3AQ7: Crystal Structure Of Truncated Hemoglobin From Tetrahymena Pyriformis, Y25f Mutant, Fe(Iii) Form

Truncated hemoglobins (trHbs) are distributed from bacteria to unicellular eukaryotes and have roles in oxygen transport and nitric oxide detoxification. It is known that trHbs exist in ciliates of the Tetrahymena group, but trHb structure and function remain poorly understood. To investigate trHb function with respect to stability of bound oxygen and protein structure, we measured the oxygen binding kinetics of Tetrahymena pyriformis trHb, and determined the crystal structure of the protein. The O(2) association and dissociation rate constants of T. pyriformis trHb were 5.5 muM(-1 )s(-1) and 0.18 s(-1), respectively. The autooxidation rate constant was 3.8 x 10(-3) h(-1). These values are similar to those of HbN from Mycobacterium tuberculosis. The three-dimensional structure of an Fe(II)-O(2) complex of T. pyriformis trHb was determined at 1.73-A resolution. Tyr25 (B10) and Gln46 (E7) were hydrogen-bonded to a heme-bound O(2) molecule. Tyr25 donated a hydrogen bond to the terminal oxygen atom, whereas Gln46 hydrogen-bonded to the proximal oxygen atom. Furthermore, Tyr25 was hydrogen-bonded to the Gln46 and Gln50 (E11) residues. Mutations at Tyr25, Gln46, and Gln50 increased the O(2) dissociation and autooxidation rate constants. An Fe(III)-H(2)O complex of T. pyriformis trHb was formed following reaction of the Fe(II)-O(2) complex of T. pyriformis trHb, in a crystal state, with nitric oxide. This suggests that T. pyriformis trHb functions in nitric oxide detoxification.
PDB ID: 3AQ7Download
MMDB ID: 89945
PDB Deposition Date: 2010/10/25
Updated in MMDB: 2011/05
Experimental Method:
x-ray diffraction
Resolution: 1.77  Å
Source Organism:
Similar Structures:
Biological Unit for 3AQ7: monomeric; determined by author and by software (PISA)
Molecular Components in 3AQ7
Label Count Molecule
Protein (1 molecule)
Group 1 Truncated Hemoglobin
Molecule annotation
Chemical (1 molecule)
* Click molecule labels to explore molecular sequence information.

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