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Characterization of Nonsymbiotic Tomato Hemoglobin

A. Iulia Ioanitescu ^*, Sylvia Dewilde , Laurent Kiger , Michael C. Marden , Luc Moens  and Sabine Van Doorslaer ^*

Departments of ^* Physics and Biomedical Sciences, University of Antwerp, Antwerp, Belgium; and INSERM, Hôpital de Bicètre, F94275 Le Kremlin- Bicètre, France

Correspondence: Address reprint requests to Sabine Van Doorslaer, Dept. of Physics, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium. Tel.: 0032-3-8202461; Fax: 0032-3-8202470; E-mail: sabine.vandoorslaer{at}ua.ac.be.

The nonsymbiotic tomato hemoglobin SOLly GLB1 (Solanum lycopersicon) is shown to form a homodimer of 36 kDa with a high affinity for oxygen. Furthermore, our combined ultraviolet/visible, resonance Raman, and continuous wave electron paramagnetic resonance (EPR) measurements reveal that a mixture of penta- and hexacoordination of the heme iron is found in the deoxy ferrous form, whereas the ferric form shows predominantly a bis-histidine ligation (F8His-Fe^2+/3+-E7His). This differs from the known forms of vertebrate hemoglobins and myoglobins. We have successfully applied our recently designed pulsed-EPR strategy to study the low-spin ferric form of tomato hemoglobin. These experiments reveal that, in ferric SOLly GLB1, one of the histidine planes is rotated 20°(±10°) away from a N_heme-Fe-N_heme axis. Additionally, the observed g-values indicate a quasicoplanarity of the histidine ligands. From the HYSCORE (hyperfine sublevel correlation) measurements, the hyperfine and nuclear quadrupole couplings of the heme and histidine nitrogens are identified and compared with known EPR/ENDOR data of vertebrate Hbs and cytochromes. Finally, the ligand binding kinetics, which also indicate that the ferrous tomato Hb is only partially hexacoordinated, will be discussed in relation with the heme-pocket structure. The similarities and differences with other known nonsymbiotic plant hemoglobins will be highlighted.

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