Biology Reference
In-Depth Information
that a globin's sole purpose is the transport or storage of O
2
for aerobic res-
piration. We now know this view of the superfamily to be incomplete. All
globins, even vertebrate haemoglobins and myoglobins, appear to have mul-
tiple functions in addition to the reversible binding of dioxygen. In the
microbial world, these auxiliary roles as well as others conveyed by the reac-
tive haem cofactor dominate the functional properties (
Vinogradov &
Moens, 2008
). This chapter summarizes the state of knowledge concerning
the globins of cyanobacteria and green algae and outlines the many oppor-
tunities that they offer for a deeper understanding of haem proteins and the
evolution of the haemoglobin superfamily.
Cyanobacterial and algal globin research is only a couple of decades old.
In contrast to vertebrate haemoglobins and myoglobins, which are imme-
diately noticeable by their colour and obtainable in high quantities by direct
extraction from the organism, cyanobacterial and algal globins occur at cat-
alytic levels in cells replete with photosynthetic pigments. It is through their
genes that they first become apparent. Thus, their discovery had to wait for
the development of modern molecular biology tools and efficient genome
sequencing techniques. With a few detailed biochemical studies of Cyano-
bacteria and Chlorophyta exemplars and a large number of sequences, we
have nevertheless caught a glimpse of the remarkable properties exhibited
by this group of proteins.
It is necessary at this stage to specify what we call 'haemoglobin'.
The term is generally reserved for protein sequences that belong to the
haemoglobin superfamily and contain the 'proximal histidine'. The proxi-
mal histidine serves as a haem axial ligand (
Fig. 6.1
) and presumably indicates
that the protein's functional state contains that particular cofactor. Sequences
related to algal globins but lacking the proximal histidine exist, though little
is known about these proteins, and their competency as haem binders is
unclear. They are only mentioned briefly in this chapter. We do not review
haem proteins related to
Escherichia coli
Dos,
Acetobacter xylinum
phosphodi-
esterase and
Bradyrhizobium japonicum
FixL, all of which have a proximal his-
tidine and are capable of reversible dioxygen binding for sensing purposes
(
Tomita, Gonzalez, Chang, Ikeda-Saito, & Gilles-Gonzalez, 2002
) but do
not belong to the haemoglobin superfamily. We also exclude the relatives
of the stress signalling protein RsbR, which does not bind haem
(
Murray, Delumeau, & Lewis, 2005
) and is a globin in fold only.
In
Section 2
, we present a historical overview of the field. Progress in the
study of globins from unicellular photosynthetic organisms is entwined
with broad advances in the structural biology and phylogeny of the
haemoglobin superfamily. After a review of these advances,
Sections 3
