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Spinach nitrite reductase: subunit composition and siroheme redox potential.

M Hirasawa1, K A Gray, J D Sung

  • 1Department of Chemistry and Biochemistry, Texas Tech University, Lubbock 79409-1061.

Archives of Biochemistry and Biophysics
|November 15, 1989
PubMed
Summary

Spinach ferredoxin:nitrite oxidoreductase (85,000 Da) comprises two subunits (61,000 and 24,000 Da). Its native siroheme has a midpoint potential of -305 mV, significantly more negative than the modified form (-30 mV).

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Area of Science:

  • Biochemistry
  • Plant Physiology
  • Enzymology

Background:

  • Ferredoxin:nitrite oxidoreductase (Nir) is a key enzyme in spinach chloroplasts.
  • This enzyme catalyzes the reduction of nitrite to ammonia, a crucial step in nitrogen assimilation.
  • Understanding its structure and redox properties is vital for plant metabolism research.

Purpose of the Study:

  • To determine the molecular weight and subunit composition of spinach ferredoxin:nitrite oxidoreductase.
  • To investigate the redox properties of the siroheme prosthetic group in different enzyme forms.

Main Methods:

  • Gel filtration chromatography under nondenaturing and denaturing conditions.
  • Gel electrophoresis in the presence of denaturants.
  • Oxidation-reduction titrations to determine midpoint potential.

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Main Results:

  • The native enzyme has a molecular weight of 85,000 Da.
  • Denaturation revealed two subunits of 61,000 Da and 24,000 Da.
  • The native siroheme midpoint potential was -305 mV, while the modified form's was -30 mV.

Conclusions:

  • Spinach ferredoxin:nitrite oxidoreductase is an 85 kDa enzyme composed of two distinct subunits.
  • The redox potential of the siroheme group is significantly influenced by the enzyme's quaternary structure.
  • These findings provide insights into the enzyme's catalytic mechanism and regulation in nitrogen assimilation.