ABSTRACT
Plant haemoglobins (Hbs), found in both symbiotic and non-symbiotic plants, are heme proteins and members of the globin superfamily. Hb genes of actinorhizal Fagales mostly belong to the non-symbiotic type of haemoglobin; however, along with the non-symbiotic Hb, Casuarina sp. posses a symbiotic one (symCgHb), which is expressed specifically in infected cells of nodules. A thorough sequence analysis of 26 plant Hb proteins, currently available in public domain, revealed a consensus motif of 29 amino acids. This motif is present in all the members of symbiotic class II Hbs including symCgHb and non-symbiotic Class II Hbs, but is totally absent in Class I symbiotic and non-symbiotic Hbs. Further, we constructed 3D structures of Hb proteins from Alnus and Casuarina through homology modelling and peeped into their structural properties. Structure-based studies revealed that the Casuarina symbiotic haemoglobin protein shows distinct stereochemical properties from that of the other Casuarina and Alnus Hb proteins. It also showed considerable structural similarities with leghemoglobin structure from yellow lupin (pdb id 1GDI). Therefore, sequence and structure analyses point to the fact that symCgHb protein shows significant resemblance to symbiotic haemoglobin found in legumes and may thus eventually play a similar role in shielding the nitrogenase from oxygen as seen in the case of leghemoglobin.
ABSTRACT
Biological nitrogen fixation is accomplished by prokaryotes through the catalytic action of complex metalloenzyme, nitrogenase. Nitrogenase is a two-protein component system comprising MoFe protein (NifD&K) and Fe protein (NifH). NifH shares structural and mechanistic similarities as well as evolutionary relationships with light-independent protochlorophyllide reductase (BchL), a photosynthesis-related metalloenzyme belonging to the same protein family. We performed a comprehensive bioinformatics analysis of the NifH/BchL family in order to elucidate the intrinsic functional diversity and the underlying evolutionary mechanism among the members. To analyse functional divergence in the NifH/ BchL family, we have conducted pair-wise estimation in altered evolutionary rates between the member proteins. We identified a number of vital amino acid sites which contribute to predicted functional diversity.We have also made use of the maximum likelihood tests for detection of positive selection at the amino acid level followed by the structure-based phylogenetic approach to draw conclusion on the ancient lineage and novel characterization of the NifH/BchL protein family. Our investigation provides ample support to the fact that NifH protein and BchL share robust structural similarities and have probably deviated from a common ancestor followed by divergence in functional properties possibly due to gene duplication.