Triosephosphate isomerase (TPI) is one of the most well characterised enzymes. However, our understanding of TPI is biased towards a few organisms, namely chicken, yeast, E. coli and T. brucei. In these organisms TPI is required for glycolysis/gluconeogenesis, while in organisms which carry out oxygenic photosynthesis TPI is involved in the regeneration phase of the Calvin-Benson-Bassham (CBB) cycle. Cyanobacteria possess a single TPI involved in both pathways, however, most photosynthetic eukaryotes possess cytoplasmic and chloroplast TPI isoforms, that are involved in glycolysis/gluconeogenesis and the CBB cycle respectively. Despite possessing a unique  chloroplast TPI isoform the TPIs from photosynthetic organisms remain comparatively under-investigated. To begin to address this gap in knowledge we have surveyed TPI diversity in photosynthetic organisms and used kinetic and structure/function paradigms to characterise seven TPIs from three photosynthetic and one derived non-photosynthetic species. The organisms whose TPIs were investigated were, the cyanobacterium Synechocystis sp. PCC6803, the model plant Arabidopsis thaliana, the red alga Porphyra umbilicalis and the non-photosynthetic parasitic plant Cuscuta australis.
We observed remarkable conservation of Michaelis–Menten steady-state kinetic parameters observing no significant difference between TPIs from different species TPI nor between cytoplasmic and chloroplast TPI isoforms from a single species. Further, the crystal structures of the C. australis TPIs demonstrate the conservation of TPI structure and provide evidence that the conformation changes which enable TPIs exquisite catalytic efficiency are conserved in the TPIs of photosynthetic organisms. Finally, we report that the chloroplast TPI of P. umbilicalis is uniquely sensitive to oxidation and present a hypothesis for a structural basis for this unique activity.