p-Hydroxybenzoate hydroxylase from Pseudomonas fluorescens. 2. Fitting of the amino-acid sequence to the tertiary structure.

The complete primary and tertiary structure of p-hydroxybenzoate hydroxylase is now known. The amino acid sequences of the two largest CNBr peptides have been fitted to the electron-density map at 0.25-nm resolution. The parts of the polypeptide chain contributing the residues to the FAD-binding site and the residues of the substrate-binding site have been identified. The active site is located in a large hydrophobic area enclosed by all domains of the enzyme structure. Here the substrate, p-hydroxybenzoate, is bound near, but not in direct contact with, the isoalloxazine ring system of FAD. Many side chains from the C-terminal part of the polypeptide chain are involved in subunit-subunit interactions. In the center of one of the largely hydrophobic contact areas between the subunits, a cluster of six aromatic amino acids was found.