Problem to solve: Haemoglobin
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The metalloprotein Haemoglobin (*Hgb*) is the iron-containing protein able
to transport oxygen, essential to get energy from aerobic metabolic
reactions, through red blood cells of almost every vertebrate. The
first atomic structure of *Hgb* was determined in 1960 by X-ray
crystallography :cite:p:`perutz1960`. *Hgb* was, alongside
myoglobin, the first structure solved by this methodology. Due to its
emblematic prominence in structural biology History, we have selected *Hgb*
to model its atomic structure.

*Hgb* is a relatively small macromolecule (molecular weight of 64 KDa) that
shows C2 symmetry. This heterotetramer is constituted by four globular
polypeptide subunits, two :math:`\alpha` and two :math:`\beta`
monomers with 141 and 146 aminoacids in human *Hgb*, respectively. Each
subunit associates to a prosthetic heme group, that consists in an
iron (Fe) ion and the heterocyclic ring of porphyrin. Although the
molecule is able of binding oxygen only in the reduced ferrous status,
human *Hgb* is commercially distributed in its nonfunctional oxidized ferric
status as *metHgb*. The atomic structure of the human *metHgb* specimen was inferred by
:cite:t:`khoshouei2017` for the first time from the
electron density volume obtained by cryo-EM and using the Volta phase
plate. The volume, at 3.2Å resolution, and its atomic interpretation
(:numref:`model_building_example`) are available in the Electron Microscopy Data Bank (*EMDB*) and Protein
Data Bank (*PDB*) with accession numbers *EMD-3488* and *PDB-5NI1*, respectively.

This tutorial will guide us in the deduction process of the human *metHgb*
atomic structure using the *Scipion* framework, the 3D map and the protein
sequences as starting input data, as well as reference atomic
structures as homologous models in the way indicated in
:cite:t:`martinez2020`.