Triosephosphate isomerase: the perfect enzyme, but how does it work?

摘要

Nearly fifty years ago the triosephosphate isomerase enzyme, from chicken, was first determined by X-ray crystallography (Banner et al., 1975). This was achieved via a multiple isomorphous replacement (MIR) phased electron-density map interpreted via a Richards box and its associated Kendrew mechanical molecular model parts (Richards, 1968). This Richards box was housed opposite my desk during my DPhil in the Laboratory of Molecular Biophysics at the University of Oxford. So, I can truly say that I once shared an office with triosephosphate isomerase! Triosephosphate isomerase was, and is, an enzyme viewed by biochemists as ‘the perfect enzyme’. Knowles Albery’s (1977) early review was entitled Perfection in enzyme catalysis: the energetics of triosphosphate isomerase. The point being that the catalytic interconversion by triosephosphate isomerase (TIM) of dihydroxyacetone phosphate (DHAP) and d-glyceraldehyde-3-phosphate (d-GAP) (Fig. 1) was diffusion limited and thereby made it a very fast enzyme catalyst. The enzyme gives a huge rate improvement (by 109) over the reaction rate without the enzyme and a simple organic base. Knowles Albery (1977) also remark: ‘the structure of the enzyme at high resolution has been solved (Banner et al., 1975); this confirms the existence of the unique activesite glutamate in a pocket in the enzyme, which also contains histidine and lysine residues whose detailed function will presumably emerge when the structure of the enzyme-DHAP complex is completed (Banner et al., 1975).’. As we see below, with many more X-ray crystal structures of TIM, the new study of Kelpsˇas et al. (2021), reported in this issue of IUCrJ and involving neutrons as a probe, was needed.