Structure and Function of a Regulated Archaeal Triosephosphate Isomerase Adapted to High Temperature

Triosephophate isomerase (TIM) is a dimeric enzyme in eucarya, bacteria and mesophilic archaea. In hyperthermophilic archaea, however, TIM exists as a tetramer composed of monomers that are about 10% shorter than other eucaryal.and bacterial TIM monomers. We report here the crystal structure of TIM from Thermoproteus tenax, a hyperthermophilic archaeon that has an optimum growth temperature of 86 degreesC. The structure was determined from both a hexagonal and an orthorhombic crystal form to resolutions of 2.5 Angstrom and 2.3 Angstrom, and refined to R-factors of 19.7% and 21.5%, respectively. In both crystal forms, T tenax TIM exists as a tetramer of the familiar (betaalpha)(8)-barrel. In solution, however, and unlike other hyperthermophilic TIMs, the T. tenax enzyme exhibits an equilibrium between inactive dimers and active tetramers, which is shifted to the tetramer state through a specific interaction with glycerol-1 -phosphate dehydrogenase of T tenax. This observation is interpreted in physiological terms as a need to reduce the build-up of thermolabile metabolic intermediates that would be susceptible to destruction by heat. A detailed structural comparison with TIMs from organisms with growth optima ranging from 15degreesC to 100degreesC emphasizes the importance in hyperthermophilic proteins of the specific location of ionic interactions for thermal stability rather than their numbers, and shows a clear correlation between the reduction of heat-labile, surface-exposed Asn and Gln residues with thermoadaptation. The comparison confirms the increase in charged surface-exposed residues at the expense of polar residues. (C) 2004 Elsevier Ltd. All rights reserved.