The Globins of Campylobacter jejuni - Advances in Microbial Physiology - page 118

Taken together, the structural and biophysical data are consistent with a
peroxidase such as 'push-pull' mechanism ( Fig. 4.4 ) that facilitates the
isomerisation of the proposed peroxynitrite intermediate (Eq. 4.2 )ina
dioxygenase reaction. However, as mentioned in Section 6.1 , the reaction
mechanismmay also proceed via a denitrosylase route whereby NO binds to
the ferrous haem first ( Fig. 4.5 ). Indeed, this has been shown to occur in vitro
with Hmp of E. coli ( Hausladen et al., 2001 ), where the unliganded ferrous
haem binds preferentially to NO ( K d ¼ 8 pM) compared to oxygen

( K d 12 nM). However, a denitrosylase reaction involves the formation

of a nitrosyl-haem complex, which requires the cleavage of an N d O bond
rather than an O d O linkage, a less thermodynamically favourable process.
Given that Cgb is able to bind NO in both the ferrous and ferric forms
( Shepherd, Bernhardt, & Poole, 2011 ), whereas only ferrous Cgb can bind
oxygen, the relative availability of NO and oxygen and the redox poise of
the cell will influence the route via which Cgb can detoxify NO.
An alternative mechanism has been investigated for Cgb, involving the
formation of an oxoferryl (Fe(IV) ] O) species ( Shepherd et al., 2011 ). Since
Campylobacter will be exposed to both hydrogen peroxide and NO during
colonisation of the host, it was proposed that ferric Cgb could react with
hydrogen peroxide to form an oxoferryl species, as is the case for horseradish
peroxidase. The oxoferryl species would subsequently react with NO to
produce nitrite and nitrate. However, although forward rate constants for
the formation of the oxoferryl species were comparable to other globins,
they were orders of magnitude slower than bona fide peroxidases such as
horseradish peroxidase. Furthermore, reaction of NO with oxoferryl Cgb
was extremely slow, which rules out this mechanism as a viable route for
NO detoxification in vivo .

¼

Figure 4.5 Proposed denitrosylase and dioxygenase routes in the catalytic cycle of Cgb.