Chloroperoxidase halogenation reactions. Chemical versus enzymic halogenating intermediates

Area of Science:

• Biochemistry
• Enzymology
• Organic Chemistry

Background:

• Chloroperoxidase catalyzes the peroxidation of chloride and bromide ions to molecular chlorine and bromine in the absence of organic substrates.
• These molecular halogens are not intermediates in the enzyme's halogenation of organic substrates.
• Enzymatic and chemical halogenation reactions exhibit different substrate specificities.

Purpose of the Study:

• To investigate the reaction mechanism and substrate specificity of chloroperoxidase in halogenation reactions.
• To compare the enzymatic halogenation pathways with non-enzymatic chemical halogenation.
• To propose a general reaction scheme for chloroperoxidase-mediated halogenation.

Main Methods:

• Comparative analysis of substrate preferences for chloroperoxidase-catalyzed reactions versus non-enzymatic reactions using hypochlorite and molecular chlorine.
• Kinetic studies comparing the rates of oxidation and halogenation.
• Spectroscopic analysis to identify reaction intermediates.

Main Results:

• Chloroperoxidase exhibits significantly higher substrate specificity for thiourea and methionine compared to 2-chlorodimedone than observed in non-enzymatic reactions.
• The rate of chloride oxidation to molecular chlorine is slower than enzymatic chlorination of substrates.
• Bromine formation rate equals acceptor bromination rate, but chloroperoxidase shows higher specificity for methionine over 2-chlorodimedone than molecular bromine.

Conclusions:

• Chloroperoxidase employs a distinct mechanism for halogenation, differing from simple chemical halogenation pathways.
• The enzyme's specificity arises from its unique catalytic mechanism, not solely from the reactivity of generated halogens.
• A proposed mechanism involves the formation of Compound I and an iron (III) hypohalite intermediate.