title: Differences in the catalytic mechanisms of mesophilic and thermophilic indole-3-glycerol phosphate synthase enzymes at their adaptive temperatures.
abstract: Thermophilic enzymes tend to be less catalytically-active at lower temperatures relative to their mesophilic counterparts, despite having very similar crystal structures. An often cited hypothesis for this general observation is that thermostable enzymes have evolved a more rigid tertiary structure in order to cope with their more extreme, natural environment, but they are also less flexible at lower temperatures, leading to their lower catalytic activity under mesophilic conditions. An alternative hypothesis, however, is that complementary thermophilic-mesophilic enzyme pairs simply operate through different evolutionary-optimized catalytic mechanisms. In this communication, we present evidence that while the steps of the catalytic mechanisms for mesophilic and thermophilic indole-3-glycerol phosphate synthase (IGPS) enzymes are fundamentally similar, the identity of the rate-determining step changes as a function of temperature. Our findings indicate that while product release is rate-determining at 25°C for thermophilic IGPS, near its adaptive temperature (75°C), a proton transfer event, involving a general acid, becomes rate-determining. The rate-determining steps for thermophilic and mesophilic IGPS enzymes are also different at their respective, adaptive temperatures with the mesophilic IGPS-catalyzed reaction being rate-limited before irreversible CO2 release, and the thermophilic IGPS-catalyzed reaction being rate limited afterwards.
authors.affiliation: Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Margot J
authors.initials: MJ
authors.lastname: Zaccardi
authors.forename: Olga
authors.initials: O
authors.lastname: Mannweiler
authors.forename: David D
authors.initials: DD
authors.lastname: Boehr
mesh_terms: D002384:Catalysis; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007209:Indole-3-Glycerol-Phosphate Synthase / Q000737:chemistry* / Q000235:genetics; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D012997:Solvents / Q000737:chemistry; D016944:Sulfolobus / Q000201:enzymology
chemical_list: D011994:Recombinant Proteins; D012997:Solvents; D007209:Indole-3-Glycerol-Phosphate Synthase