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Characterization of manganese(II) binding site mutants of manganese peroxidase
K Kishi1, M Kusters-van Someren, M B Mayfield
1Department of Chemistry, Biochemistry, and Molecular Biology, Oregon Graduate Institute of Science & Technology, Portland 97291-1000, USA.
Abstract:
A series of site-directed mutants, E35Q, E39Q, and E35Q-D179N, in the gene encoding manganese peroxidase isozyme 1 (mnp1) from Phanerochaete chrysosporium, was created by overlap extension, using the polymerase chain reaction. The mutant genes were expressed in P. chrysosporium during primary metabolic growth under the control of the glyceraldehyde-3-phosphate dehydrogenase promoter. The mutant manganese peroxidases (MnPs) were purified and characterized. The molecular masses of the mutant proteins, as well as UV-vis spectral features of their oxidized states, were very similar to those of the wild-type enzyme. Resonance Raman spectral results indicated that the heme environment of the mutant MnP proteins also was similar to that of the wild-type protein. Steady-state kinetic analyses of the E35Q and E39Q mutant MnPs yielded K(m) values for the substrate MnII that were approximately 50-fold greater than the corresponding K(m) value for the wild-type enzyme. Likewise, the kcat values for MnII oxidation were approximately 300-fold lower than that for wild-type MnP. With the E35Q-D179N double mutant, the K(m) value for MnII was approximately 120-fold greater, and the kcat value was approximately 1000-fold less than that for the wild-type MnP1. Transient-state kinetic analysis of the reduction of MnP compound II by MnII allowed the determination of the equilibrium dissociation constants (KD) and first- order rate constants for the mutant proteins. The KD values were approximately 100-fold higher for the single mutants and approximately 200-fold higher for the double mutant, as compared with the wild-type enzyme. The first-order rate constants for the single and double mutants were approximately 200-fold and approximately 4000-fold less, respectively, than that of the wild-type enzyme. In contrast, the K(m) values for H2O2 and the rates of compound I formation were similar for the mutant and wild-type MnPs. The second-order rate constants for p-cresol and ferrocyanide reduction of the mutant compounds II also were similar to those of the wild-type enzyme.
Insights
Site-directed mutations in manganese peroxidase (MnP) significantly reduced its ability to oxidize manganese (MnII). These findings highlight the critical roles of specific residues in MnP catalytic activity and substrate binding.
Area of Science:
- Biochemistry
- Enzymology
- Molecular Biology
Background:
- Manganese peroxidase (MnP) is a key enzyme in lignin degradation and detoxification.
- Understanding MnP's catalytic mechanism is crucial for biotechnological applications.
Purpose of the Study:
- To investigate the role of specific residues (E35, E39, D179) in the catalytic activity of Phanerochaete chrysosporium manganese peroxidase isozyme 1 (MnP1).
- To characterize the kinetic properties of MnP1 mutants to elucidate substrate binding and oxidation mechanisms.
Main Methods:
- Site-directed mutagenesis was used to create E35Q, E39Q, and E35Q-D179N mutants of MnP1.
- Mutant MnP1 enzymes were expressed, purified, and characterized using steady-state and transient-state kinetic analyses.
- Spectroscopic methods (UV-vis, Resonance Raman) were employed to assess protein structure and heme environment.
Main Results:
- Mutant MnP1 enzymes showed significantly altered kinetics for Mn(II) oxidation, with 50-120 fold higher K(m) and 300-1000 fold lower k(cat) values compared to wild-type.
- Equilibrium dissociation constants (KD) for Mn(II) were 100-200 fold higher, and rate constants for MnP compound II reduction were 200-4000 fold lower in mutants.
- No significant changes were observed in K(m) for H2O2, compound I formation, or reduction rates of compound II by other substrates.
Conclusions:
- Residues E35, E39, and D179 are critical for efficient Mn(II) oxidation and substrate binding in MnP1.
- These mutations primarily affect the enzyme's interaction with its substrate Mn(II), rather than H2O2 activation or subsequent reaction steps.