2.7.1 Halogenation
Despite the impressive number of halometabolites identified so far, only a few
types of halogenating enzymes have been characterized to date [1951–1954]:
Whereas flavin- [1955] and α-ketoglutarate-dependent (nonheme) ironhalogenases [1956] are rather substrate specific, haloperoxidases show a broad
substrate scope and thus had a dominant impact in biotransformations
[1451, 1471, 1957–1960]. These enzymes are widely distributed in nature and
enable a multitude of electrophilic halogenation reactions following the general
equation shown in Scheme 2.224, where X stands for halide (Cl
À
, Br
À and I
À , but
not F
À
,
56 [1961]). The individual enzymes are called chloro-, bromo-, and
iodoperoxidase. The name reflects the smallest halide ion that they can oxidize,
in correlation to the corresponding redox potential. For redox reactions catalyzed
by haloperoxidases which do not involve a halide (such as hydroxylation, epoxidation, or sulfoxidation) see Sect. 2.3.4.
Despite their mechanistic differences, the overall net reaction of haloperoxidases
consists of a two-electron oxidation of halide anion at the expense of H 2 O 2 yielding
[X
+
] (Scheme 2.224).
57 Depending on the type of halide and the reaction conditions, such as pH, the electrophilic species may be halonium (X
+
, X 3
+
), halogen
(X 2 ) or hypohalous acid/hypohalite (HOX/OX
À ). Two major types of
haloperoxidases depend on a catalytic metal:
• The mechanism of heme-iron-dependent enzymes is closely related to that of
peroxygenases, i.e. H 2 O 2 -dependent oxidation of Fe
3+ in the enzyme’s resting
state yields the Fe
4+ -species Compound I (Scheme 2.171). The latter oxidizes
halide in a two-electron transfer step (Scheme 2.224) [1962, 1963].
• In contrast, vanadium-depending haloperoxidases do not change the oxidation
state of V
5+ during the catalytic cycle, but switch between vanadate and peroxovanadate [1964, 1965].
For both enzymes, the fate of X
+ generated and the existence of a metal-bound
hypohalite adduct is under debate, for heme-dependent haloperoxidases this elusive
species is ironically denoted as ‘Compound X’ [1966]. The actual halogenation
reaction is believed to take place outside of the active site and consequently, any
asymmetric induction observed in haloperoxidase-catalyzed reactions is
usually low.
56 The high electronegativity of fluorine renders the formation of F
+ energetically prohibitive.
Enzymatic (nucleophilic) fluorination is extremely rare and requires S-adenosylmethionine (SAM)
as cofactor.
57 In a side reaction, X
+ may react with H 2 O 2 to form singlet oxygen (X
+ + H 2 O 2 !
1
O 2 + X
À + H
+
).
2.7 Halogenation and Dehalogenation Reactions
251
Despite the impressive number of halometabolites identified so far, only a few
types of halogenating enzymes have been characterized to date [1951–1954]:
Whereas flavin- [1955] and α-ketoglutarate-dependent (nonheme) ironhalogenases [1956] are rather substrate specific, haloperoxidases show a broad
substrate scope and thus had a dominant impact in biotransformations
[1451, 1471, 1957–1960]. These enzymes are widely distributed in nature and
enable a multitude of electrophilic halogenation reactions following the general
equation shown in Scheme 2.224, where X stands for halide (Cl
À
, Br
À and I
À , but
not F
À
,
56 [1961]). The individual enzymes are called chloro-, bromo-, and
iodoperoxidase. The name reflects the smallest halide ion that they can oxidize,
in correlation to the corresponding redox potential. For redox reactions catalyzed
by haloperoxidases which do not involve a halide (such as hydroxylation, epoxidation, or sulfoxidation) see Sect. 2.3.4.
Despite their mechanistic differences, the overall net reaction of haloperoxidases
consists of a two-electron oxidation of halide anion at the expense of H 2 O 2 yielding
[X
+
] (Scheme 2.224).
57 Depending on the type of halide and the reaction conditions, such as pH, the electrophilic species may be halonium (X
+
, X 3
+
), halogen
(X 2 ) or hypohalous acid/hypohalite (HOX/OX
À ). Two major types of
haloperoxidases depend on a catalytic metal:
• The mechanism of heme-iron-dependent enzymes is closely related to that of
peroxygenases, i.e. H 2 O 2 -dependent oxidation of Fe
3+ in the enzyme’s resting
state yields the Fe
4+ -species Compound I (Scheme 2.171). The latter oxidizes
halide in a two-electron transfer step (Scheme 2.224) [1962, 1963].
• In contrast, vanadium-depending haloperoxidases do not change the oxidation
state of V
5+ during the catalytic cycle, but switch between vanadate and peroxovanadate [1964, 1965].
For both enzymes, the fate of X
+ generated and the existence of a metal-bound
hypohalite adduct is under debate, for heme-dependent haloperoxidases this elusive
species is ironically denoted as ‘Compound X’ [1966]. The actual halogenation
reaction is believed to take place outside of the active site and consequently, any
asymmetric induction observed in haloperoxidase-catalyzed reactions is
usually low.
56 The high electronegativity of fluorine renders the formation of F
+ energetically prohibitive.
Enzymatic (nucleophilic) fluorination is extremely rare and requires S-adenosylmethionine (SAM)
as cofactor.
57 In a side reaction, X
+ may react with H 2 O 2 to form singlet oxygen (X
+ + H 2 O 2 !
1
O 2 + X
À + H
+
).
2.7 Halogenation and Dehalogenation Reactions
251