Abstract
The single-cell green alga Chlamydomonas reinhardtii harbors twelve truncated hemoglobins (Cr-TrHbs). Cr-TrHb1-1 and Cr-TrHb1-8 have been postulated to be parts of the nitrogen assimilation pathway, and of a NO-dependent signaling pathway, respectively. Here, spectroscopic and reactivity properties of Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4, all belonging to clsss 1 (previously known as group N or group I), are reported. The ferric form of Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4 displays a stable 6cLS heme-Fe atom, whereas the hexa-coordination of the ferrous derivative appears less strongly stabilized. Accordingly, kinetics of azide binding to ferric Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4 are independent of the ligand concentration. Conversely, kinetics of CO or NO2 − binding to ferrous Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4 are ligand-dependent at low CO or NO2 − concentrations, tending to level off at high ligand concentrations, suggesting the presence of a rate-limiting step. In agreement with the different heme-Fe environments, the pH-dependent kinetics for CO and NO2−binding to ferrous Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4 are characterized by different ligand-linked protonation events. This raises the question of whether the simultaneous presence in C. reinhardtii of multiple TrHb1s may be related to different regulatory roles.
Highlights
Based on phylogeny, the globin superfamily contains three lineages: (i) flavohemoglobins and single domain globins, (ii) protoglobins and globin coupled sensors, and (iii) truncatedPLOS ONE | DOI:10.1371/journal.pone.0125005 May 20, 2015Truncated Hemoglobins of Chlamydomonas reinhardtii
The best template identified by I-TASSER for Cr-TrHb1-1 and Cr-TrHb1-4 was the structure of the Chlamydomonas eugametos TrHb1 (PDB code: 1DLY) [9], while for CrTrHb1-2 the best template was the structure of the Tetrahymena piriformis TrHb1 (PDB code: 3AQ5) [21]
Analysis of the molecular models of Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4 indicate that, in spite of the fact that in the I-TASSER threading procedure different templates have been selected for Cr-TrHb1-1, Cr-TrHb1-2, and Cr-TrHb1-4, the overall fold of the core region of the three proteins is very similar, and they all belong to group I TrHb1s [22]
Summary
The globin superfamily contains three lineages: (i) flavohemoglobins and single domain globins, (ii) protoglobins and globin coupled sensors, and (iii) truncated. Except for the proximal His(F8) residue bound to the heme iron and the Phe(B9)-Tyr(B10) pair, commonly found in the distal cavity, the other residues constituting the heme surrounding are not conserved in TrHbs [3] Such a variability in the protein matrix reflects different ligand binding properties of members from different groups of TrHbs, suggesting biological functions related to O2/NO chemistry [7]. Chlamydomonas reinhardtii is a unicellular green alga which has provided a valuable reference system for the investigation of fundamental biological functions associated with both the plant and the animal lineages This is due to the ease of handling and manipulating this organism in the laboratory, and to the peculiar evolutionary history of the genus Chlamydomonas. Truncated Hemoglobins of Chlamydomonas reinhardtii catalyze the NO2− conversion to NO very efficiently, representing a viable pathway for NO generation under anaerobic conditions
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