Abstract

The triosephosphate isomerase (TIM) barrel superfamily is a broad family of proteins, most of which are enzymes. At the amino-acid-sequence level, many of the members of this family share little, if any, homology. Yet, they adopt the same three-dimensional (βα)₈ fold. The TIM barrel fold seems to be a good framework for many different kinds of enzymes, providing unique possibilities for both natural and human-designed evolution, as the catalytic center and the stabilizing features are separated to different ends of the barrel. Indeed, in the light of most recent studies, it seems likely that at least most of the different TIM barrel enzymes, catalyzing a huge variety of reactions, have evolved from a common ancestor.

TIM can be considered a real text-book enzyme — its catalytic properties and stucture-function relationships have been studied for decades. Still, at present, we are quite far from understanding the structural features that make TIM and other enzymes such superior catalysts in both efficiency and precision. TIM is a dimeric enzyme that consists of two identical subunits of 250 residues. It catalyzes the interconversion of dihydroxyacetone phosphate and D-glyceraldehyde-3-phosphate in glycolysis. The basics of this reaction are well known, but there is ongoing discussion about the details of the proton transfer steps, and three alternative pathways have been suggested. In addition, it is a fascinating question how the enzyme succeeds in abstracting a highly stable proton from a carbon atom of the substrate.

This study was undertaken to shed light on some of the questions concerning the structure-function relationships in TIM. The most important findings are the elucidation of the role of Asn11 as a catalytic residue and the meaning of the flexibility of both the catalytic Glu167 side chain as well as the substrate during catalysis, and the presence of a low-barrier hydrogen bond between Glu167 and a transition-state analogue, 2-phosphoglycolate. Furthermore, significant results were obtained on the importance of a conserved salt bridge, 20 Å away from the active site and the dimer interface, for the stability and folding of TIM as well as on the factors influencing the opening of the flexible loop 6 upon product release.