Biology Reference
In-Depth Information
1. INTRODUCTION
The globin family has long been known from studies of vertebrate
myoglobin (Mb) and haemoglobins (Hbs), which are haemoproteins typi-
cally composed of about 150 amino acids. In the past 30 years, the Hb super-
family has been enriched by the discovery of Hbs and related haemoproteins
in virtually all kingdoms of life; among these non-symbiotic Hbs in plants
and symbiotic Hbs in plants other than legumes, chimaeric flavoHbs com-
prised of an N-terminal globin linked to a FAD reductase domain in bacteria
and yeasts, neuroglobins and cytoglobins in vertebrates, globin-coupled sen-
sors and protoglobins in eubacteria and in archaea, and globins that fall in the
110-130 amino acid range (per haem), which have been called 'truncated
Hbs', in protozoa and in bacteria. Such large variety of Hb types suggests
an ancient origin for their genes and stresses the concept that Hbs/globins
cover functions that stretch well beyond that of simple oxygen carriers.
In this review, we focus our attention on the truncated Hb family based
on the extensive sequence and crystallographic investigations performed in
various laboratories starting from the early 2000s. The term 'truncated Hb'
was first introduced to refer to the size of these small globins. However,
structural considerations underline the fact that the tertiary structure of these
proteins results from careful editing of the classical globin fold through an
evolutionary/engineering process that affects the whole polypeptide chain,
rather than just acting through simple truncation of the N- and C-terminal
ends. For this reason, the historical term 'truncated Hb' was recently rep-
laced by '2/2Hbs' (read 2-on-2 Hbs) in relation to specific features of their
folds (see below).
The 2/2Hbs are small oxygen-binding haemoproteins, identified in bac-
teria, higher plants, and in certain unicellular eukaryotes, building a clear
separate cluster within the haemoglobin superfamily (
Nardini, Pesce,
Milani, & Bolognesi, 2007; Vinogradov, Tinajero-Trejo, Poole, &
Hoogewijs, 2013; Vuletich & Lecomte, 2006; Wittenberg, Bolognesi,
Wittenberg, & Guertin, 2002
). 2/2Hbs display amino acid sequences that
are 20-40 residues shorter than (non-)vertebrate Hbs, to which they are
loosely related by sequence similarity (sequence identity to vertebrate
Hbs falls well below 20%). Based on amino acid sequence analysis, three
2/2Hbs phylogenetic groups (groups I, II, and III, whose members are des-
ignated by the N, O, and P suffixes, respectively) were recognized, proteins
being orthologous within each group and paralogous across the groups