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Biochemistry. 2009 Feb 3;48(4):720-8. doi: 10.1021/bi801918u.

Crystallographic, spectroscopic, and computational analysis of a flavin C4a-oxygen adduct in choline oxidase.

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  • 1Biology Department, Brookhaven National Laboratory, Upton, New York 11973-5000, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.


Flavin C4a-OO(H) and C4a-OH adducts are critical intermediates proposed in many flavoenzyme reaction mechanisms, but they are rarely detected even by rapid transient kinetics methods. We observe a trapped flavin C4a-OH or C4a-OO(H) adduct by single-crystal spectroscopic methods and in the 1.86 A resolution X-ray crystal structure of choline oxidase. The microspectrophotometry results show that the adduct forms rapidly in situ at 100 K upon exposure to X-rays. Density functional theory calculations establish the electronic structures for the flavin C4a-OH and C4a-OO(H) adducts and estimate the stabilization energy of several active site hydrogen bonds deduced from the crystal structure. We propose that the enzyme-bound FAD is reduced in the X-ray beam. The aerobic crystals then form either a C4a-OH or C4a-OO(H) adduct, but an insufficient proton inventory prevents their decay at cryogenic temperatures.

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