Effects of zinc on creatine kinase: activity changes, conformational changes, and aggregation

Kinetics for Zinc Ion Induced Sepia Pharaonis Arginine Kinase Inactivation and Aggregation

Arginine kinase is an essential enzyme which is closely related to energy metabolism in marine invertebrates. Arginine kinase provides a significant role in quick response to environmental change and stress. In this study, we simulated a tertiary structure of Sepia pharaonis arginine kinase (SPAK) based on the gene sequence and conducted the molecular dynamics simulations between SPAK and Zn²⁺. Using these results, the Zn²⁺ binding sites were predicted and the initial effect of Zn²⁺ on the SPAK structure was elucidated. Subsequently, the experimental kinetic results were compared with the simulation results. Zn²⁺ markedly inhibited the activity of SPAK in a manner of non-competitive inhibitions for both arginine and ATP. We also found that Zn²⁺ binding to SPAK resulted in tertiary conformational change accompanying with the hydrophobic residues exposure. These changes caused SPAK aggregation directly. We screened two protectants, glycine and proline, which effectively prevented SPAK aggregation and recovered the structure and activity. Overall, our study suggested the inhibitory effect of Zn²⁺ on SPAK and Zn²⁺ can trigger SPAK aggregation after exposing large extent of hydrophobic surface. The protective effects of glycine and proline against Zn²⁺ on SPAK folding were also demonstrated.

Zinc induces unfolding and aggregation of dimeric arginine kinase by trapping reversible unfolding intermediate

Arginine kinase plays an important role in the cellular energy metabolism of invertebrates. Dimeric arginine kinase (dAK) is unique in some marine invertebrates. The effects of Zn²(+) on the unfolding and aggregation of dAK from the sea cucumber Stichopus japonicus were investigated. Our results indicated that Zn²(+) caused dAK inactivation accompanied by conformational unfolding, the exposure of hydrophobic surface, and aggregation. Kinetic studies showed the inactivation and unfolding of dAK followed biphasic kinetic courses. Zn²(+) can affect unfolding and refolding of dAK by trapping the reversible intermediate. Our study provides important information regarding the effect of Zn²(+) on metabolic enzymes in marine invertebrates.

Towards creatine kinase aggregation due to the cysteine modification at the flexible active site and refolding pathway

The dimeric native state of creatine kinase (CK) was aggregated at conspicuous levels during cysteine modification at the active site with using 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) under a high enzyme concentration. Measuring the ANS-binding fluorescence revealed that the hydrophobic surface of CK was increased by cysteine modification due to the flexible active site, and this resulted in insoluble aggregation, probably via non-specific hydrophobic interactions. To determine whether the aggregates can be refolded, 3M guanidine hydrochloride (GdnHCl) was used to dissolve the aggregates into the denatured form. Refolding of the solubilized enzyme sample was then conducted, accompanied by deprivation of DTNB from the CK in the presence of DTT. As a result, CK was reactivated by up to 40% with partial recovery of the tertiary (78%) and secondary structures (77%). To further elucidate its kinetic refolding pathway, both time interval measurements and a continuous substrate reaction were performed. The results showed that the refolding behavior was similar to the manner of normal CK folding with respect to the following two-phase kinetic courses. Additionally, the rate constants for the dimerization of the unfolded CK were dependent on the enzyme concentration and this was irrespective to the DTT concentrations, suggesting the rate-limiting steps of CK reassociation. The present study will expand our insight into the flexibility of the enzyme active site, which might act as a risk factor for inducing the unfavorable aggregation and partial refolding pathway of CK, as well as inducing an intermediate-like state recovery from aggregation.

Monomeric Creatine Kinase Aggregation and Sodium Dodecyl Sulfate-cyclodextrin Assisted Refolding

The monomeric state of creatine kinase (CK) was stably captured at the equilibrium state by employing cysteine residue modifications in the presence of a denaturant, and at a partially folded state. The partially folded monomeric CK (PF-CK) was aggregated with kinetic order, which was mainly caused by the hydrophobic surface interactions between the CK subunits. The artificial chaperone, described as a SDS-cyclodextrin, was applied to prevent aggregation as well as to refold the PF-CK: SDS treatment onto the monomeric CK can significantly block aggregation and can be successfully refolded in the solutions containing cyclodextrins and DTT. Three types of cyclodextrins such as alpha-, beta-, and gamma-cyclodextrins were applied to strip SDS from the enzyme molecule, and each kinetic course was measured. The intrinsic fluorescence changes showed that reactivation occurred and this accompanied the conformational changes. The size exclusion chromatography detected the variously trapped monomeric CKs such as the thiol residue modified PF-CK, the SDS-binding PF-CK, the cyclodextrin treated PF-CK, and the DTT treated SDS-binding PF-CK. Our study demonstrated monomer CK aggregation for the first time; we also demonstrated the complex reassociation of CK during refolding with the aid of the SDS-cyclodextrin, and these pathways followed first-order kinetics.

Expression of microsomal epoxide hydrolase is elevated in Alzheimer's hippocampus and induced by exogenous β-amyloid and trimethyl-tin

The brain is a potential target for drugs and environmental toxins. Microsomal epoxide hydrolase (mEH) is one of several critical biotransformation enzymes in xenobiotic metabolism and detoxification. In the present study, we report that the expression of mEH is significantly elevated in the hippocampus and associated cortex, but not in the cerebellum, in Alzheimer's disease (AD) patients. A large proportion of the mEH-positive cells are located around beta-amyloid plaques. The mEH-positive-staining cells are astrocytes and pyramidal neurons. Western blotting analysis confirmed increased expression of mEH in AD hippocampal tissues. In primary hippocampal glial culture, beta-amyloid aggregation stimulated mEH expression in the astrocytes, which displayed a patchy distribution. An environmental neurotoxic agent, trimethyl-tin, also activated mEH expression in rat hippocampus and entorhinal cortex. The present study demonstrates a significant increase in mEH expression in the AD hippocampus, a region showing abundant neuropathology in AD. The expression of mEH could also be elevated by exposure to exogenous beta-amyloid in vitro and environmental toxins in vivo. Our studies suggest that mEH may play a role in pathogenesis of neurodegeneration in response to environmental stress.

Inhibition of the Escherichia coli RecA protein: zinc(II), copper(II) and mercury(II) trap RecA as inactive aggregates

In bacteria, the RecA protein plays important roles in a number of DNA recombination and repair processes, including homologous recombination, SOS induction and recombinational DNA repair. We have explored the idea that the Escherichia coli RecA protein's functions could be controlled by small molecules. We investigated the 2:1 complex of zinc(II) with 1,4-dithio-l-threitol (l-DTT) that inhibits the E. coli rho transcription terminator, which is a hexameric ATP motor protein and is structurally homologous to RecA. We found that both the complex and ZnCl(2) inhibit the single-stranded DNA-dependent ATPase activity of RecA at sub-millimolar concentrations. Investigation of a variety of metal dications (0.4 mM final concentration) determined that zinc(II), copper(II) and mercury(II) all induce the precipitation of RecA, while the dichloride salts of calcium, manganese, barium, cobalt, and nickel do not. The inhibition of RecA activity by Zn(II), Cu(II) and Hg(II) results from the metal-dependent initiation of RecA aggregation. These observations may have implications for the design of biophysical experiments requiring solid-phase RecA protein, for a more complete understanding of metal toxicities, and for the design of metal-chelate inhibitors of prokaryotic DNA repair.

Aggregation and folding of recombinant human creatine kinase

The processes of aggregation and refolding of recombinant human creatine kinase (rHCK) were studied. Most of the rHCK expressed in E. coli was present in the insoluble traction and it could be solubilized in 6 M urea solution. Unfolding of rHCK in 6 M urea showed biphasic kinetic courses (kappa1 = 6.5 x 10(-3) s(-1); kappa2 = 0.54 x 10(-3) s(-1)) as observed by maximum fluorescence wavelength change. During refolding of the rHCK dissolved in urea, significant aggregation was noticed following first-order kinetics. Aggregation rate constants were influenced by the concentration of NaCl, which increased the difference in transition-free energy (deltadeltaG), showing that stabilization of folding intermediates by NaCl could efficiently reduce the formation of insoluble aggregates. Formations of aggregate were also reduced by adjusting temperature, pH, and concentration of rHCK. Refolding of rHCK under the optimized condition which prevented the aggregation also showed multi-kinetic phases (kappa1 = 3.0 x 10(-3) s(-1); kappa2 = 0.64 x 10(-3) s(-1)). Under optimized conditions applied in this study, rHCK could correctly refold retrieving the high specific enzymatic activity.

Inhibitive effect of zinc ion on fatty acid synthase from chicken liver

Fatty acid synthase (FAS; acyl-CoA:malonyl-CoA C-acyltransferase [decarboxylating, oxoacyl- and enoyl-reducing and thioester-hydrolyzing], EC 2.3.1.85) is an important enzyme participating in energy metabolism in vivo which is related to adiposis and cancer [Cancer Lett. 167 (1) (2001) 99; Nat. Med. 8 (4) (2002) 335]. Tests of fast- and slow-binding inhibitions showed that fatty acid synthase of chicken liver is rapidly and irreversibly inactivated by low Zn(2+) concentrations. Electrophoresis and FPLC results showed that FAS cross-links occurred in the presence of high Zn(2+) concentrations (>4 microM) which may be another reason that FAS lost its activity. The modification velocity of FAS by DTNB decreased with increasing Zn(2+) concentration, which confirmed that Zn(2+) interacted with SH groups. Substrate protective experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that all three substrates tested had some protective effects on FAS in the presence of Zn(2+), and malonyl-CoA was the most effective of the three substrates. In the presence of malonyl-CoA, the activity loss of FAS decreased sharply and almost no cross-link was observed in SDS-PAGE. This suggests that the phosphopantetheine SH group is the critical group in the cross-link and inhibition of FAS in the presence of Zn(2+).