Stress conditions (e.g. anoxia) frequently result in a decrease of [ATP] and in an increase of [ADP] and [AMP], with a concomitant increase of [Mg2+] and other cations, e.g. Ca2+. The elevation of [Mg2+] is linked to the shift in the apparent equilibrium of adenylate kinase. As a result, enzymes that use Mg2+ as a cofactor are activated, Ca2+ activates calcium-dependent signalling pathways, and PPi can serve as an alternative energy source in its active form of MgPPi or Mg2PPi. Under anoxic conditions in plants, an important source of PPi may come as a result of combined reactions of PK (pyruvate kinase) and PPDK (pyruvate, phosphate dikinase). The PPi formed in the PPDK/PK cycle ignites glycolysis in conditions of low [ATP] by involving PPi-dependent reactions. This saves ATP and makes metabolism under stress conditions more energy efficient.

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