Glyceraldehyde-3-Phosphate Dehydrogenases from Archaea: Objects for Studying Protein Thermoadaptation

Protein stabilization by compatible solutes

Heteronuclear NMR relaxation measurements and hydrogen exchange data have been used to characterize protein dynamics in the presence or absence of stabilizing solutes from hyperthermophiles. Rubredoxin from Desulfovibrio gigas was selected as a model protein and the effect of diglycerol phosphate on its dynamic behaviour was studied. The presence of 100 mM diglycerol phosphate induces a fourfold increase in the half-life for thermal denaturation of D. gigas rubredoxin. A model-free analysis of the protein backbone relaxation parameters shows an average increase of generalized order parameters of 0.015 reflecting a small overall reduction in mobility of fast-scale motions. Hydrogen exchange data acquired over a temperature span of 20 degrees C yielded thermodynamic parameters for the structural opening reactions that allow for the exchange. This shows that the closed form of the protein is stabilized by an additional 1.6 kJ x mol(-1) in the presence of the solute. The results seem to indicate that the stabilizing effect is due mainly to a reduction in mobility of the slower, larger-scale motions within the protein structure with an associated increase in the enthalpy of interactions.

Sequence and structural comparison of thermophilic phosphoglycerate kinases with a mesophilic equivalent

The monomeric glycolytic enzyme phosphoglycerate kinase (PGK) has been used as a model system to study protein thermostability. The primary sequence of this enzyme has been elucidated from 47 species to date. Although only 42 amino acids are totally conserved, most of which line the active site cleft, the protein is structurally conserved. This is achieved by making conservative changes to maintain the same secondary and tertiary folds. The crystal structures of 5 PGK enzymes have been solved by X-ray diffraction methods. This paper seeks to use the available information to understand protein thermostability. Although some general mechanisms to increase stability can be determined, different species have adopted a variety of subtle additive changes to achieve greater protein stability. Comparisons have been directly made between the PGK enzyme from yeast, the moderate thermophilic bacterium Bacillus stearothermophilus, the hyperthermophilic bacteria Thermus thermophilus, Thermotoga maritima, and the hyperthermophilic archaea Sulpholobus solfataricus and Methanothermus fervidus.

Phosphoribosyl anthranilate isomerase from Thermotoga maritima is an extremely stable and active homodimer

The metabolism of hyperthermophilic microorganisms can function properly at temperatures close to 100 degrees C. It follows that they are equipped with both thermostable enzymes and mechanisms that handle labile metabolites. We wanted to understand how stable and active phosphoribosyl anthranilate isomerase (tPRAI) from the hyperthermophile Thermotoga maritima is at its optimum growth temperature of 80 degrees C, and how its thermolabile substrate, N-(5'-phosphoribosyl)-anthranilate (PRA), is protected from rapid decomposition. To this end, the trpF gene of T. maritima was expressed heterologously in Escherichia coli and tPRAI was purified. In contrast to most PRAIs from mesophiles, which are monomers with the eightfold beta alpha (or TIM) barrel fold, tPRAI is a homodimer. It is strongly resistant toward inactivation by temperatures up to 95 degrees C, by acidification to pH 3.2, and by proteases in the presence and absence of detergents. tPRAI is about 35-fold more active at its physiologic temperature than is the enzyme from E. coli (ePRAI) at 37 degrees C. This high catalytic efficiency of tPRAI is likely to complete successfully with the rapid spontaneous hydrolysis of PRA at 80 degrees C. Thus, with respect to both stability and function, tPRAI appears well adapted to the extreme habitat of T. maritima. Single crystals of tPRAI have been obtained that are suitable for X-ray analysis at high resolution.