CATALYSIS BY EXTREMOZYMES: COMPARING OROTIDINE 5?- MONOPHOSPHATE DECARBOXYLASES FROM PSYCHROPHILES, MESOPHILES, AND THERMOPHILES
Roy, Alexander C
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Low temperatures impose a unique set of restrictions on the thermodynamic strategies available for enzymatic catalysis. The specific thermodynamic consequences of cold environments were determined for two psychrophile-derived variants of the highly-proficient enzyme, orotidine 5?–monophosphate decarboxylase (ODCase) — one from Psychrobacter arcticus 273–4 (PaODCase), and another from Colwellia psychrerythraea 34H (CpODCase). Determination of the kinetic parameters of these psychrozymes as a function of temperature indicated that PaODCase operates through entropy-driven ground-state-destabilisation, while CpODCase operates primarily through enthalpy-driven transition-state-stabilisation. In the context of prior studies conducted with mesozyme and thermozyme ODCase-variants, a large value of kcat was found to be the most consistent hallmark of a psychrozyme. Interestingly, a low value of an enzyme's melting temperature (Tm) appeared to correlate weakly with low-temperature activity. On the whole, the trends identified herein afford greater understanding of the unique challenges to providing catalysis at low-temperatures overcome by psychrozymes.