Tiny TIM: A small, tetrameric, hyperthermostable triosephosphate isomerase

Comparative structural studies on proteins derived from organisms with grow th optima ranging from 15 to 100 degreesC are beginning to shed light on th e mechanisms of protein thermoadaptation. One means of sustaining hyperther mostability is for proteins to exist in higher oligomeric forms than their mesophilic homologues. Triosephosphate isomerase (TIM) is one of the most s tudied enzymes, whose fold represents one of nature's most common protein a rchitectures. Most TIMs are dimers of approximately 250 amino acid residues per monomer. Here, we report the 2.7 Angstrom resolution crystal structure of the extremely thermostable TIM from Pyrococcus woesei, a hyperthermophi lic archaeon growing optimally at 100 degreesC, representing the first arch aeal TIM structure. P. woesei TIM exists as a tetramer comprising monomers of only 228 amino acid residues. Structural comparisons with other less the rmostable TIMs show that although the central P-barrel is largely conserved , severe pruning of several helices and truncation of some loops give rise to a much more compact monomer in the small hyperthermophilic TIM. The clas sical TIM dimer formation is conserved in P. woesei TIM. The extreme thermo stability of PwTIM appears to be achieved by the creation of a compact tetr amer where two classical TIM dimers interact via an extensive hydrophobic i nterface. The tetramer is formed through largely hydrophobic interactions b etween some of the pruned helical regions. The equivalent helical regions i n less thermostable dimeric TIMs represent regions of high average temperat ure factor. The PwTIM seems to have removed these regions of potential inst ability in the formation of the tetramer. This study of PwTIM provides furt her support for the role of higher oligomerisation states in extreme therma l stabilisation. (C) 2001 Academic Press.