Flavin-dependent hydroxylases

PHBH in its native structure is a homodimer that contains one FAD per monomer, and it has a monomer molecular weight of 45,000. The active site of each monomer is constructed from the flavin and amino acid residues in that monomer. There is no clear evidence that a dimer is necessary for activity. PHBH is one member of a large family of similar flavoproteins that probably have a common evolutionary descent, based on structural and sequence similarities. This family is often referred to as the one-component aromatic hydroxylases. This aspect distinguishes them from another large and diverse group of flavoprotein hydroxylases, which is the two-component flavin-dependent hydroxylases that use two different proteins to carry out the catalysis of hydroxylation. The latter group, which is described in a separate section, has no common ancestry with the one-component enzymes. 

Overall Reaction Catalyzed by a Flavin-Dependent Hydroxylase. 

Degradation of L-arginine by Streptomyces griseus is initiated by a hydroxylase that causes decarboxylation and conversion of the amino acid into an amide (Eq. 24-26), a reaction analogous to that catalyzed by the flavin-dependent lysine oxygenase (Eq. 18-41). The... 

Modelling can pinpoint functional groups and analyse catalytic interactions. In several enzymes, catalytic interactions have been identified via calculation. For example, in the flavin-dependent monooxygenases, para-hydroxybenzoate hydroxylase and phenol hydroxylase, a conserved proline residue was found from QM/MM modelling, which specifically stabilizes the transition state for aromatic hydroxy-lation.12,13... 

Most aromatic hydroxylases are either cytochrome- or flavin-dependent enzymes the three enzymes that catalyze hydroxylation of the aromatic amino acids phenylalanine, tyrosine, and tryptophan are apparently unique in... 

Ballou DP, Entsch B, Cole LJ. Dynamics involved in catalysis by single-component and two-component flavin-dependent aromatic hydroxylases. Biochem. Biophys. Res. Comm. 2005 338 590-598. 

Some flavin-dependent aromatic hydroxylases and the monooxygenase reactions catalyzed... 

There are three hydroxylation reactions on the ubiquinone biosynthetic pathway. All three oxygens are derived from molecular oxygen 1198] and heme is not required [199]. This suggests that these hydroxylations proceed by a mechanism analogous to that of the flavin dependent tyrosine hydroxylase (Fig. 42) [14]. 

Eppink MHM, SA Boeren, J Vervoort, WJH van Berkel (1997) Purification and properties of 4-hydroxyben-zoate 1-hydroxylase (decarboxylating), a novel flavin adenein dinucleotide-dependent monooxygenase from Candida parapilosis CBS604. J Bacteriol 179 6680-6687. 

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

The monooxygenase group of enzymes includes the non-P450 hydroxylases which catalyze the insertion of a hydroxyl group to replace a hydrogen atom at a saturated carbon [6-8] and the non-heme-dependent oxygenases such as the flavin-molybdenum-cobalt-dependent xanthine oxidase and aldehyde oxidase... 

What pterin-dependent hydroxylation reactions are important to the human body Point out similarities and differences between flavin and pterin hydroxylase mechanisms. 

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