Biology Reference
In-Depth Information
CHAPTER TWO
The Diversity of 2/2 (Truncated)
Globins
Alessandra Pesce
*
, Martino Bolognesi
†
,
‡
, Marco Nardini
†
,1
*
Department of Physics, University of Genova, Genova, Italy
†
Department of Biosciences, University of Milano, Milano, Italy
‡
CIMAINA and CNR Institute of Biophysics, Milano, Italy
1
Corresponding author: e-mail address: marco.nardini@unimi.it
Contents
1.
Introduction
50
2. Fold and Fold Variation in 2/2Hb Groups I, II, III
52
3. The Haem Environment
54
4. Tunnels and Cavities Through 2/2Hb Protein Matrix
60
5. Proposed Functions for the 2/2Hb Family
66
6. Conclusions
71
References
72
Abstract
Small size globins that have been defined as
'
truncated haemoglobins
'
or as
'
2/2
haemoglobins
have increasingly been discovered in microorganisms since the early
1990s. Analysis of amino acid sequences allowed to distinguish three groups that collect
proteins with specific and common structural properties. All three groups display 3D
structures that are based on four main
a
-helices, which are a subset of the conventional
eight-helices globin fold. Specific features, such as the presence of protein matrix tun-
nels that are held to promote diffusion of functional ligands to/from the haem, distin-
guish members of the three groups. Haem distal sites vary for their accessibility, local
structures, polarity, and ligand stabilization mechanisms, suggesting functional roles
that are related to O
2
/NO chemistry. In a few cases, such activities have been proven
in vitro and in vivo through deletion mutants. The issue of 2/2 haemoglobin varied
biological functions throughout the three groups remains however fully open.
'
ABBREVIATIONS
2/2Hb
2-on-2 globin
Hb
haemoglobin
Mb
myoglobin
(non-)vertebrate Hbs
vertebrate and non-vertebrate Hbs
amino acid residues have been labelled using their three-letter codes and the topological site
they occupy within the globin fold
