Thermal-unfolding Reaction of Triosephosphate Isomerase from Trypanosoma cruzi

Structural insights from a novel invertebrate triosephosphate isomerase from Litopenaeus vannamei

Triosephosphate isomerase (TIM; EC 5.3.1.1) is a key enzyme involved in glycolysis and gluconeogenesis. Glycolysis is one of the most regulated metabolic pathways, however little is known about the structural mechanisms for its regulation in non-model organisms, like crustaceans. To understand the structure and function of this enzyme in invertebrates, we obtained the crystal structure of triosephosphate isomerase from the marine Pacific whiteleg shrimp (Litopenaeus vannamei, LvTIM) in complex with its inhibitor 2-phosphogyceric acid (2-PG) at 1.7Å resolution. LvTIM assembles as a homodimer with residues 166-176 covering the active site and residue Glu166 interacting with the inhibitor. We found that LvTIM is the least stable TIM characterized to date, with the lowest range of melting temperatures, and with the lowest activation enthalpy associated with the thermal unfolding process reported. In TIMs dimer stabilization is maintained by an interaction of loop 3 by a set of hydrophobic contacts between subunits. Within these contacts, the side chain of a hydrophobic residue of one subunit fits into a cavity created by a set of hydrophobic residues in the neighboring subunit, via a "ball and socket" interaction. LvTIM presents a Cys47 at the "ball" inter-subunit contact indicating that the character of this residue is responsible for the decrease in dimer stability. Mutational studies show that this residue plays a role in dimer stability but is not a solely determinant for dimer formation.

Complex kinetics and residual structure in the thermal unfolding of yeast triosephosphate isomerase

BACKGROUND

Saccharomyces cerevisiae triosephosphate isomerase (yTIM) is a dimeric protein that shows noncoincident unfolding and refolding transitions (hysteresis) in temperature scans, a phenomenon indicative of the slow forward and backward reactions of the native-unfolded process. Thermal unfolding scans suggest that no stable intermediates appear in the unfolding of yTIM. However, reported evidence points to the presence of residual structure in the denatured monomer at high temperature.

RESULTS

Thermally denatured yTIM showed a clear trend towards the formation of aggregation-prone, β-strand-like residual structure when pH decreased from 8.0 to 6.0, even though thermal unfolding profiles retained a simple monophasic appearance regardless of pH. However, kinetic studies performed over a relatively wide temperature range revealed a complex unfolding mechanism comprising up to three observable phases, with largely different time constants, each accompanied by changes in secondary structure. Besides, a simple sequential mechanism is unlikely to explain the observed variation of amplitudes and rate constants with temperature. This kinetic complexity is, however, not linked to the appearance of residual structure. Furthermore, the rate constant for the main unfolding phase shows small, rather unvarying values in the pH region where denatured yTIM gradually acquires a β-strand-like conformation. It appears, therefore, that the residual structure has no influence on the kinetic stability of the native protein. However, the presence of residual structure is clearly associated with increased irreversibility.

CONCLUSIONS

The slow temperature-induced unfolding of yeast TIM shows three kinetic phases. Rather than a simple sequential pathway, a complex mechanism involving off-pathway intermediates or even parallel pathways may be operating. β-strand-type residual structure, which appears below pH 8.0, is likely to be associated with increased irreversible aggregation of the unfolded protein. However, this denatured form apparently accelerates the refolding process.

Reversibility and two state behaviour in the thermal unfolding of oligomeric TIM barrel proteins

The reversible thermal unfolding of oligomeric TIM barrels results from a delicate balance of physicochemical properties related to the sequence, the native and unfolded states and the transition between them.

Triosephosphate isomerase I170V alters catalytic site, enhances stability and induces pathology in a Drosophila model of TPI deficiency

Triosephosphate isomerase (TPI) is a glycolytic enzyme which homodimerizes for full catalytic activity. Mutations of the TPI gene elicit a disease known as TPI Deficiency, a glycolytic enzymopathy noted for its unique severity of neurological symptoms. Evidence suggests that TPI Deficiency pathogenesis may be due to conformational changes of the protein, likely affecting dimerization and protein stability. In this report, we genetically and physically characterize a human disease-associated TPI mutation caused by an I170V substitution. Human TPI(I170V) elicits behavioral abnormalities in Drosophila. An examination of hTPI(I170V) enzyme kinetics revealed this substitution reduced catalytic turnover, while assessments of thermal stability demonstrated an increase in enzyme stability. The crystal structure of the homodimeric I170V mutant reveals changes in the geometry of critical residues within the catalytic pocket. Collectively these data reveal new observations of the structural and kinetic determinants of TPI Deficiency pathology, providing new insights into disease pathogenesis.

Structural and thermodynamic folding characterization of triosephosphate isomerases from Trichomonas vaginalis reveals the role of destabilizing mutations following gene duplication

We report the structures and thermodynamic analysis of the unfolding of two triosephosphate isomerases (TvTIM1 and TvTIM2) from Trichomonas vaginalis. Both isoforms differ by the character of four amino acids: E/Q 18, I/V 24, I/V 45, and P/A 239. Despite the high sequence and structural similarities between both isoforms, they display substantial differences in their stabilities. TvTIM1 (E18, I24, I45, and P239) is more stable and less dissociable than TvTIM2 (Q18, V24, V45, and A239). We postulate that the identities of residues 24 and 45 are responsible for the differences in monomer stability and dimer dissociability, respectively. The structural difference between both amino acids is one methyl group. In TvTIMs, residue 24 is involved in packing α-helix 1 against α-helix 2 of each monomer and residue 45 is located at the center of the dimer interface forming a "ball and socket" interplay with a hydrophobic cavity. The mutation of valine at position 45 for an alanine in TvTIM2 produces a protein that migrates as a monomer by gel filtration. A comparison with known TIM structures indicates that this kind of interplay is a conserved feature that stabilizes dimeric TIM structures. In addition, TvTIMs are located in the cytoplasm and in the membrane. As TvTIM2 is an easily dissociable dimer, the dual localization of TvTIMs may be related to the acquisition of a moonlighting activity of monomeric TvTIM2. To our knowledge, this is the simplest example of how a single amino acid substitution can provide alternative function to a TIM barrel protein.