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

The Energetics of Streptococcal Enolase Octamer Formation: The Quantitative Contributions of the Last Eight Amino Acids at the Carboxy-Terminus

The enolase produced by Streptococcus pyogenes is a homo-octamer whose overall shape resembles that of a donut. The octamer is best described as a tetramer of dimers. As such, it contains two types of interfaces. The first is common to almost all enolases as most enolases that have been studied are dimers. The second is unique to the octamers and includes residues near the carboxy-terminus. The primary sequence of the enolase contains 435 residues with an added 19 as an N-terminal hexahistine tag. We have systematically truncated the carboxy-terminus, individually removing the first 8 residues. This gave rise to a series of eight structures containing respectively, 435, 434, 433, 432, 431, 430, 429 and 427 residues. The truncations cause the protein to gradually dissociate from octamers to enzymatically inactive monomers with very small amounts of intermediate tetramers and dimers. We have evaluated the contributions of the missing residues to the monomer/octamer equilibrium using a combination of analytical ultracentrifugation and activity assays. For the dissociation reaction, octamer ⇐⇒ 8 monomer truncation of all eight C-terminal residues resulted in a diminution in the standard Gibbs energy of dissociation of about 59 kJ/mole of octamer relative to the full length protein. Considering that this change is spread over eight subunits, this translates to a change in standard Gibbs interaction energy of less than 8 kJ/mole of monomer distributed over the eight monomers. The resulting proteins, containing 434, 433, 432, 431, 430, 429 and 427 residues per monomer, showed intermediate free energies of dissociation. Finally, three other mutations were introduced into our reference protein to establish how they influenced the equilibrium. The main importance of this work is it shows that for homo-multimeric proteins a small change in the standard Gibbs interaction energy between subunits can have major physiological effects.

Effects of the G376E and G157D mutations on the stability of yeast enolase--a model for human muscle enolase deficiency

The first known human enolase deficiency was reported in 2001 [Comi GP, Fortunato F, Lucchiari S, Bordoni A, Prelle A, Jann S, Keller A, Ciscato P, Galbiati S, Chiveri L et al. (2001) Ann Neurol50, 202-207]. The subject had inherited two mutated genes for beta-enolase. These mutations changed glycine 156 to aspartate and glycine 374 to glutamate. In order to study the effects of these changes on the structure and stability of enolase, we have introduced the corresponding changes (G157D and G376E) into yeast enolase. The two variants are correctly folded. They are less stable than wild-type enolase with respect to thermal denaturation, and both have increased Kd values for subunit dissociation. At 37 degrees C, in the presence of salt, both are partially dissociated and are extensively cleaved by trypsin. Under the same conditions, wild-type enolase is fully dimeric and is only slightly cleaved by trypsin. However, wild-type enolase is also extensively cleaved if it is partially dissociated. The identification of the cleavage sites and spectral studies of enolase have revealed some of the structural differences between the dimeric and monomeric forms of this enzyme.

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.

Role of Serine 286 in cosubstrate binding and catalysis of a flavonol O-methyltransferase

O-Methyltransferases catalyze the transfer of the methyl groups of S-adenosyl-L-methionine to specific hydroxyl groups of several classes of flavonoid compounds. Of the several cDNA clones isolated from a Chrysosplenium americanum library, FOMT3' encodes the 3'/5'-O-methylation of partially methylated flavonols. The recombinant protein of another clone, FOMTx which differs from FOMT3' by a single amino acid residue (Ser286Arg) exhibits no enzymatic activity towards any of the flavonoid substrates tested. Replacement of Ser 286 in FOMT3' with either Ala, Leu, Lys or Thr, almost abolished O-methyltransferase activity. In contrast with FOMT3', no photoaffinity labeling could be achieved using [(14)CH(3)]AdoMet with the mutant recombinant proteins indicating that Ser 286 is also required for cosubstrate binding. These results are corroborated by isothermal titration microcalorimetry measurements. Circular dichroism spectra ruled out any significant conformational differences in the secondary structures of both FOMT3' and Ser286Arg. Modeling FOMT3' on the structure of chalcone methyltransferase indicates that serine 286 is greater than 10 A from any of the residues of the active site or the AdoMet binding site of FOMT3'. At the same time, residues 282 to 290 are conserved in most of the Chrysosplenium americanum OMTs. These residues form a large part of the subunit interface, and at least five of these residues are within 4 A of the opposing subunit. It would appear, therefore, that mutations in Ser286 exert their influence by altering the contacts between the subunits and that these contacts are necessary for maintaining the integrety of the AdoMet binding site and active site of this group of enzymes.

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.

Cloning, expression and mutagenesis of a subunit contact of rabbit muscle-specific (betabeta) enolase

The cDNA for rabbit muscle-specific (betabeta) enolase was cloned, sequenced and expressed in Escherichia coli. This betabeta-enolase differs at eight positions from that sequenced by Chin (17). Site-directed mutagenesis was used to change residue 414 from glutamate to leucine, thereby abolishing a salt bridge involved in subunit contacts. Recombinant wild-type and mutant enolase were purified from E. coli and compared to enolase isolated from rabbit muscle. Molecular weights were determined by mass spectrometry. All three betabeta-enolases had similar kinetics, and UV and circular dichroism (CD) spectra. The mutant enolase was far more sensitive to inactivation by pressure, by KCl or EDTA, and by sodium perchlorate. We confirmed, by analytical ultracentrifugation, that the sodium perchlorate inactivation was due to dissociation. DeltaG(o) for dissociation of enolase was decreased from 49.7 kJ/mol for the wild-type enzyme to 42.3 kJ/mol for the mutant. In contrast to the wild-type enzyme, perchlorate inactivation of E414L was accompanied by a small loss of secondary structure.

The binding of Na(+) to apo-enolase permits the binding of substrate

Enolase from rabbit muscle (betabeta-enolase) is inactivated by NaClO(4). Enolase free of divalent cations is more susceptible to inactivation by NaClO(4) than is enolase in the presence of Mg(2+). We find that substrate protects apo-enolase against inactivation, indicating that substrate can bind to enolase in the absence of a divalent cation. This binding is not due to contamination by trace levels of divalent cations since (1) it occurs even in the presence of EDTA or EGTA and (2) metal analysis by ICP (inductively coupled plasma) mass spectrometry did not reveal sufficient contamination to account for the protection. The binding of PGA to apo-enolase did require Na(+). When TMAClO(4) was used instead of NaClO(4), there was no protection by PGA. Protection was restored when TMAClO(4) plus NaCl were used. The inactivation of apo-enolase by NaClO(4) is due to dissociation into inactive monomers. We conclude that Na(+) binds to apo-enolase, permitting substrate to then bind. Of the three known Me(2+) binding sites on enolase, we believe the most likely binding site for Na(+) is the carboxylate cluster of site 1, the highest affinity site of enolase.

High salt concentrations induce dissociation of dimeric rabbit muscle creatine kinase. Physico-chemical characterization of the monomeric species

Incubation of dimeric MM-creatine kinase (MM-CK) with high NaCl or LiCl concentrations results in dissociation of the subunits and complete enzyme inactivation. In NaCl, this process, which depends on protein concentration, may be described according to a two-state model where the dimer can be reversibly converted into compact folded monomers (D <--> 2M). At LiCl concentrations higher than 2-2.5 M, MM-CK is recovered in two monomeric states: an inactive compact species (M) and a more expanded form (EF), which represents 15-20% of the population. Thus, in LiCl, a three-state model (D <--> 2M --> 2EF) more adequately accounts for our experimental results. The monomeric species (M) obtained in NaCl and LiCl exhibits some properties of the molten globule state described in guanidine hydrochloride. Indeed, this form is compact and devoid of any enzymatic activity; it maintains a high degree of secondary structure and binds 8-anilino-1-naphthalenesulfonate. The formation of this intermediate induces the exposure of a second tryptophan (among the four present) which is located at the monomer-monomer interface in the native structure. In LiCl, the monomeric species (M) is irreversibly converted into a less compact form (EF) which seems to have lost a large part of its secondary structure.