Heat Stability of a Tetrameric Enzyme, D-Glyceraldehyde-3-PhosphateDehydrogenase

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Abstract
The tetrameric enzyme D‐glyceraldehyde‐3‐phosphate dehydrogenase from the moderate thermophile Bacillus stearothermophilus is more stable to thermal denaturation than its counterpart from lobster muscle [Harris et al. (1980) Eur. J. Biochem. 108, 535–547]. Extra buried ionic bonds between subunits of the thermophilic enzyme make an important contribution to thermal stabilisation. Further stabilisation of the tetrameric enzyme is derived from additional hydrophobic interactions between the S‐loops at the core of the tetramer. In the enzyme from the extreme thermophile Thermus aquaticus, which is even more thermostable, intersubunit ion pairs must also play a role but changes in interactions at the surface appear to be equally important. Thus additional hydrophobic interactions at the edge of subunit interfaces would prevent access of water to the interior of the molecule. Furthermore, the arrangement of charged residues on the surface of the T. aquaticus enzyme would allow maximal surface ion pair formation. The presence of surface ion pairs in other proteins correlates well with thermal stability [Perutz, M. F. and Raidt, H. (1975) Nature (Lond.) 255, 256–258] and would provide a general stabilising influence on the subunit in this case.