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The rho-hydroxybenzoate hydroxylase (PHBH)-mediated conversion of tetrafluoro-4-hydroxybenzoate (F4-POHB) involves a number of consecutive oxygenation and dehalogenation steps. These include hydroxylation at the substrate C2 position. The enzymatic conversion of F3-DOHB to 5,6-F2-TOHB implies that F3-DOHB binding leads to the stabilization of the flavin C(4a)-hydroperoxide. It also indicates that the peroxide bond of the oxygenated flavin intermediate becomes oriented in such a way as to allow C2 hydroxylation.

The side chain of Tyr222 in p-hydroxybenzoate hydroxylase interacts with the carboxy moiety of the substrate. Studies on the Tyr222→Phe mutant, [F222]p-hydroxybenzoate hydroxylase, have shown that disruption of this interaction hampers the hydroxylation of 4-hydroxybenzoate. Tyr222 is possibly involved in flavin motion, which may facilitate the exchange of substrate and product during catalysis. To elucidate the function of Tyr222 in more detail, in the present study the substrate and product during catalysis. To elucidate the function of Tyr222 in more detail, in the present study the substrate and effector specificity of the Tyr222→Ala mutant, [A222]p-hydroxybenzoate hydroxylase, was investigated. Replacement of Tyr222 by Ala impairs the binding of the physiological substrate 4-hydroxybenzoate and the substrate analog 4-aminobenzoate. With these compounds, hydroxylase tightly interacts with 2,4-dihydroxybenzoate and 2-hydroxy-4-aminobenzoate. Crystallographic data [Schreuder, H.A., Mattevi, A., Oblomova, g., Kalk, K. H., Hol, W.G.J., van der Bolt, F.J.T. & van Berkel, W.J.H. (1994) Biochemsitry 33, 10161-10170] suggest that this is due to motion of the flavin ring out of the active site, allowing hydrogen-bond interaction between the 2-hydroxy group of the substrate analogs and N3 of the flaving. [A222]p-Hydroxybenzoate hydroxylase produces about 0.6 mol 2,3,4-trihydroxybenzoate from 2,4-dihydroxybenzoate/mol NADPH oxidized. This indicates that reduction of the Tyr222→Ala mutant shifts the equilibrium of flavin conformers towards the productive 'in' position. [A222]p-Hydroxybenzoate hydroxylase converts 2-fluoro-4-hydroxybenzoate to 2-fluor-3,4-dihydroxybenzoate. The regioselectivity of hydroxylation suggests that [A222]p-hydroxybenzoate hydroxylase binds the fluorinated substrate in the same orientation as wild-type. Spectral studies suggest that wild-type and [A222]p-hydroxybenzoate hydroxylase bind 2-fluoro-4-hydroxybenzoate in the phenolate form with the flavin ring preferring the 'out' conformation. Despite activation of the fluorinated substrate and in contrast to the wild-type enzyme. [A222]p-hydroxybenzoate hydroxylase largely produces hydrogen peroxide. The effector specificity of p-hydroxybenzoate hydroxylase is not changed by the Tyr222→Ala replacement. This supports the idea that the effector specificity is mainly dictated by the protein-substrate interactions at the re-side of the flavin ring. [ABSTRACT FROM AUTHOR]