Natural diversity of FAD-dependent 4-hydroxybenzoate hydroxylases

4-Hydroxybenzoate 3-hydroxylase (PHBH) is the most extensively studied group A flavoprotein monooxygenase (FPMO). PHBH is almost exclusively found in prokaryotes, where its induction, usually as a consequence of lignin degradation, results in the regioselective formation of protocatechuate, one of the central intermediates in the global carbon cycle. In this contribution we introduce several less known FAD-dependent 4-hydroxybenzoate hydroxylases. Phylogenetic analysis showed that the enzymes discussed here reside in distinct clades of the group A FPMO family, indicating their separate divergence from a common ancestor. Protein homology modelling revealed that the fungal 4-hydroxybenzoate 3-hydroxylase PhhA is structurally related to phenol hydroxylase (PHHY) and 3-hydroxybenzoate 4-hydroxylase (3HB4H). 4-Hydroxybenzoate 1-hydroxylase (4HB1H) from yeast catalyzes an oxidative decarboxylation reaction and is structurally similar to 3-hydroxybenzoate 6-hydroxylase (3HB6H), salicylate hydroxylase (SALH) and 6-hydroxynicotinate 3-monooxygenase (6HNMO). Genome mining suggests that the 4HB1H activity is widespread in the fungal kingdom and might be responsible for the oxidative decarboxylation of vanillate, an import intermediate in lignin degradation. 4-Hydroxybenzoyl-CoA 1-hydroxylase (PhgA) catalyzes an intramolecular migration reaction (NIH shift) during the three-step conversion of 4-hydroxybenzoate to gentisate in certain Bacillus species. PhgA is phylogenetically related to 4-hydroxyphenylacetate 1-hydroxylase (4HPA1H). In summary, this paper shines light on the natural diversity of group A FPMOs that are involved in the aerobic microbial catabolism of 4-hydroxybenzoate.

Antioxidant activity and calcium binding of isomeric hydroxybenzoates

The association constant for calcium binding to hydroxybenzoates in aqueous 0.16 M NaCl at 25 °C was found electrochemically to have the value K_ass = 280 mol L⁻¹ with ΔH^o = −51 kJ mol⁻¹, ΔS^o = −122 J mol⁻¹ K⁻¹ for the 2-isomer (salicylate), K_ass = 7 mol L⁻¹ with ΔH^o = −39 kJ mol⁻¹, ΔS^o = −116 J mol⁻¹ K⁻¹ for the 3-isomer, and K_ass = 8 mol L⁻¹ with ΔH^o = −51 kJ mol⁻¹, ΔS^o = −155 J mol⁻¹ K⁻¹ for the 4-isomer. The 3- and 4-isomers were found more efficient as antioxidants than the 2-isomer in decreasing oxygen consumption rate in a peroxidating methyl linoleate emulsion and less sensitive to presence of calcium. All isomers were found prooxidative for iron-catalyzed initiation of oxidation due to enhanced radical formation as shown by electron spin resonance spectroscopy. Calcium salicylate was found to have low solubility with a solubility product K_sp = 4.49·10⁻⁶ based on activity with ΔH^o = 67 kJ mol⁻¹, ΔS^o = 123 J mol⁻¹ K⁻¹ for dissolution in water, when corrected for the strong complex formation. Calcium in food and beverages may thus lower antioxidant activity of plant phenols through complexation or by precipitation.

Development and Validation of High Performance Liquid Chromatographic Method for Determination of Gentisic Acid and Related Renal Cell Carcinoma Biomarkers in Urine

A reversed phase liquid chromatographic (RPLC) method was developed to simultaneously detect and quantify creatinine, quinolinic acid, gentisic acid and 4-hydroxybenzoic acid in urine. These four bio-markers are present in relatively high concentrations in urine. Using a 5% methanol in water mobile phase with 0.6% acetic acid and a Zorbax C₁₈ column, baseline resolution for all four biomarkers in synthetic urine was achieved. Better resolution was obtained for the separation of these four compounds when water rich mobile phases were used. Detection of the four biomarkers in urine using the proposed RPLC method is limited by background from the urine matrix for the later eluting compounds and from the dead marker for earlier eluting compounds.