Coupling of ATP pyrophosphorolysis to transport of Ca2+ in mitochondria

Abstract In the coupling of ATP pyrophosphorolysis to Ca 2+ transport in beef heart mitochondria, for each molecule of ATP cleaved, one proton is released and one Ca 2+ is transported into the interior space. With the use of tritium labelled ATP, it could be shown that ATP is pyrophosphorylyzed into a species equivalent to Pi that moves inward, and a species equivalent to ADP that is extruded into the aqueous space on the exterior of the mitochondrion. The species equivalent to Pi has been identified as a negatively charged form of Pi (PO − ) and the species equivalent to ADP as a positively charged form (ADP + ). The inward flow of PO − is coupled to the inward flow of Ca 2+ in 1:1 stoichiometry—a token that Ca 2+ must enter in the form of Ca 2+ A − , where A − is a monovalent anion. During ATP pyrophosphorolysis protons are released on the I side and taken up on the M side—one proton for each molecule of ATP cleaved. The alkalinization of the matrix space leads to the deposition of Ca 3 (PO 4 ) 2 and to the extrusion of the two species released by this deposition (Pi, K + ). Two thirds of the PO − is trapped as Ca 3 (PO 4 ) 2 in the matrix space and one third is extruded into the external space. The extrusion of K + provides a mechanism by which protons can be continuously brought into the matrix space to sustain a high rate of coupled pyrophosphorolysis of ATP. The coupling pattern for Ca 2+ transport driven by ATP pyrophosphorolysis is identical with the corresponding pattern for Ca 2+ transport driven by electron transfer. This identity is suggestive that coupling mediated by the Fo-F 1 system and coupling mediated by electron transfer complexes are mechanistically indistinguishable.