Biology Reference
In-Depth Information
Tarricone, & Ascenzi, 1997; Perutz, 1979
). The third lineage encompasses
the “truncated” globins that are structurally distinct from 3/3 globins. This
lineage consists of three subgroups, all sharing an abbreviated single globin
domain folded as a 2/2
a
-helical sandwich (2/2Hb), characterised by a very
short or absent A-helix, a brief CE inter-helical region and most of the
F-helix occurring as a loop, with only the B-, E-, G- and H-helices left
to surround the haem group (
Nardini, Pesce, Milani, & Bolognesi, 2007;
Pesce, Bolognesi, & Nardini, 2013; Vuletich & Lecomte, 2006
).
Pgbs belong phylogenetically to the same cluster as the GCS group
within the globin superfamily. However, while GCSs are chimeric haem-
proteins, tentatively classified either as aerotactic or gene regulating, which
couple a globin-like sensor domain (usually larger than that of Mb) to a
transmitter domain of variable structure and function, Pgbs are single-
domain variants devoid of the transmitter domain (
Freitas et al., 2003,
2004, 2005; Hou et al., 2001
).
Pgb derives its name from a very first phylogenetic analysis of the GCS
family, which suggested that each GCS globin domain evolved indepen-
dently, with its particular signalling partner, and predicted the existence
of an ancestor globin, or Pgb, within more primitive organisms. As such,
Pgb was first characterised in two Archaea, in the obligate aerobic hyper-
thermophile
Aeropyrum pernix
(
Ap
Pgb) and in the strictly anaerobic meth-
anogen
Methanosarcina acetivorans
(
Ma
Pgb). At that time the SDSgbs had
not been identified yet, and Pgb was postulated to be the single-domain
ancestor not only for GCS but also possibly for all contemporary Hbs
(
Freitas et al., 2004
). Such prototypic view of Pgb soon appeared unlikely,
based on more extended surveys of putative globins in sequenced genomes:
the identification of SDSgbs within the GCS lineage that could explain the
emergence of chimeric GCSs, the surprisingly distant relationship between
Pgbs and chimeric sensor sequences, and the sequence homogeneity among
Pgbs support the idea that Pgbs are a relatively recent group of globins
emerged in globin evolution (
Vinogradov et al., 2007
). Within this evolu-
tionary scenario, a model for globin evolution finding its root in a bacterial
FHb-like single-domain ancestral globin was proposed (
Vinogradov et al.,
2007
). Thus, the FHb family globins displaying enzymatic functions in
the early bacteria are held to have evolved
via
horizontal gene transfer in
multi-cellular eukaryotes; such event brought about new properties, includ-
ing reversible binding of important diatomic ligands, such as O
2
, NO and
sulphide, enabling the evolution of
transport and storage functions
(
Vinogradov & Moens, 2008
).
