JXB Advance Access originally published online on June 23, 2006
Journal of Experimental Botany 2006 57(10):2133-2141; doi:10.1093/jxb/erl006
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Opinion Paper |
Equilibration of adenylates in the mitochondrial intermembrane space maintains respiration and regulates cytosolic metabolism
1Department of Plant Science, Faculty of Agricultural and Food Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
2Department of Plant Physiology, Umeå Plant Science Centre, University of Umeå, 901 87 Umeå, Sweden
*To whom correspondence should be addressed. E-mail: igamberd{at}cc.umanitoba.ca
Adenylate kinase (AK) uses one each of Mg-complexed and free adenylates as substrates in both directions of its reaction. It is very active in the mitochondrial intermembrane space (IMS), but is absent from the mitochondrial matrix where low [ADP] upon intensive respiration limits the respiratory rate. AK activity in the IMS is linked to ATP/ADP exchange across the inner mitochondrial membrane by using ATP (imported from the matrix) and AMP as substrates, the latter provided by apyrase and other AMP-generating reactions. The ADP formed by AK is exported to the matrix (in exchange for ATP), providing a mechanism for regeneration of ADP during respiration. From the AK equilibrium, and taking pH values characteristic of subcellular compartments, [Mg2+] in the IMS is calculated as 0.4–0.5 mM and in the cytosol as 0.2–0.3 mM, whereas the MgATP:MgADP ratio in the IMS and cytosol is 6–9 and 10–15, respectively. These represent optimal conditions for transport of adenylates (via the maintenance of an ATPfree:ADPfree ratio close to 1) and mitochondrial respiratory rates (via the maintenance of submillimolar [ADPfree] in the IMS). This, in turn, has important consequences for mitochondrial and cytosolic metabolism, including regulation of the protein phosphorylation rate (via changes in the MgATP:AMPfree ratio) and allosteric regulation of mitochondrial and cytosolic enzymes. Metabolomic consequences are discussed in connection with the calculation of metabolic fluxes from subcompartmental distributions of total adenylates and Mg2+.
Key words: Adenylate kinase, apyrase, free magnesium, intermembrane space, metabolomics, mitochondrion, respiration
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