Plant sulfite reductase: molecular structure, catalytic function and interaction with ferredoxin

Area of Science:

• Biochemistry
• Enzymology
• Plant molecular biology

Background:

• Plant sulfite reductase (SiR) is a key enzyme in nitrogen and sulfur assimilation.
• It contains siroheme and a [4Fe-4S] cluster, catalyzing six-electron reductions of sulfite and nitrite.
• Electron transfer is mediated by ferredoxin.

Purpose of the Study:

• To investigate structure-function relationships of plant sulfite reductase.
• To elucidate the roles of specific amino acid residues in substrate binding and specificity.
• To characterize ferredoxin mutants affecting electron donation.

Main Methods:

• Heterologous expression of maize sulfite reductase cDNA in E. coli.
• Site-directed mutagenesis of SiR to alter substrate-binding residues.
• Construction and characterization of ferredoxin mutants.
• Kinetic analysis of wild-type and mutant enzymes.

Main Results:

• Amino acid substitutions near the siroheme significantly altered substrate specificity for sulfite and nitrite.
• Mutations affecting ferredoxin's enzyme recognition or redox properties reduced electron donation efficiency.
• These findings highlight the importance of specific residues in enzyme catalysis and interaction.

Conclusions:

• Specific residues are critical for determining plant sulfite reductase substrate specificity.
• Ferredoxin's interaction and redox state are vital for efficient electron transfer to sulfite reductase.
• Understanding these structure-function relationships provides insights into plant metabolic pathways.