Vanadate-dependent bromoperoxidases from Ascophyllum nodosum in the synthesis of brominated phenols and pyrroles

Abstract

Bromoperoxidases from the brown alga Ascophyllum nodosum, abbreviated as V(Br)PO(AnI) and V(Br)PO(AnII), show 41% sequence homology and differ by a factor of two in the percentage of α-helical secondary structures. Protein monomers organize into homodimers for V(Br)PO(AnI) and hexamers for V(Br)PO(AnII). Bromoperoxidase II binds hydrogen peroxide and bromide by approximately one order of magnitude stronger than V(Br)PO(AnI). In oxidation catalysis, bromoperoxidases I and II turn over hydrogen peroxide and bromide similarly fast, yielding in morpholine-4-ethanesulfonic acid (MES)-buffered aqueous tert-butanol (pH 6.2) molecular bromine as reagent for electrophilic hydrocarbon bromination. Alternative compounds, such as tribromide and hypobromous acid are not sufficiently electrophilic for being directly involved in carbon-bromine bond formation. A decrease in electrophilicity from bromine via hypobromous acid to tribromide correlates in a frontier molecular orbital (FMO) analysis with larger energy gaps between the π-type HOMO of, for example, an alkene and the σ*(Br,X)-type LUMO of the bromination reagent. By using this approach, the reactivity of substrates and selectivity for carbon-bromine bond formation in reactions mediated by vanadate-dependent bromoperoxidases become predictable, as exemplified by the synthesis of bromopyrroles occurring naturally in marine sponges of the genera Agelas, Acanthella, and Axinella.

Metagenomic discovery of polybrominated diphenyl ether biosynthesis by marine sponges

“…Second, and in concert with our previous description of PBDE biogenesis from marine γ -proteobacteria 4 , chemical structures of naturally produced PBDEs and dioxins imply that genes encoding their biosynthesis cannot be expected to resemble modular natural product assembly lines that lend themselves to computational mining and discovery from genomic and metagenomic datasets 27 . Furthermore, in light of published literature 28,29 invoking the contribution of an entirely different class of halogenating enzymes in the biosynthesis of polybrominated phenolics than what we had characterized previously from marine Pseudoalteromonas and Marinomonas bacteria 4 , it was not apparent that the biochemical logic for the production in PBDEs in marine γ- proteobacteria would be preserved in Dysideidae sponges.…”

Biocatalytic Oxidation Reactions: A Chemist's Perspective

“…Haloperoxidases,f or example,i nitiate the halogenation of activated aromatic compounds.T he catalytic mechanism involves oxidative activation of halides to hypohalites,w hich then, in an on-enzymatic follow-up reaction, form the halogenated products (Scheme 12). Aside from phenols,a lso pyrroles [52] and indoles [53] have been converted in this fashion. As the halogenation does not occur within the enzyme active site,the selectivity of the halogenation reaction is dictated by the chemical reactivity of the starting material rather than by the enzyme,c hallenging its preparative scope.N evertheless,t he very high catalytic activities (TN > 2000 000 and TF average > 25 s À1 )t hat have been achieved are very convincing.…”

Thymol Bromination – A Comparison between Enzymatic and Chemical Catalysis

“…[3] Also oxidative bromination reactions of phenol and substituted phenols have been performed, and these reactions have usually led to selective para-bromination with respect to the hydroxy group together with formation of the corresponding ortho,para-diBr derivatives, both resulting from the classical electrophilic aromatic substitution. [8,9] No products of sidechain bromination were detected. The role of vanadium haloperoxidases in the formation of volatile brominated compounds from natural sources has been recently discussed in a Perspective Article, evidencing their impact on the environment.…”

“…In particular, the first one may take place in a hydrophilic portion close to the active site, considering that at least two molecules of water and a proton are required to oxidize bromide, [8] while the functionalization of the substrate may occur in a more hydrophobic location.…”