Effects of Reversing the Protein Positive Charge in the Proximity of the Flavin N(1) Locus of Choline Oxidase

Abstract

A protein positive charge near the flavin N(1) locus is a distinguishing feature of most flavoprotein oxidases, with mechanistic implications for the modulation of flavin reactivity. A recent study showed that in the active site of choline oxidase the protein positive charge is provided by His(466). Here, we have reversed the charge by substitution with aspartate (CHO-H466D) and, for the first time, characterized a flavoprotein oxidase with a negative charge near the flavin N(1) locus. CHO-H466D formed a stable complex with choline but lost the ability to oxidize the substrate. In contrast to the wild-type enzyme, which binds FAD covalently in a 1:1 ratio, CHO-H466D contained approximately 0.3 FAD per protein, of which 75% was not covalently bound to the enzyme. Anaerobic reduction of CHO-H466D resulted in the formation of a neutral hydroquinone, with no stabilization of the flavin semiquinone; in contrast, the anionic semiquinone and hydroquinone species were observed with the wild type and a H466A variant of the enzyme. The midpoint reduction potential for the oxidized-reduced couple in CHO-H466D was approximately 160 mV lower than that of the wild-type enzyme. Finally, CHO-H466D lost the ability to form complexes with glycine betaine or sulfite. Thus, with a reversal of the protein charge near the FAD N(1) locus, choline oxidase lost the ability to stabilize negative charges in the active site, irrespective of whether they develop on the flavin or are borne on ligands, resulting in defective flavinylation of the protein, the decreased electrophilicity of the flavin, and the consequent loss of catalytic activity.

Structure of glycerol-3-phosphate dehydrogenase, an essential monotopic membrane enzyme involved in respiration and metabolism

“…8). The catalytic center is shielded from solvent, providing an apolar microenvironment required for catalysis that has been observed in other flavoenzymes (23,24). In GlpD, the isoalloxazine ring forms numerous interactions with the protein polypeptide on the si-side; residues in proximity of 3.4 Å or less are Ser-46, Leu-48, His-50, Leu-355, and Thr-356.…”

“…4), which is positioned under the isoalloxazine ring at 3.7 Å from N1 with additional stabilization by the backbone nitrogen of Leu-355. A protein positive charge near the flavin N1 locus is a distinguishing feature of most flavoprotein oxidases, with mechanistic implications for the modulation of flavin reactivity (24). Comparative analysis with other flavoenzyme structures clusters the site of oxidative attack with substrates Ϸ3.5 Å distance from N5, defining an angle with the N5-N10 atoms in the narrow range of 96-177° (23).…”

A Novel Photoreaction Mechanism for the Circadian Blue Light Photoreceptor Drosophila Cryptochrome

“…isoalloxazine ring needs to be stabilized in the radical state of dCRY. For this purpose, all structurally characterized flavoprotein oxidases have either a positively charged residue or a helix dipole oriented toward the N(1)-C(2)ϭO region of the enzyme-bound flavin (49). In photolyases, the U-shaped conformation of the FAD chromophore may contribute to the stabilization of the negative charge of the two electron-reduced FADH Ϫ by the 3Ј hydroxyl group of the riboflavin moiety, which is in H-bonding distance to the nitrogen N(1) of the isoalloxazine ring (25).…”

Formate Oxidase, an Enzyme of the Glucose-Methanol-Choline Oxidoreductase Family, Has a His-Arg Pair and 8-Formyl-FAD at the Catalytic Site

“…38) In choline oxidase, the His 466 with protonated N "2 atom and Asn 510 have been found to contribute to stabilization of the intermediate. 39,41) During the modification of FAD in FOD AO , the intermediate might be transiently formed and stabilized by the protonated side chains of His 511 and Arg 554 , which are situated at $4:5 Å from the N1 and O4 atoms of the isoalloxazine ring (Fig. 5B).…”