Mechanism and kinetics of haloperoxidase-catalyzed bromination reaction.

Abstract

Haloperoxidase-catalyzed bromination of tyrosine was studied. Three haloperoxidases from different sources were used. When chloroperoxidase (CPO) was used as the enzyme, and when {dollar}\rm was H\sb2O\sb2{dollar} was present, the conversion of Br{dollar}\sp-{dollar} to Br{dollar}\sb3\sp-{dollar}/Br{dollar}\sb2{dollar} species was an enzyme-catalyzed reaction, and the enzyme, CPO, was irreversibly deactivated by the free molecular bromine formed in that reaction. It was observed that the above deactivation of CPO was associated with the irreversible destruction of CPO's heme structure. The minimum Br{dollar}\sb2{dollar}/CPO ratio to cause that deactivation was obtained. The actual bromination reaction was concluded to be a nonenzymatic reaction between Br{dollar}\sb2{dollar} and tyrosine, Also observed was the reaction between {dollar}\rm H\sb2O\sb2{dollar} and Br{dollar}\sb3\sp-{dollar}/Br{dollar}\sb2{dollar} species to produce O{dollar}\sb2{dollar} and Br{dollar}\sp-,{dollar} if the Br{dollar}\sp-{dollar}/{dollar}\rm H\sb2O\sb2{dollar} ratio was not high enough. The minimum Br{dollar}\sp-{dollar}/{dollar}\rm H\sb2O\sb2{dollar} ratio required to inhibit the production of O{dollar}\sb2{dollar} and Br{dollar}\sp-{dollar} was determined. Inhibition of this reaction would allow Br{dollar}\sb3\sp-{dollar}/Br{dollar}\sb2{dollar} to accumulate. The maximum Br{dollar}\sp-{dollar}/{dollar}\rm H\sb2O\sb2{dollar} ratio that would allow the production of O{dollar}\sb2{dollar} and Br{dollar}\sp-{dollar} was also determine. This would prevent the Br{dollar}\sb3\sp-{dollar}/Br{dollar}\sb2{dollar} species from accumulating.

In the horseradish peroxidase-catalyzed bromination system, horseradish peroxidase (HPO) would not be deactivated irreversibly by Br{dollar}\sb2{dollar} under the condition of the highest Br{dollar}\sb2{dollar} concentration used in our system. When HPO was mixed with Br{dollar}\sb2{dollar} or when HPO was used as the enzyme in the bromination reaction system, the same shift of HPO's Soret band from its original position was observed, which indicated a similar change of HPO's heme structure. It was also observed that HPO could "recover" from the above structural change and shift back to its original position when HPO was exposed to Br{dollar}\sb2{dollar} long enough or when the bromination reaction was close to equilibrium. A probable reaction mechanism of the HPO-catalyzed bromination was proposed, which suggested that the Br{dollar}\sb2{dollar}/Br{dollar}\sb3\sp-{dollar} species bound to HPO to form an enzyme intermediate, and that intermediate catalyzed the actual bromination reaction.

When lactoperoxidase (LPO) was used as the enzyme, it was observed that LPO was deactivated irreversibly by Br{dollar}\sb2{dollar}; but, the deactivation process was different from that of CPO.