MOLECULAR EVOLUTIONARY ANALYSIS OF THE THIAMINE-DIPHOSPHATE-DEPENDENTENZYME, TRANSKETOLASE

Members of the transketolase group of thiamine-diphosphate-dependent e nzymes from 17 different organisms including mammals, yeast, bacteria, and plants have been used for phylogenetic reconstruction. Alignment of the amino acid and DNA sequences for 21 transketolase enzymes and o ne putative transketolase reveals a number of highly conserved regions and invariant residues that are of predicted importance for enzyme ac tivity, based on the crystal structure of yeast transketolase. One par ticular sequence of 36 residues has some similarities to the nucleotid e-binding motif and we designate it as the transketolase motif. We rep ort further evidence that the recP protein from Streptococcus pneumoni ae might be a transketolase and we list a number of invariant residues which might be involved in substrate binding. Phylogenies derived fro m the nucleotide and the amino acid sequences by various methods show a conventional clustering for mammalian, plant, and gramnegative bacte rial transketolases, The branching order of the gram-positive bacteria could not be inferred reliably. The formaldehyde transketolase (somet imes known as dihydroxyacetone synthase) of the yeast Hansenula polymo rpha appears to be orthologous to the mammalian enzymes but paralogous to the other yeast transketolases. The occurrence of more than one tr ansketolase gene in some organisms is consistent with several gene dup lications. The high degree of similarity in functionally important res idues and the fact that the same kinetic mechanism is applicable to al l characterized transketolase enzymes is consistent with the propositi on that they are all derived from one common ancestral gene. Transketo lase appears to be an ancient enzyme that has evolved slowly and might serve as a model for a molecular clock, at least within the mammalian clade.