Homology model of human corticosteroid binding globulin: a study of its steroid binding ability and a plausible mechanism of steroid hormone release at the site of inflammation

Abstract

Corticosteroid binding globulin (CBG) and thyroxin binding globulin (TBG) both belong to the same SERPIN superfamily of serine-proteinase inhibitors but in the course of evolution CBG has adapted to its new role as a transport agent of insoluble hormones. CBG binds corticosteroids in plasma, delivering them to sites of inflammation to modify the inflammatory response. CBG is an effective drug carrier for genetic manipulation, and hence there is immense biological interest in the location of the hormone binding site. The crystal structure of human CBG (hCBG) has not been determined, but sequence alignment with other SERPINs suggests that it conforms as a whole to the tertiary structure shared by the superfamily. Human CBG shares 52.15% and 55.50% sequence similarity with alpha1-antitrypsin and alpha1-antichymotrypsin, respectively. Multiple sequence alignment among the three sequences shows 73 conserved regions. The molecular structures of alpha1-antitrypsin and alpha1-antichymotrypsin, the archetype of the SERPIN superfamily, obtained by X-ray diffraction methods are used to develop a homology model of hCBG. Energy minimization was applied to the model to refine the structure further. The homology model of hCBG contains 371 residues (His13 to Val383 ). The secondary structure comprises 11 helices, 15 turns and 11 sheets. The putative corticosteroid binding region is found to exist in a pocket between beta-sheets S4, S10, S11 and alpha helix H10. Both cortisol and aldosterone are docked to the elongated hydrophobic ligand binding pocket with the polar residues at the two extremities. A difference accessible surface area (DASA) study revealed that cortisol binds with the native hCBG more tightly than aldosterone. Cleavage at the Val379-Met380 peptide bond causes a deformation of hCBG (also revealed through a DASA study). This deformation could probably trigger the release of the bound hormone. Figure Stereoscopic view of the ribbon diagram of hCBG complexed with cortisol. The bound cortisol is shown in space filling model in blue. Helices and sheets are shown in red and magenta respectively. Turns are shown in yellow.

Functional Implication of an Arg307Gly Substitution in Corticosteroid-Binding Globulin, a Candidate Gene for a Quantitative Trait Locus Associated With Cortisol Variability and Obesity in Pig

“…The alignment of pig and rat amino acid sequences shows that the pig Arg307 amino acid is located at two residues of the substitution Met276Ile in the rat that is implicated in reduced steroid binding affinity of the BioBreeding rat (Smith and Hammond 1991). According to the deduced tertiary structure of CBG (Dey and Roychowdhury 2003), amino acids Gln232, Asn238, Ile263, and Trp266 seem involved in ligand binding in humans and are well conserved across species. In our pig model, Thr257Met and Ile265Val are very close to these important amino acids and within the b-barrel, which has been involved in steroid binding (Dey and Roychowdhury 2003).…”

“…According to the deduced tertiary structure of CBG (Dey and Roychowdhury 2003), amino acids Gln232, Asn238, Ile263, and Trp266 seem involved in ligand binding in humans and are well conserved across species. In our pig model, Thr257Met and Ile265Val are very close to these important amino acids and within the b-barrel, which has been involved in steroid binding (Dey and Roychowdhury 2003).…”

Expression of Corticosteroid-Binding Globulin in Human Astrocytoma Cell Line

“…CBG has a single steroid-binding site with high affinity to glucocorticoids, such as cortisol in humans and corticosterone in rat (Dey and Roychowdhury 2003;Hammond et al 1991;Demey-Ponsart et al 1982), while dexamethasone does not bind to CBG (Lindenbaum and Chatterton 1981). We observed CBG in neuroendocrine hypothalamic neurons, hippocampal pyramidal cells, and cerebellar neurons (Möpert et al 2006;Sivukhina et al 2006).…”

“…Several studies suggest the existence of a membrane associated receptor for CBG (Maitra et al 1993;Dey and Roychowdhury 2003). The role of such receptors has not been completely defined, but it is possible that they aid in capturing and/or internalizing CBG.…”

Modeling of Human Corticosteroid Binding Globulin. Use of Structure–Activity Relations in Soft Steroid Binding to Refine the Structure

“…Based on a photoaffinity labeling study in which it was found that a tryptophan residue, #371, was selectively labeled by various ligands (14), the ligand binding site was proposed to involve residues near the carboxyl end of the protein. Finally, models have been proposed (15,16), in which, if there is any cysteine involved in the binding site, it would have to be Cys 228.…”