modulates APC turnover rate

Model accurately fits data from rat mitochondria experiments

Abstract

Mitochondrial adenine nucleotide (AdN) content is regulated through the Ca2+-activated, electroneutral ATP-Mg/Pi carrier (APC). The APC is a protein in the mitochondrial carrier super family that localizes to the inner mitochondrial membrane (IMM). It is known to modulate a number of processes that depend on mitochondrial AdN content, such as gluconeogenesis, protein synthesis, and citrulline synthesis. Despite this critical role, a kinetic model of the underlying mechanism has not been developed and validated. Here, a biophysical model of the APC is developed that is thermodynamically balanced and accurately reproduces a number of reported data sets from isolated rat liver and rat kidney mitochondria. The model is based on an ordered bi-bi mechanism for hetero-exchange of ATP and Pi and also includes homo-exchanges of ATP and Pi to explain both the initial rate and time course data on ATP and Pi transport via the APC. The model invokes seven kinetic parameters regarding the APC mechanism and three parameters related to matrix pH regulation by external Pi. These parameters are estimated based on nineteen independent data curves; the estimated parameters are validated using six additional data curves. The model takes into account the effects of pH, Mg2+ and Ca2+ on ATP and Pi transport via the APC and supports the conclusion that the pH gradient across the IMM serves as the primary driving force for AdN uptake or efflux. Moreover, computer simulations demonstrate that extra-matrix Ca2+ modulates the turnover rate of the APC and not the binding affinity of ATP, as previously suggested.