Comparative structural modeling of a monothiol GRX from chickpea: insight in iron-sulfur cluster assembly

Glutaredoxins (GRXs) are small, ubiquitous, multifunctional, heat-stable and glutathione-dependent thiol-disulphide oxidoreductases, classified under thioredoxin-fold superfamily. In the green lineage, GRXs constitute a complex family of proteins. Based on their active site, GRXs are classified into two subfamilies: dithiol and monothiol. Monothiol GRXs contain 'CGFS' as a redox active motif and assist in maintaining redox state and iron homeostasis within the cell. Using RACE strategy, a full length cDNA of chickpea (Cicer arietinum) glutaredoxin 3 (CarGRX3) was cloned and sequenced. The cDNA contains open reading frame of 537 bp encoding 178 amino acids and exhibits features of other known 'CGFS' type GRXs. Based on the multiple sequence alignment among CarGRX3 and monothiol GRXs of other photosynthetic organisms, the characteristic motif (KGX4PXCGFSX([29/30/32])KX4WPTXPQX4GX3GGXDI) with 18 invariant residues was observed. The proposed structure of CarGRX3 was compared with structurally resolved monothiol GRXs of other organisms. The CarGRX3 and nearest Arabidopsis homolog (AtGRXcp) shares 76% sequence identity which was reflected by their 3D-structure conservation. The structure of chickpea monothiol GRX (CarGRX3) coordinates glutathione ligated [2Fe-2S] cluster in a homodimeric form, highlighting the structural basis for iron-sulfur cluster (ISC) assembly and delivery to acceptor proteins. The present study on CarGRX3 model highlighted the utility of the theoretical approaches to understand complex biological phenomena such as glutathione docking and incorporation of GSH-ligated [2Fe-2S] cluster.