The salt-induced dissociation and inactivation of a mammalian enolase: evidence for the formation of active monomers

Differential susceptibility of Plasmodium falciparum versus yeast and mammalian enolases to dissociation into active monomers

In the past, several unsuccessful attempts have been made to dissociate homodimeric enolases into their active monomeric forms. The main objective of these studies had been to understand whether intersubunit interactions are essential for the catalytic and structural stability of enolases. Further motivation to investigate the properties of monomeric enolase has arisen from several recent reports on the involvement of enolase in diverse nonglycolytic (moonlighting) functions, where it may occur in monomeric form. Here, we report successful dissociation of dimeric enolases from Plasmodium falciparum, yeast and rabbit muscle into active and isolatable monomers. Dimeric enolases could be dissociated into monomers by high concentrations ( approximately 250 mm) of imidazole and/or hydrogen ions. Two forms were separated using Superdex-75 gel filtration chromatography. A detailed comparison of the kinetic and structural properties of monomeric and dimeric forms of recombinant P. falciparum enolase showed differences in specific activity, salt-induced inhibition and inactivation, thermal stability, etc. Furthermore, we found that enolases from the three species differ in their dimer dissociation profiles. Specifically, on challenge with imidazole, Mg(II) protected the enolases of yeast and rabbit muscle but not of P. falciparum from dissociation. The observed differential stability of the P. falciparum enolase dimer interface with respect to mammalian enolases could be exploited to selectively dissociate the dimeric parasite enzyme into its catalytically inefficient, thermally unstable monomeric form. Thus enolase could be a novel therapeutic target for malaria.

The powerful high pressure tool for protein conformational studies

The pressure behavior of proteins may be summarized as a the pressure-induced disordering of their structures. This thermodynamic parameter has effects on proteins that are similar but not identical to those induced by temperature, the other thermodynamic parameter. Of particular importance are the intermolecular interactions that follow partial protein unfolding and that give rise to the formation of fibrils. Because some proteins do not form fibrils under pressure, these observations can be related to the shape of the stability diagram. Weak interactions which are differently affected by hydrostatic pressure or temperature play a determinant role in protein stability. Pressure acts on the 2 degrees, 3 degrees and 4 degrees structures of proteins which are maintained by electrostatic and hydrophobic interactions and by hydrogen bonds. We present some typical examples of how pressure affects the tertiary structure of proteins (the case of prion proteins), induces unfolding (ataxin), is a convenient tool to study enzyme dissociation (enolase), and provides arguments to understand the role of the partial volume of an enzyme (butyrylcholinesterase). This approach may have important implications for the understanding of the basic mechanism of protein diseases and for the development of preventive and therapeutic measures.

Use of hydrostatic pressure to produce ‘native’ monomers of yeast enolase

The effects of hydrostatic pressure on yeast enolase have been studied in the presence of 1 mm Mn(2+). When compared with apo-enolase, and Mg-enolase, the Mn-enzyme differs from the others in three ways. Exposure to hydrostatic pressure does not inactivate the enzyme. If the experiments are performed in the presence of 1 mm Mg(2+), or with apo-enzyme, the enzyme is inactivated [Kornblatt, M.J., Lange R., Balny C. (1998) Eur. J. Biochem 251, 775-780]. The UV spectra of the high pressure forms of the Mg(2+)- and apo-forms of enolase are identical and distinct from the spectrum of the form obtained in the presence of 1 mm Mn(2+); this suggests that Mn(2+) remains bound to the high pressure form of enolase. With Mn-enolase, the various spectral changes do not occur in the same pressure range, indicating that multiple processes are occurring. Pressure experiments were performed as a function of [Mn(2+)] and [protein]. One of the changes in the UV spectra shows a dependence on protein concentration, indicating that enolase is dissociating into monomers. The small changes in the UV spectrum and the retention of activity lead to a model in which enolase, in the presence of high concentrations of Mn(2+), dissociates into native monomers; upon release of pressure, the enzyme is fully active. Although further spectral changes occur at higher pressures, there is no inactivation as long as Mn(2+) remains bound. We propose that the relatively small and polar nature of the subunit interface of yeast enolase, including the presence of several salt bridges, is responsible for the ability of hydrostatic pressure to dissociate this enzyme into monomers with a native-like structure.

A new continuous spectrophotometric assay method for DOPA oxidase activity of tyrosinase

Sensitive assay methods for tyrosinase are essential not only for the understanding the process of pigment production but also for the development of effective inhibitors of tyrosinase. To develop an efficient assay method, we applied thymol blue to reaction mixtures. The enzyme kinetic study revealed that DOPA oxidase activity of tyrosinase in thymol blue-applied reaction system was more sensitively measured, even under lower enzyme units compared with the previous report with significant enhancement of Vmax while affinity change on substrate was not observed. To test whether this method could be applicable to the inhibition and the inactivation kinetic study of tyrosinase, the effect of kojic acid, a well-known tyrosinase inhibitor, and sodium chloride respectively, have been studied. Conclusively, thymol blue method can assay tyrosinase activity with sensitivity and is applicable to the inhibition and the inactivation study of tyrosinase.

The effects of sodium perchlorate on rabbit muscle enolase--Spectral characterization of the monomer

Incubation of rabbit beta beta enolase in NaClO4 (< or = O.3 M) results in a loss of enzymatic activity and striking changes in the second-derivative ultraviolet spectrum of enolase. HPLC gel filtration shows that dissociation of the dimeric enzyme is occurring. We have used molecular modelling, fluorescence and circular dichroic spectroscopy to examine the structural differences between the monomeric and dimeric forms of this protein. In the dimer, the tyrosine residues are in a non-polar environment; upon dissociation, two of them that were at the dimer interface become exposed. This results in large changes in the second-derivative spectrum. Both the tryptophan fluorescence emission spectrum and the aromatic region of the CD spectrum indicate that there are also changes in the environment of other aromatic residues. No perturbations in the peptide bond region of the CD spectrum are observed. We propose that the major structural effect of NaClO4 is to increase the flexibility of the loops connecting the helices and strands of the alpha/beta barrel of enolase. These loops, which contain about half of the aromatic residues, contain some of the residues of the active site and other residues involved in subunit contacts. Increased flexibility of the loops could disrupt both subunit interactions and the structure of the active site.