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that of the basic form of cytochrome
c
, which has a histidine and a lysine for
axial ligands (
Brautigan et al., 1977
). The likely candidate is Lys E10 in a
demonstration that the orientation of the E helix can be adjusted under
the driving force of ligation. The exact configuration of the haem pocket
in the reduced state is not known.
The chemical properties of haem proteins are exquisitely dependent on
structural details. The power of prediction—regarding how an amino acid
replacement, a change in pH, or some apparently benign perturbation, will
affect reactivity—is therefore remarkably limited. Despite the educated
expectations based on extensive sequence information, knowledge of
canonical globin behaviour and spectral comparison, the T globins presented
uncharted and mysterious territory at the time of the first
N. commune
and
C. eugametos
studies. Three-dimensional structural information was greatly
needed to rationalize spectroscopic data. The first structures of T globins,
from
P. caudatum
and
C. eugametos
(
Pesce et al., 2000
), were therefore much
welcomed by the globin research community.
The three-dimensional structure of CtrHb revealed how the 'truncated'
fold differs from the canonical myoglobin fold (
Fig. 6.2
). The latter is
described as a 3/3 orthogonal sandwich, placing three of eight
a
helices
(A, E, F in the Perutz nomenclature) against another three (B, G, H). In
the truncated globin domain, the A helix is cut short, reduced from over
four turns to a single one, and loss of secondary structure occurs in the
F helix region. Thus, the sandwich is best described with B-E/G-H pairing,
and therefore the 2/2 denomination. There are other distinctive features,
specifically two glycines separating the A and B helices, the absence of
the D helix and the resulting tight connection between the C helix and
the E helix, and the unstructured long loop between the E and F helices
(
Nardini et al., 2007; Vuletich & Lecomte, 2006
). The protein, however,
is large enough to provide the haem group with a sheltered environment
limiting solvent access to the distal side.
For stability reasons, the protein form of CtrHb that was chosen for crys-
tallization was the cyanomet complex. Haem orientational isomerism is
observed, the proportions of the two forms being roughly 1:1 (
Pesce
et al., 2000
). Compared with the
N. commune
GlbN results and assuming that
the equilibrium was reached in both cases, this indicates a less discriminating
haem pocket in the algal protein. It is tempting to attribute the difference to
the immediate neighbours of the haem group. A survey of
b
haem proteins
and their variants, however, indicates that remote effects can be responsible
for the imbalance (
La Mar et al., 2000
). At this stage, the significance of the
