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cofactors, largely because of the influence of the distal and proximal charge
balance upon the vibrational spectroscopy of the iron d CO bond. Essen-
tially, the Fe-C-O moiety can exist in one of two extreme resonance struc-
tures, depending upon the extent to which the d p orbital of the iron donates
electron density to the
p * orbital of CO shown in Eq. (4.3) :
Fe d
O d þ I
His
C
ðÞ!
His
Fe
¼
C
¼
OII
ðÞ
ð
4
:
3
Þ
Generally, a positive polar environment destabilises form (I) and pro-
motes the 'backbonding' interaction, leading to a stronger Fe d CO bond
and a weaker C d O bond. Hence, the frequency of the
n
Fe-CO mode is typ-
n
ically inversely correlated with that of the
C-O mode in a linear fashion
( Egawa & Yeh, 2005; Rousseau, Li, Couture, & Yeh, 2005; Spiro &
Wasbotten, 2005 ). Form II is referred to as closed, where the haem-bound
CO is stabilised by B10Tyr and E7Gln, whereas form I is known as the open
conformation that lacks these hydrogen-bonding interactions. In the case of
Cgb, both resonance conformers were observed: CO was proposed to be
stabilised by a hydrogen bond from the Tyr-28 (B10)/Gln-52 (E7) pair
(closed), or the ligand remained bound without a hydrogen bond (open).
Globins that can exist in both these resonance forms, such as Cgb, Hmp
and HbN from M. tuberculosis , have all been implicated in protecting the
respective organisms from nitrosative stress ( Couture et al., 1999; Frey &
Kallio, 2005; Mukai et al., 2001 ).
Further evidence for the peroxidase-like character of Cgb was investi-
gated through testing the reactivity towards peroxides. Surprisingly, Cgb
exhibited lower reactivity towards hydrogen peroxide compared to Mb,
although rapid decomposition of the organic peroxide m -chloroperbenzoic
acid by Cgb demonstrates its potential function as an organic peroxidase
(Cgb was fivefold faster than Mb). In addition, peroxidase-like
haemoproteins with imidazolate axial ligands typically have lower redox
midpoints than haem proteins with neutral axial ligands. For example,
horseradish peroxidase has a midpoint potential of
250 mV ( Yamada,
Makino, & Yamazaki, 1975 ), compared to 58 mV for Mb ( Rayner,
Stocker, Lay, & Witting, 2004 ). Unsurprisingly, Cgb has a low redox mid-
point of
134 mV at pH 7 ( Shepherd et al., 2010 ), reflecting the negative
charge imposed upon the axial histidine by the proximal hydrogen-bonding
network. Indeed, mutation of the H23Glu residue increased the redox mid-
point to
110 mV, reflecting coordination of the haem by a more neutral
axial histidine.
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