A new folding intermediate of apomyoglobin from Aplysia limacina: stepwise formation of a molten globule

Oxidative Implications of Substituting a Conserved Cysteine Residue in Sugar Beet Phytoglobin BvPgb 1.2

Phytoglobins (Pgbs) are plant-originating heme proteins of the globin superfamily with varying degrees of hexacoordination. Pgbs have a conserved cysteine residue, the role of which is poorly understood. In this paper, we investigated the functional and structural role of cysteine in BvPgb1.2, a Class 1 Pgb from sugar beet (Beta vulgaris), by constructing an alanine-substituted mutant (Cys86Ala). The substitution had little impact on structure, dimerization, and heme loss as determined by X-ray crystallography, size-exclusion chromatography, and an apomyoglobin-based heme-loss assay, respectively. The substitution significantly affected other important biochemical properties. The autoxidation rate increased 16.7- and 14.4-fold for the mutant versus the native protein at 25 °C and 37 °C, respectively. Thermal stability similarly increased for the mutant by ~2.5 °C as measured by nano-differential scanning fluorimetry. Monitoring peroxidase activity over 7 days showed a 60% activity decrease in the native protein, from 33.7 to 20.2 U/mg protein. When comparing the two proteins, the mutant displayed a remarkable enzymatic stability as activity remained relatively constant throughout, albeit at a lower level, ~12 U/mg protein. This suggests that cysteine plays an important role in BvPgb1.2 function and stability, despite having seemingly little effect on its tertiary and quaternary structure.

[Prediction of protein thermostability from their primary structure: the current state and development factors]

The construction of proteins and peptides with desired properties, including resistance to high temperatures, as well as optimization of their amino acid composition, is an important and complex task, which attracts much attention in various branches of the basic sciences, and also in biomedicine and biotechnology. This raises the question: what method is more relevant for the at the pilot stage of research in order to estimate the influence of the planned amino acid substitutions on the thermostability of the resultant protein construct? In this brief review we have classified existing basic practical and theoretical approaches used in studies and predicting the thermal stability of native and recombinant polypeptides. Particular attention has been paid to the predictive potential of statistical methods for studying the thermodynamic parameters of the primary protein structure and prospects of their use.

Mapping local structural perturbations in the native state of stefin B (cystatin B) under amyloid forming conditions

Unlike a number of amyloid-forming proteins, stefins, and in particular stefin B (cystatin B) form amyloids under conditions where the native state predominates. In order to trigger oligomerization processes, the stability of the protein needs to be compromised, favoring structural re-arrangement however, accelerating fibril formation is not a simple function of protein stability. We report here on how optimal conditions for amyloid formation lead to the destabilization of dimeric and tetrameric states of the protein in favor of the monomer. Small, highly localized structural changes can be mapped out that allow us to visualize directly areas of the protein which eventually become responsible for triggering amyloid formation. These regions of the protein overlap with the Cu (II)-binding sites which we identify here for the first time. We hypothesize that in vivo modulators of amyloid formation may act similarly to painstakingly optimized solvent conditions developed in vitro. We discuss these data in the light of current structural models of stefin B amyloid fibrils based on H-exchange data, where the detachment of the helical part and the extension of loops were observed.

Predicting melting temperature directly from protein sequences

Proteins of both hyperthermophilic and mesophilic microorganisms generally constitute from the same 20 amino acids; however, the extent of thermal tolerance of any given protein is an inherent property of its amino acid sequence. The present study is the first to report a rapid method for predicting Tm (melting temperature), the temperature at which 50% of the protein is unfolded, directly from protein sequences (the Tm Index program is available at http://tm.life.nthu.edu.tw/). We examined 75 complete microbial genomes using the Tm Index, and the analysis clearly differentiated hyperthermophilic from mesophilic microorganisms on this global genomic basis. These results are consistent with the previous hypothesis that hyperthermophiles express a greater number of high Tm proteins compared with mesophiles. The Tm Index will be valuable for modifying existing proteins (enzymes, protein drugs and vaccines) or designing novel proteins having a desired melting temperature.

Identification and characterization of protein folding intermediates

In order to understand the mechanism by which a polypeptide chain folds into its functionally active native state it is necessary to characterize in detail all the species accumulated along the pathway. The elusive nature of protein folding intermediates poses their identification and characterization as an extremely difficult task in the protein folding field. In the case of small single domain proteins, the direct measurement of the thermodynamics and structural parameters of protein folding intermediates has provided new insights on the nature of the forces involved in the stabilization of nascent protein structures. Here we summarize some of the experimental approaches aimed at the detection and characterization of folding intermediates along with a discussion of some general structural features emerging from these studies.

1H-NMR study of the effect of temperature through reversible unfolding on the heme pocket molecular structure and magnetic properties of aplysia limacina cyano-metmyoglobin

Two-dimensional 1H NMR spectroscopy over a range of temperature through thermal unfolding has been applied to the low-spin, ferric cyanide complex of myoglobin from Aplysia limacina to search for intermediates in the unfolding and to characterize the effect of temperature on the magnetic properties and electronic structure of the heme iron. The observation of strictly linear behavior from 5 to 80 C degrees through the unfolding transition for all hyperfine-shifted resonances indicates the absence of significant populations of intermediate states to the cooperative unfolding with Tm approximately 80 degrees C. The magnetic anisotropies and orientation of the magnetic axes for the complete range of temperatures were also determined for the complex. The anisotropies have very similar magnitudes, and exhibit the expected characteristic temperature dependence, previously observed in the isoelectronic sperm whale myoglobin complex. In contrast to sperm whale Mb, where the orientation of the magnetic axis was completely temperature-independent, the tilt of the major magnetic axis, which correlates with the Fe-CN tilt, decreases at high temperature in Aplysia limacina Mb, indicating a molecular structure that is conserved with temperature, although more plastic than that of sperm whale Mb. The pattern of contact shifts reflects a conserved Fe-His(F8) bond and pi-spin delocalization into the heme, as expected for the orientation of the axial His imidazole.

Molecular mechanism underlying the thermal stability and pH-induced unfolding of CHABII

The 37-residue alpha/beta protein CHABII was previously demonstrated to undergo a gradual pH-induced unfolding. It has been shown that even at pH 4.0 CHABII still retained a highly native-like secondary structure and tertiary topology although its tight side-chain packing was severely disrupted, typical of the molten globule state. Here, we have expressed and refolded the recombinant proteins of CHABII and its mutant [Phe21]-CHABII, and subsequently conducted extensive CD and NMR characterizations. The results indicated: (1) replacement of His21 by Phe in [Phe21]-CHABII eliminated the pH-induced unfolding from pH 6.5 to 4.0, indicating that His21 was responsible for the observed pH-induced unfolding of CHABII. Further examinations revealed that although the pH-induced unfolding of CHABII was also triggered by the protonation of the His residue as previously uncovered for apomyoglobin, their molecular mechanisms are different. (2) Monitoring the pH-induced unfolding by 1H-15N HSQC spectroscopy allowed us to visualize the gradual development of the CHABII molten globule. At pH 4.0, the HSQC spectrum of CHABII was poorly dispersed with dispersions of approximately 1 ppm over proton dimension and 10 ppm over 15N dimension, characteristic of severely or even "completely unfolded" proteins. One the other hand, unambiguous assignments of the NOESY spectra of CHABII led to the identification of the persistent medium and long-range NOEs at pH 4.0, which define a highly native-like secondary structure and tertiary packing. This implies that the degree of the native-like topology might be underestimated in the previous characterization of partially folded and even completely unfolded proteins. (3) Replacement of His21 by Phe with higher side-chain hydrophobicity only caused a minor structural rearrangement but considerably enhanced the packing interaction of the hydrophobic core, as evident from a dramatic increase in NOE contacts in [Phe21]-CHABII. The enhancement led to an increase of the thermal stability of [Phe21]-CHABII by approximately 17 deg. C.

Folding of Aplysia limacina apomyoglobin involves an intermediate in common with other evolutionarily distant globins

In the globin family, similarities in the folding mechanism have been found among different mammalian apomyoglobins (apoMb). The best-characterized intermediate of sperm whale apoMb, called I(AGH), is mainly stabilized by nativelike contacts among the A, G, and H helices involving a cluster of hydrophobic residues that includes two conserved tryptophans. To verify the hypothesis of a common intermediate in the folding of all members of the globin family, we have extensively studied a site-directed mutant of the myoglobin from Aplysia limacina, distantly related to the mammalian counterpart, in which one of the two tryptophans in the A-G-H cluster [i.e., Trp(H8)130] has been mutated to tyrosine. The results presented here show that this mutation destabilizes both the native state and the acid intermediate I(A) but exerts little or no effect on the thermally stable core of an intermediate species (called I(T)) peculiar to Aplysia apomyoglobin. Dynamic quenching of Trp emission by acrylamide provides information on the accessibility of the chromophores at the native and the intermediate states of wild-type and mutant Aplysia apomyoglobin, consistent with the thermodynamics. Our results agree well with those obtained for the corresponding topological position of apomyoglobin from sperm whale and clearly show that the H8 position is involved in the stabilization of the main intermediate in both apoproteins. This residue thus plays a role which is evolutionarily conserved in the globin family from invertebrates to mammals; our results support the contention that the A-G-H cluster is important in the folding pathway of different globins.

Unifying features in protein-folding mechanisms

We compare the folding of representative members of a protein superfamily by experiment and simulation to investigate common features in folding mechanisms. The homeodomain superfamily of three-helical, single-domain proteins exhibits a spectrum of folding processes that spans the complete transition from concurrent secondary and tertiary structure formation (nucleation-condensation mechanism) to sequential secondary and tertiary formation (framework mechanism). The unifying factor in their mechanisms is that the transition state for (un)folding is expanded and very native-like, with the proportion and degree of formation of secondary and tertiary interactions varying. There is a transition, or slide, from the framework to nucleation-condensation mechanism with decreasing stability of the secondary structure. Thus, framework and nucleation-condensation are different manifestations of an underlying common mechanism.

Construction and characterization of a chimeric myoglobin

In order to investigate the functional and structural role of modular structure in globins, we have engineered a chimeric myoglobin (ChimMb) in which the first and third exon come from the gene coding for the sperm whale Mb and the second exon from the gene coding for Aplysia limacina Mb. This ChimMb, fused to the Maltose Binding Protein (MBP) and expressed in Escherichia coli as an apoprotein, binds protoheme in a 1:1 stoichiometric ratio. Based on some functional and spectroscopic properties, we conclude that the central core of the ChimMb (which derives from A. limacina) is native-like. On the other hand, the ChimMb deprived (by proteolytic digestion) of the fused MBP displays a considerably reduced stability. These results suggest that the sperm whale A-G-H nucleus does not contribute significantly to the overall stability of the ChimMb.

Unfolding of the loggerhead sea turtle (Caretta caretta) myoglobin: A (1)H-NMR and electronic absorbance study

The effect of urea concentration on the backbone solution structure of the cyanide derivative of ferric Caretta caretta myoglobin (at pH 5.4) is reported. By addition of urea, sequential and long-range nuclear Overhauser effects (NOEs) are gradually lost. By using the residual NOE constraints to build the molecular model, a picture of the unfolding pathway was obtained. When the urea concentration is raised to 2.2 M, helices A and B appear largely disordered; helices C, D, and F loose structural constraints at 3.0 M urea. At urea concentration >6 M, the protein appears to be fully unfolded, including the GH hairpin and helix E stabilizing the prosthetic group. Reversible and cooperative denaturation isotherms obtained by following NOE peaks are considerably different from those obtained by monitoring electronic absorption changes. The reversible and cooperative urea-dependent folding-unfolding process of C. caretta myoglobin follows the minimum three-state mechanism N long left and right arrow X long left and right arrow D, where X represents a disordered globin structure (occurring at approximately 4 M urea) that still binds the heme.

A stable dimer in the pH-induced equilibrium unfolding of the homo-hexameric enzyme 4-oxalocrotonate tautomerase (4-OT)

4-Oxalocrotonate tautomerase (4-OT) is a multimeric, bacterial enzyme comprised of 6 identical 62-amino acid subunits, which associate under native conditions to form a homo-hexameric structure stabilized entirely by noncovalent interactions. We have previously shown that the GuHCl-induced equilibrium unfolding of 4-OT at pH 8.5 is well modeled as a two-state process involving only hexamer and unfolded monomer; and we have obtained spectroscopic evidence that intermediate state(s) is (are) populated in the equilibrium unfolding reaction at pHs 6.0 and 7.4 [Silinski, P., Allingham, M. J., and Fitzgerald, M. C. (2001) Biochemistry 40, 4493-4502]. Here, we report on the pH-induced equilibrium unfolding of 4-OT using size-exclusion chromatography (SEC), far-UV-circular dichroism (CD) spectroscopy, and catalytic activity measurements over the pH range from 1.5 to 10.1. Our results indicate that the native hexamer of 4-OT is the predominant species in solution at pHs > or =6.2, that a partially folded dimeric state of 4-OT is stabilized in solution at pH 4.8, and that the enzyme is largely denatured in strongly acidic solutions (pH < or =3.1). GuHCl-induced equilibrium unfolding studies on 4-OT at pH 4.8 indicate that the folded 4-OT dimer populated at this pH is stabilized by 11.7 kcal.mol(-1). The results of biophysical studies on a fluorescent analogue of the enzyme, 4-OT(F50Y), and the results of UV photo-cross-linking studies on a synthetically derived 4-OT analogue, 4-OT(P1Bpa), suggest the polypeptide chains in the 4-OT dimer are nativelike in structure with the exception of their C-termini.