Parakeet Hemoglobin – Its Crystal Structure and Oxygen Affinity in Relation to Some Avian Hemoglobins
Authors: Jaimohan S.M. , Naresh M.D. , Mandal A.B. 
Published: March 2020

Background:
“Avians” often show efficient oxygen management to meet the demands
of their metabolism. Hemoglobin, a transporter protein consists of four non-covalently linked subunits
contain haem binding hydrophobic pocket serves as a site of allosteric cooperativity. The physiology
and anatomy of both mammals and avian are functionally different, in birds, the respiratory
system formed by small air sacs that serve as tidal ventilation for the lungs and have no significant
exchange across their cells. Parakeet (Psittacula krameri) a tropical and non-migrating species and
it is easily adapted to living in disturbed habitat. The sequence analysis reveals that α and β chain
of parakeet hemoglobin highly similar grey lag goose and bar headed goose hemoglobin respectively.
Thus it has been tempted us to study in to analyzing the sequence and structural comparison of
this hemoglobin to find out the physiological capabilities of parakeet hemoglobin.

Objective:
The structure determination studies of parakeet hemoglobin by X-ray diffraction. The sequence
and structure are compared with goose, chicken and human Hb, emphasizing the role of
amino acids in the subunit contacts that facilitate survival by low oxygen demand.

Methods:
The Hb was purified and crystallized by hanging drop vapor diffusion method using poly
ethylene glycol (PEG) 3350 and sodium phosphate buffer. X-ray diffracted data set was collected
at 3Å resolution, the data was processed in Automar and molecular replacement, refinements,
model building was carried out in CCP4i program package. The final refined model was deposited
in protein data bank with accession id 2zfb.

Results:
The tertiary structure of Parakeet Hb is compared with the met form of BHG Hb (1c40)
and oxy form of GLG (1faw) and oxy form of human Hbs (1hho). Superimposing parakeet Hb α1β1
subunit with ‘R’ state human Hb shows an r.m.s.d of 0.98 Å and for BHG and GLG Hb, the r.m.s.d
shows 0.72 and 0.61 Å. The replacement of α115Asp in parakeet Hb as against the α115Glu in human
Hb results in the movement of GH corners. The amino acid proline at α50 present only in Parakeet
Hb and Chicken HbD and not present in any other avian family which includes human Hb.
The residue α78Thr located in EF corner loop region, which slightly diverge when superimposing
with human and BHG Hb and also replacement of α113Asn present only in Parakeet Hb placed
near the FG helix corner.

Conclusion:
The present study describes the structure determination of parakeet hemoglobin and
its structural features to understand its oxygen affinity characteristics. The crystals were obtained
by buffered low-salt conditions, like those of chicken HbD, carbonmonoxy and cyanomet human
Hb. The present study reveals several interesting and unique modifications in the finer aspects of
the quaternary structure of parakeet Hb, which are involved in oxygen affinity characteristics and
the α1β1 subunit contacts. Crystallization of parakeet Hb with allosteric effectors like Inositol pentaphosphate
may bring further understanding of the influence of physiological and environmental
factors on the quaternary structure.