RESUMEN
Nonsymbiotic hemoglobins (nsHbs) and leghemoglobins (Lbs) are plant proteins that can reversibly bind O(2) and other ligands. The nsHbs are hexacoordinate and appear to modulate cellular concentrations of NO and maintain energy levels under hypoxic conditions. The Lbs are pentacoordinate and facilitate the diffusion of O(2) to symbiotic bacteroids within legume root nodules. Multiple lines of evidence suggest that all plant Hbs evolved from a common ancestor and that Lbs originated from nsHbs. However, little is known about the structural intermediates that occurred during the evolution of pentacoordinate Lbs from hexacoordinate nsHbs. We have cloned and characterized a Hb (ppHb) from the root nodules of the ancient caesalpinoid legume Chamaecrista fasciculata. Protein sequence, modeling data, and spectral analysis indicated that the properties of ppHb are intermediate between that of nsHb and Lb, suggesting that ppHb resembles a putative ancestral Lb. Predicted structural changes that appear to have occurred during the nsHb to Lb transition were a compaction of the CD-loop and decreased mobility of the distal His inhibiting its ability to coordinate directly with the heme-Fe, leading to a pentacoordinate protein. Other predicted changes include shortening of the N- and C-termini, compaction of the protein into a globular structure, disappearance of positive charges outside the heme pocket and appearance of negative charges in an area located between the N- and C-termini. A major consequence for some of these changes appears to be the decrease in O(2)-affinity of ancestral nsHb, which resulted in the origin of the symbiotic function of Lbs.
Asunto(s)
Chamaecrista/genética , Hemoglobinas/química , Hemoglobinas/genética , Leghemoglobina/química , Simbiosis , Secuencia de Aminoácidos , Secuencia de Bases , Clonación Molecular , ADN Complementario , Hemoglobinas/metabolismo , Modelos Moleculares , Datos de Secuencia Molecular , Oryza/química , Filogenia , Estructura Secundaria de Proteína , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Alineación de Secuencia , Glycine max/química , Análisis EspectralRESUMEN
Ferrous oxygenated hemoglobins (Hb2+O2) autoxidize to ferric Hb3+, but Hb3+ is reduced to Hb2+ by enzymatic and non-enzymatic mechanisms. We characterized the interaction between the soybean ferric leghemoglobin reductase 2 (FLbR2) and ferric rice non-symbiotic Hb1 (Hb13+). Spectroscopic analysis showed that FLbR2 reduces Hb13+. Analysis by tryptophan fluorescence quenching showed that FLbR2 interacts with Hb13+, however the use of ITC and IEF techniques revealed that this interaction is weak. In silico modeling showed that predicted FLbR2 and native Hb13+ interact at the FAD-binding domain of FLbR2 and the CD-loop and helix F of Hb13+.