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MgADP and free Pi as the substrates and the Mg2+ requirement for photophosphorylation

J M Zhou1, P D Boyer

  • 1Molecular Biology Institute, University of California, Los Angeles 90024-1570.

Biochemistry
|March 31, 1992
PubMed

Insights

Magnesium-bound ADP (MgADP) is the primary substrate for photophosphorylation, not free ADP or Mg2+. This study clarifies the roles of these molecules in energy production within chloroplasts.

Area of Science:

  • Biochemistry
  • Plant Physiology

Background:

  • Photophosphorylation is crucial for ATP synthesis in chloroplasts.
  • The precise substrates and requirements for Mg2+ and ADP in this process remain debated.

Purpose of the Study:

  • To determine whether ADP or Pi, or their complexes with Mg2+, are substrates for photophosphorylation.
  • To ascertain the requirement for free Mg2+ or ADP during photophosphorylation.

Main Methods:

  • Enzyme kinetics studies using spinach chloroplasts.
  • Varying concentrations of Mg2+ and different nucleotides (ADP, GDP, UDP) were employed.

Main Results:

  • MgADP, MgGDP, and MgUDP were identified as substrates.
  • Observed reaction velocities were dependent on MgADP, not free ADP concentration.
  • Low concentrations of free Mg2+ or ADP (<2-3 microM) are sufficient for high phosphorylation rates.
  • Free ADP does not inhibit MgADP binding to catalytic sites during active photophosphorylation.

Conclusions:

  • MgADP is the principal substrate for photophosphorylation in spinach chloroplasts.
  • Minimal free Mg2+ or ADP is required for efficient photophosphorylation.
  • The binding dynamics of nucleotides to catalytic sites are complex and depend on the occupancy state.

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