Temperature-dependent ΔC0p generated by a shift in equilibrium between macrostates of an enzyme

• Published in: Nature (London) 1981-07, Vol.292 (5820), p.271-272
• Main Authors: Fisher, Harvey F., Colen, Alan H., Medary, Richard T.
• Format: Article
• Language: English
• Subjects: Glutamate Dehydrogenase; Humanities and Social Sciences; letter; Macromolecular Substances; multidisciplinary; NADP; Protein Conformation; Science; Science (multidisciplinary); Temperature; Thermodynamics
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/292271a0

Substantial negative heat capacity changes (Δ C 0 p ′s) have frequently been observed to accompany the formation of protein–ligand complexes 1,2 . Glutamate dehydrogenase 3 and horse liver alcohol dehydrogenase 4 , however, have been reported to form binary complexes with coenzyme with negligible Δ H 0 ′ and only small Δ C 0 p ′s. Although many intriguing mechanisms have been proposed to account for the observed phenomena, there is little direct experimental evidence available which might provide a basis for evaluating the contributions of Δ C 0 p ′s of complex formation from the various mechanistic sources or even for distinguishing between them. However, if, as Eftink and Biltonen 5 have suggested, a shift in equilibrium between macrostates contributes significantly to an observed Δ C 0 p ′s for a given reaction, it should be possible to characterize such a system by measuring the temperature dependence of the Δ C 0 p ′s. Despite this, few studies have determined Δ H 0 ′ values at more than two temperatures. We have now measured the temperature dependence of the Δ H 0 ′ (and, thereby, that of the Δ C 0 p ′s) of the formation of an enzyme-reduced coenzyme complex in an attempt to provide such a basis and have found that the entire Δ C 0 p ′s of complex formation is accounted for by a temperature-induced shift of an equilibrium between the different forms of the free enzyme.