Evolution of the TPP - dependent enzyme family

The evolutionary relationships of the thiamine pyrophosphate (TPP) dependent family of enzymes was investigated by generation of a neighbor joining (NJ) phylogenetic tree using sequences from the conserved pyrophosphate (PP) and pyrimidine (Pyr) binding domains of 17 TPP-dependent enzymes. This represents the most comprehensive analysis of TPP-dependent enzyme evolution to date. The phylogeny was shown to be robust by comparison with maximum likelihood (ML) trees generated for each individual enzyme. The study discerned the order in which enzymes diverged in the progenote population and several instances where enzyme types diverged later, and broadly confirms the evolutionary history proposed recently from structural comparisons alone (Duggleby 2006). The relationship between the PP-and Pyr-domains and the recruitment of additional protein domains was examined using the transketolase C-terminal (TKC) domain as an example. This domain has been recruited by several members of the family and yet forms no part of the active site and has unknown function. Removal of the TKC-domain was found to increase activity towards β-hydroxypyruvate (HPA) and glycolaldehyde (GA). Further truncations of the Pyr-domain yielded several variants with retained activity. This suggests the influence of TKC-domain recruitment on the evolution of the mechanism and specificity of transketolase (TK) has been minor, and that the smallest functioning unit of TK comprises the PP-and Pyr-domains whose evolutionary histories extend to all TPP-dependent enzymes.