Functional Conversion of Acetyl-Coenzyme a Synthase to a Nickel Superoxide Dismutase via Rational Design of Coordination Microenvironment for the Nid-Site

The Ni_d site coordination microenvironment of a truncated acetyl-coenzyme A synthase has been designed systematically for functional conversion to a Ni-SOD-like enzyme. To this end, the first strategy is to introduce an axial histidine ligand, using mutations F598H, S594H and S594H-GP individually. The resulting three mutants obtained Ni-SOD-like activity successfully, although the catalytic activity was about 10-fold lower than in native Ni-SOD. The second strategy is to mimic the H-bond network in the second sphere coordination microenvironment of the native Ni-SOD. Two mutations based on F598H (EFG-F598H and YGP-F598H) were designed. The successful EFG-F598H exhibited ~3-fold Ni-SOD-like activity of F598H. These designed Ni-SOD-like metalloproteins were characterized by UV/Vis, EPR and Cyclic voltammetry while F598H was also characterized by X-ray protein crystallography. The pH titrations were performed to reveal the source of the two protons required for forming H₂O₂ in the SOD catalytic reaction. Based on all of the results, a proposed catalytic mechanism for the Ni-SOD-like metalloproteins is presented.

Efficient preparation and metal specificity of the regulatory protein TroR from the human pathogen Treponema pallidum

TroR is a putative metal-dependent regulatory protein that has been linked to the virulence of the human pathogen Treponema pallidum. It shares high homology with the well-known iron-dependent regulatory protein DtxR from Corynebacterium diphtheriae, as well as the manganese-dependent MntR from Bacillus subtilis. However, it has been uncertain whether manganese or zinc is the natural cofactor of TroR to date. Herein, we established an efficient method named "double-fusion tagging" to obtain soluble TroR for the first time. A series of studies, including ICP, CD, fluorescence, ITC, and electrophoresis mobility shift assay (EMSA), were performed to resolve the discrepancies in its metal-binding specificity. In addition, bioinformatic analysis as well as mutation studies were carried out to find the genetic relationships of TroR with its homology proteins. In conclusion, our findings indicate that TroR is a manganese-dependent rather than a zinc-dependent regulatory protein.

Structural and functional investigation into acetyl-coenzyme A synthase and methyltransferase from human pathogen Clostridium difficile

Methyltransferase (MeTrCd) and acetyl-coenzyme A synthase (ACSCd) are two key enzymes in the acetyl-coenzyme A synthesis pathway of the human pathogen Clostridium difficile. The pathway is absent in humans and is essential for the survival of the pathogen. MeTrCd and ACSCd were cloned, expressed in E. coli, and characterized for the first time. Structural and functional investigations of the two enzymes were performed using homology structure modeling, fluorescence spectroscopy, and steady state/pre-steady state kinetics. The conformational change and methyl transfer activity of MeTrCd were shown to be pH dependent. The kinetic studies of MeTrCd at the optimal pH 5.1 yield the parameters kcat (2.63 s(-1)), Km (17.8 μM) and kcat/Km (0.15 μM(-1) s(-1)). The active site metal cluster (A-cluster) of ACSCd, [Fe4S4][NipNid], was characterized using metal analysis, structural modeling, and UV/Vis spectra of the characteristic features of [Fe4S4] cubane. Nip, as a labile metal, can be removed by treatment with chelators, resulting in the loss of ACS activity. Three bidentate chelators (1,10-phenanthroline, 8-hydroxyquinoline, and 2,2-dipyridyl) exhibited excellent inhibition effects on ACSCd methyl group transfer and acetyl-coenzyme A synthesis activity. These inhibitory effects were further examined using antibacterial activity assays against Clostridium difficile. These results provide a new strategy to find new potential antibiotics for the treatment of CDI.