Monomeric Camelus dromedarius GSTM1 at low pH is structurally more thermostable than its native dimeric form.

Ajamaluddin Malik,Mohamed S Daoud,Farid S Ataya,Nikolaos E Labrou,Javed M Khan,Dalia Fouad,Mohamed O Abdelkader,Salman F Alamery

doi:10.1371/journal.pone.0205274

Abstract

Glutathione S‒transferases (GSTs) are multifunctional enzymes that play an important role in detoxification, cellular signalling, and the stress response. Camelus dromedarius is well-adapted to survive in extreme desert climate and it has GSTs, for which limited information is available. This study investigated the structure-function and thermodynamic properties of a mu-class camel GST (CdGSTM1) at different pH. Recombinant CdGSTM1 (25.7 kDa) was expressed in E. coli and purified to homogeneity. Dimeric CdGSTM1 dissociated into stable but inactive monomeric subunits at low pH. Conformational and thermodynamic changes during the thermal unfolding pathway of dimeric and monomeric CdGSTM1 were characterised via a thermal shift assay and dynamic multimode spectroscopy (DMS). The thermal shift assay based on intrinsic tryptophan fluorescence revealed that CdGSTM1 underwent a two-state unfolding pathway at pH 1.0–10.0. Its Tm value varied with varying pH. Another orthogonal technique based on far-UV CD also exhibited two-state unfolding in the dimeric and monomeric states. Generally, proteins tend to lose structural integrity and stability at low pH; however, monomeric CdGSTM1 at pH 2.0 was thermally more stable and unfolded with lower van't Hoff enthalpy. The present findings provide essential information regarding the structural, functional, and thermodynamic properties of CdGSTM1 at pH 1.0–10.0.

Highlights

IntroductionGlutathione S–transferases (GSTs) belong to a supergene family of multifunctional enzymes and are grouped into different species-independent gene classes [3,4,5]
The amino acid sequence of CdGSTM1 was aligned with ten homologous mammalian GSTM1 by MAFFT Multiple Sequence Alignment [40] (Fig 1A)
Multiple sequence alignment showed that CdGSTM1 has the highest sequence identity with V. pacos GSTM1 (96%)

Summary

Introduction

GSTs belong to a supergene family of multifunctional enzymes and are grouped into different species-independent gene classes [3,4,5]. GSTs are further sub-grouped into a cytosolic family and membrane-bound microsomal family [12,13,14]. Cytosolic GST subunits are approximately 22–28 kDa in size and are divided into sixteen classes based on amino acid sequence similarity, substrate specificity, inhibitor sensitivity, and immunological cross-reactivity. Eight (Alpha [α], Mu [μ], Pi [π], Sigma [σ], Theta [θ], Omega [O], Kappa [κ], and Zeta [z]) of these 16 classes are found in mammals [12, 16,17,18]

Methods

Results

Conclusion