Mechanism of Action of p-Hydroxybenzoate Hydroxylase from Pseudomonas putida

Amino acid sequences around the pyridoxal-5'-phosphate-binding sites of phenol hydroxylase

Phenol hydroxylase was labelled with pyridoxal 5'-phosphate. A radioactive label was introduced by using sodium boro[3H]hydride to reduce the initially formed Schiff's base. The labelled enzyme was digested with Staphylococcus V8 protease. Labelled peptides were isolated and their sequences were determined. The label could be located to three different lysyl residues. Sequence similarities with the known structures of p-hydroxybenzoate hydroxylase and glutathione reductase are discussed. The positions of the labelled sequences, relative to the bound ligands at the active site, are proposed on the basis of such sequence similarities.

Crystal structure of p-hydroxybenzoate hydroxylase complexed with its reaction product 3,4-dihydroxybenzoate

Crystals of the flavin-containing enzyme p-hydroxybenzoate hydroxylase (PHBHase) complexed with its reaction product were investigated in order to obtain insight into the catalytic cycle of this enzyme involving two substrates and two cofactors. PHBHase was crystallized initially with its substrate, p-hydroxybenzoate and the substrate was then converted into the product 3,4-dihydroxybenzoate by allowing the catalytic reaction to proceed in the crystals. In addition, crystals were soaked in mother liquor containing a high concentration of this product. Data up to 2.3 A (1 A = 0.1 nm) were collected by the oscillation method and the structure of the enzyme product complex was refined by alternate restrained least-squares procedures and model building by computer graphics techniques. A total of 273 solvent molecules could be located, four of them being presumably sulfate ions. The R-factor for 14,339 reflections between 6.0 A and 2.3 A is 19.3%. The 3-hydroxyl group of the product introduced by the enzyme is clearly visible in the electron density, showing unambiguously which carbon atom of the substrate is hydroxylated. A clear picture of the hydroxylation site is obtained. The plane of the product is rotated 21 degrees with respect to the plane of the substrate in the current model of enzyme-substrate complex. The 4-hydroxyl group of the product is hydrogen bonded to the hydroxyl group of Tyr201, its carboxyl group is interacting with the side-chains of Tyr222, Arg214 and Ser212, while the newly introduced 3-hydroxyl group makes a hydrogen bond with the backbone carbonyl oxygen of Pro293.

Chemical modification of sulfhydryl groups in p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens. Involvement in catalysis and assignment in the sequence

The cysteine residues in p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens were modified with several cysteine reagents. One of the five sulfhydryl groups reacts rapidly and specifically with N-ethylmaleimide without inactivation of the enzyme. Cysteine-116 was found to be the reactive cysteine by isolation of a labeled tryptic peptide. The enzyme is easily inactivated by mercurial compounds. The original activity can be fully restored by treatment of the modified enzyme with sulfhydryl-containing compounds. The rate of incorporation of mercurial compounds is pH-independent and is pseudo-first-order up to 90-95% loss of activity. The reaction shows saturation kinetics. The substrate p-hydroxybenzoate protects the enzyme from fast inactivation. The mercurial compounds themselves inhibit the inactivation reaction at concentrations higher than 80 microM. A spin-labeled derivative of p-chloromercuribenzoate reacts fairly specifically with only Cys-152 on use of enzyme prelabeled with N-ethylmaleimide, in contrast to p-chloromercuribenzoate which reacts with additional cysteine residues, i.e. Cys-211 and Cys-158. From these results it is concluded that modification of Cys-152 decreases drastically the affinity of the enzyme for the substrate. The results strongly indicate that the substrate binding site and Cys-152 are interdependent. This observation is not obvious when the three-dimensional data only are considered. The modified enzyme exhibits a somewhat higher affinity for NADPH than the native enzyme. Modification of N-ethylmaleimide-prelabeled enzyme by p-chloromercuribenzoate leads to absorbance difference spectra showing maxima at 250 nm, 290 nm and 360 nm. The intensities of the absorbance difference maxima at 290 nm and 360 nm are strongly dependent on the pH value of the solution. The intensities are very low at low pH values and increase with increasing pH values, reaching a maximum at about pH = 9. The ionizing group shows a pK value of about 7.6. The maximal molar difference absorption coefficient at 290 nm is 3200 M-1cm-1 at pH 9, suggesting that tyrosine residues ionize under the conditions of modification of the enzyme. The results are discussed in the light of the known three-dimensional structure.