The research team, led by Dr. Anthony Maurelli, of the Department of Environmental and Global Health and Emerging Pathogens Institute, that resolved the 60-year-old Chlamydia anomaly has solved another long-standing mystery of this important bacterial pathogen.
Chlamydia trachomatis is the leading cause of bacteria sexually transmitted infections (STIs) in the United States. It is responsible for over 1.5 million reported cases in 2016 and more than 94,000 cases in Florida. The Chlamydia anomaly was based on the observation that this bacterium is sensitive to penicillin and other antibiotics that attack formation of the bacterial cell yet it had no detectable cell wall. Scientists had been unable to find proof of a cell wall in Chlamydia until 2014 when Maurelli’s team provided definitive evidence of this cell wall in Chlamydia.
Almost all known bacteria, including Chlamydia, make a cell wall that is composed of certain unusual chemical components. These building blocks are not found in humans or animals so the enzymes that are used to construct the cell wall are excellent targets for antibiotics, like penicillin. One of these unique building blocks of the bacterial cell wall is D-glutamate. This amino acid is the mirror image of L-glutamate, one of the 20 amino acids that are used to make proteins in all living organisms. One of the remaining mysteries in how Chlamydia makes its cell wall is how it makes D-glutamate.
Dr. Maurelli’s team collaborated with Dr. George Liechti at the Uniformed Services University in Bethesda, Maryland to tackle the problem. They used bioinformatics to analyze more than 200 fully sequenced genomes of Chlamydia to identify any gene that could encode the enzyme that all bacteria use to synthesize D-glutamate from L-glutamate. The enzyme, known as a racemase, inverts the stereochemistry of an asymmetric carbon from the L-isomer form to its mirror image D-isomer form. The UF team found that no Chlamydia genome had the gene for glutamate racemase. Instead, they found that Chlamydia uses a different enzyme to carry out the reaction of the missing racemase. Surprisingly, this enzyme, diaminopimelate epimerase (DapF), was already known and had been described previously by Dr. Maurelli’s group. DapF carries out a critical step in the synthesis of diaminopimelate (DAP), another unique chemical building block of the bacterial cell wall. Like D-glutamate, DAP is only found in bacterial cell walls and is not made by humans and animals. Thus, the enzymes involved in the synthesis of DAP are novel targets for antibacterial drug development since no drug currently attacks this pathway.
An epimerase like DapF carries out a reaction similar to that of a racemase but it inverts the asymmetric carbon atom in a compound that has more than one center of asymmetry. The remarkable promiscuity of the chlamydial DapF to act both as an epimerase to make diaminopimelate and as a racemase to make D-glutamate is thought to be the hallmark of early microbial life on this planet.
Thus, Dr. Maurelli concludes that the chlamydial DapF is a primordial isomerase that functions as both racemase and epimerase and that the specialized D-glutamate racemases found in other bacteria never evolved in Chlamydia. While this study has important implications in the evolution of bacterial pathogens, it also points to a potential novel target for anti-chlamydial drug development. A drug that can block the action of DapF would cripple cell wall synthesis in Chlamydia by blocking synthesis of two essential components of the cell wall. The absence of any human enzyme similar to DapF would also make the drug selective for targeting Chlamydia without harming host processes.
The full report of this discovery can be found in the April issue of mBio.