Proline 293 of p-hydroxybenzoate hydroxylase from Pseudomonas aeruginosa is in a highly conserved region of the flavoprotein aromatic hydroxylases. It is thought to impart rigidity to the backbone, as it partially cradles the FAD in these hydroxylases. Thus, this residue has been substituted with serine by site-directed mutagenesis to investigate the importance of flexibility of the peptide segment in catalysis. Differential scanning calorimetry demonstrated that the mutation has decreased the stability of the folded mutant protein compared to the wild-type PHBH. The increased flexibility in the protein backbone enhanced the accessibility of the flavin hydroperoxide intermediate to the solvent, causing an increase in the elimination of H2O2 from this labile intermediate and, consequently, a decrease in the efficiency of substrate hydroxylation. Additionally, the increased accessibility of this mutant form of the enzyme makes it more susceptible than the wild-type enzyme to being trapped in the hydroxyflavin intermediate form in the presence of high levels of p-hydroxybenzoate. The mutation also lowers the pKa of the phenolic oxygen of bound p-hydroxybenzoate, and eliminates the pH dependence of the rate constant for flavin reduction by NADPH. These experimental observations lead to a model that explains how the wild-type protein can sense the charge of the 4-substituent of the aromatic ligand and link this charge to a flavin conformational change that is required for reaction with NADPH: (i) The peptide oxygen of Pro 293 is repelled by the negative charge of the phenolic oxygen of p-hydroxybenzoate. (ii) This repulsion is transmitted through the peptide backbone, causing the movement of Asn 300. (iii) The change in the position of Asn 300 triggers the movement of the flavin from the largely buried "in" conformation to the exposed, reactive "out" conformation.
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