Glucose metabolism in Chlamydia trachomatis: the 'energy parasite' hypothesis revisited

Chlamydia trachomatis is an obligate intracellular eubacteria that is dependent on a eukaryotic host cell for a variety of metabolites. For years, it has been speculated that chlamydiae are energy parasites, totally dependent on their host cell for ATP and other high-energy intermediates. To determine whether C. trachomatis contains functional enzymes that produce energy or reducing power, four enzymes involved in glycolysis or the pentose phosphate pathway, specifically pyruvate kinase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase, were cloned, sequenced and expressed as recombinant proteins in Escherichia coli. The deduced amino acid sequences obtained show high homology to other pyruvate kinase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase enzymes. In contrast to numerous other bacterial species, chlamydial glycolytic genes are not arranged in an operon, but are dispersed throughout the genome. Results from reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicate that all four genes are maximally expressed in the middle of the chlamydial developmental cycle. The chlamydial genes are capable of complementing mutant E. coli strains lacking the respective enzyme activities. In vitro enzyme analysis indicates that recombinant chlamydial enzymes expressed in E. coli are active and, interestingly, recombinant chlamydial pyruvate kinase is not regulated allosterically by fructose 1,6 bisphosphate or AMP, as found with other bacterial pyruvate kinases. In summary, identification and characterization of these glucose-catabolizing enzymes indicate that chlamydia contains the functional capacity to produce its own ATP and reducing power.