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Monitoring the Reductive and Oxidative Half-Reactions of a Flavin-Dependent Monooxygenase using Stopped-Flow Spectrophotometry

Monitoring the Reductive and Oxidative Half-Reactions of a Flavin-Dependent Monooxygenase using Stopped-Flow Spectrophotometry

Published on: March 18, 2012

Role of the covalent flavin linkage in monomeric sarcosine oxidase

Alshaimaa Hassan-Abdallah¹, Guohua Zhao, Marilyn Schuman Jorns

¹Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, Pennsylvania 19102, USA.

|August 2, 2006

Abstract:

Monomeric sarcosine oxidase (MSOX) is a prototypical member of a recently recognized family of amine-oxidizing enzymes that all contain covalently bound flavin. Mutation of the covalent flavin attachment site in MSOX produces a catalytically inactive apoprotein (apoCys315Ala) that forms an unstable complex with FAD (K(d) = 100 muM), similar to that observed with wild-type apoMSOX where the complex is formed as an intermediate during covalent flavin attachment. In situ reconstitution of sarcosine oxidase activity is achieved by assaying apoCys315Ala in the presence of FAD or 8-nor-8-chloroFAD, an analogue with an approximately 55 mV higher reduction potential. After correction for an estimated 65% reconstitutable apoprotein, the specific activity of apoCys315Ala in the presence of excess FAD or 8-nor-8-chloroFAD is 14% or 80%, respectively, of that observed with wild-type MSOX. Unlike oxidized flavin, apoCys315Ala exhibits a high affinity for reduced flavin, as judged by results obtained with reduced 5-deazaFAD (5-deazaFADH(2)) where the estimated binding stoichiometry is unaffected by dialysis. The Cys315Ala.5-deazaFADH(2) complex is also air-stable but is readily oxidized by sarcosine imine, a reaction accompanied by release of weakly bound oxidized 5-deazaFAD. The dramatic difference in the binding affinity of apoCys315Ala for oxidized and reduced flavin indicates that the protein environment must induce a sizable increase in the reduction potential of noncovalently bound flavin (DeltaE(m) approximately 120 mV). The covalent flavin linkage prevents loss of weakly bound oxidized FAD and also modulates the flavin reduction potential in conjunction with the protein environment.

Insights

The covalent attachment of flavin in sarcosine oxidase (MSOX) is crucial for enzyme activity and stability. Mutating this site reveals how the protein environment modulates flavin

Area of Science:

Biochemistry
Enzymology
Protein Chemistry

Background:

Monomeric sarcosine oxidase (MSOX) is a key enzyme in the amine-oxidizing enzyme family, characterized by covalently bound flavin.
The covalent flavin linkage is essential for enzyme stability and catalytic efficiency.
Understanding the role of this covalent bond requires studying mutated variants lacking it.

Purpose of the Study:

To investigate the role of the covalent flavin attachment site in MSOX function.
To characterize the properties of a mutated MSOX apoprotein (apoCys315Ala) lacking the covalent flavin.
To elucidate the contribution of the protein environment to flavin binding and redox potential modulation.

Main Methods:

Site-directed mutagenesis to create apoCys315Ala.
Enzyme activity assays using FAD and flavin analogues (8-nor-8-chloroFAD, 5-deazaFAD).
Binding affinity studies for oxidized and reduced flavins.
Analysis of flavin reduction potential modulation.

Main Results:

apoCys315Ala showed significantly reduced activity (14-80%) compared to wild-type MSOX, even with added flavins.
apoCys315Ala exhibited high affinity for reduced flavin but low affinity for oxidized flavin.
The protein environment was found to increase the reduction potential of noncovalently bound flavin by approximately 120 mV.
The covalent linkage prevents flavin loss and fine-tunes the redox potential.

Conclusions:

The covalent flavin linkage in MSOX is critical for maintaining high catalytic activity and preventing flavin dissociation.
The protein environment plays a significant role in modulating the reduction potential of the noncovalently bound flavin cofactor.
These findings provide insights into the mechanism of flavin-dependent enzymes and the importance of covalent cofactor attachment.

Read full analysis

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