Ferredoxin-dependent oxidative decarboxylases of Halobacterium salinarum

Two key enzymatic steps in central metabolism are the conversion of pyruvate to acetyl-CoA and of alpha-ketoglutarate to succinyl-CoA. These enzymatic reactions result in oxidative decarboxylation of the substrate. In Halobacterium salinarum and many archaea these are catalyzed by ferredoxin-dependent protein complexes while in Escherichia coli and most bacteria and eukaryotes these are catalyzed by lipoamide-dependent enzyme complexes.

I. Pyruvate dehydrogenase (ferredoxin)

Physiological role:

• Pyruvate dehydrogenase converts pyruvate to acetyl-CoA which is then completely oxidyzed through the citric acid cycle.

Reaction catalyzed:

• pyruvate + HSCoA + fdx(ox) --> acetyl-CoA + CO2 + fdx(red)

Protein complex:

• Pyruvate dehydrogenase is a heterotetramer.

Subunits:

• alpha chain: porA OE2623R PIR:S22396

• beta chain: porB OE2622R PIR:S22397

Genes:

• the genes porA-porB form a transcription unit.

Coenzymes and redox partners:

• ferredoxin fdx OE4217R

• coenzyme A

• thiamine pyrophosphate

Experimental data:

• The protein was purified and its catalytic properties were determined.

• The molecular weight of the protein complex and its subunits as well as the amino acid composition was determined.

• Protein sequencing of the N-terminus and of internal peptides was achieved.

• The presence of the coenzyme thiamine pyrophosphate and the absence of the coenzyme lipoamide was verified.

References:

• W. Plaga, F. Lottspeich, D. Oesterhelt:
  Improved Purification, Crystallization and Primary Structure of Pyruvate:Ferredoxin Oxidoreductase from Halobacterium halobium.
  Eur. J. Biochem. 205, 391-397 (1992)
• L. Kerscher, D. Oesterhelt:
  Purification and Properties of Two 2-Oxoacid:Ferredoxin Oxidoreductases from Halobacterium halobium.
  J. Biochem. 116, 587-594 (1981)
• L. Kerscher, D. Oesterhelt:
  The Catalytic Mechanism of 2-Oxoacid:Ferredoxin Oxidoreductases from Halobacterium halobium: One-Electron Transfer at two Distinct Steps of the Catalytic Cycle.
  Eur. J. Biochem. 116, 595-600 (1981)

II. Alpha-ketoglutarate dehydrogenase (ferredoxin)

Physiological role:

• alpha-detoglutarate dehydrogenase is one step of the citric acid cycle.

Reaction catalyzed:

• alpha-ketoglutarate + HSCoA + fdx(ox) --> succinyl-CoA + CO₂ + fdx(red)

Subunits:

• alpha chain: korA OE1711R
• beta chain: korB OE1710R

Genes:

• the genes korA-korB form a transcription unit.

Coenzymes and redox partners:

• ferredoxin fdx OE4217R
• coenzyme A
• thiamine pyrophosphate

Experimental data:

• The protein was purified and its catalytic properties were determined.
• The molecular weight of the protein complex and its subunits as well as the amino acid composition was determined.
• The presence of the coenzyme thiamine pyrophosphate and the absence of the coenzyme lipoamide was verified.
• The genes for the subunits were cloned and sequenced.

References

• L. Kerscher, D. Oesterhelt:
  Purification and Properties of Two 2-Oxoacid:Ferredoxin Oxidoreductases from Halobacterium halobium.
  J. Biochem. 116, 587-594 (1981)
• L. Kerscher, D. Oesterhelt:
  The Catalytic Mechanism of 2-Oxoacid:Ferredoxin Oxidoreductases from Halobacterium halobium: One-Electron Transfer at two Distinct Steps of the Catalytic Cycle.
  Eur. J. Biochem. 116, 595-600 (1981)