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Fluorescence probing of the function-specific cysteines of rat microsomal NADPH-cytochrome P-450 reductase
Abstract:
Titration of NADPH-cytochrome P-450 reductase with a fluorigenic maleimide suggests that approximately four cysteines are initially accessible and in close proximity to four tryptophans. Perturbation of the cysteines and/or tryptophans results in concomitant decreases in enzymic activity. These cysteines were correlated with functional components by binding studies and subsequent tryptic peptide mapping on the acid mobile phase-reverse phase HPLC. Adenine nucleotides and cytochrome c block labelling of the more hydrophilic peptides, while detergents facilitate labelling of the more hydrophobic peptides. The more hydrophobic peptides contain the microsomal binding site of cytochrome P-450. Removal of the prosthetic flavins exposes more cysteines in the more hydrophilic and hydrophobic regions of the peptide map, associating the former with FAD and the latter with FMN binding sites.
Insights
NADPH-cytochrome P-450 reductase activity is linked to specific cysteine and tryptophan residues. Modifying these sites impacts enzyme function, revealing connections to nucleotide binding and cytochrome P-450 interactions.
Area of Science:
- Biochemistry
- Enzymology
- Molecular Biology
Background:
- NADPH-cytochrome P-450 reductase is a key enzyme in xenobiotic metabolism.
- Understanding its active sites is crucial for drug development and toxicology.
- Previous studies have implicated cysteine and tryptophan residues in enzyme function.
Purpose of the Study:
- To identify and characterize the functional cysteine and tryptophan residues in NADPH-cytochrome P-450 reductase.
- To correlate these residues with specific binding sites for cofactors and substrates.
- To elucidate the structural basis of enzyme activity regulation.
Main Methods:
- Titration with a fluorigenic maleimide to probe accessible cysteines.
- Enzyme activity assays before and after modification.
- Binding studies with adenine nucleotides, cytochrome c, and detergents.
- Tryptic peptide mapping using acid mobile phase-reverse phase HPLC.
- Analysis of cysteine accessibility after prosthetic flavin removal.
Main Results:
- Approximately four accessible cysteines are in close proximity to four tryptophans.
- Modification of these residues leads to decreased enzymatic activity.
- Adenine nucleotides and cytochrome c protect hydrophilic peptides from labeling.
- Detergents enhance labeling of hydrophobic peptides containing the cytochrome P-450 microsomal binding site.
- Flavin removal exposes additional cysteines, linking hydrophilic regions to FAD and hydrophobic regions to FMN binding sites.
Conclusions:
- Specific cysteine and tryptophan residues are essential for NADPH-cytochrome P-450 reductase activity.
- These residues are strategically located within distinct functional domains, including cofactor and substrate binding sites.
- The findings provide a structural basis for understanding enzyme regulation and interactions within the cytochrome P-450 system.