Phosphoketolase, a neglected enzyme of microbial carbohydrate metabolism

Abstract

Phosphoketolases are thiamine diphosphate (ThDP) dependent enzymes of the phosphoketolase (PK) pathway of heterofermentative and facultatively homofermentative lactic acid bacteria and of the fructose 6-phosphate shunt of bifidobacteria. PK activity was also measured in protein extracts of other microorganisms including yeasts. The dual substrate xylulose 5-phosphate/fructose 6-phosphate phosphoketolase (Xfp) from the 'probiotic' Bifidobacterium lactis was purified, and its encoding gene (xfp) was cloned and sequenced. Comparisons with public databases revealed an unexpectedly wide spread of more than 30 homologous xfp sequences in the kingdom of the bacteria, but not of the archaea. We assigned amino acid motifs typically found in PKs. Two of them (G-P-G-H-G and E-G-G-E-L-G-Y, respectively) discriminate PKs from transketolases, which have at least one ThDP binding site in common. On the basis of further comparative analyses we conclude that the PK prevalence among diverse organisms is due to longitudinal and not horizontal gene transmission.