BioWorld International Correspondent
LONDON - Researchers in Germany believe that they might have found a chink in the armor of the bacterium that causes multidrug-resistant tuberculosis. Their studies showed that an enzyme, called LipB, is vital to the function of the bacterium and particularly active in human cells infected with multidrug-resistant Mycobacterium tuberculosis.
The team already characterized a site within the enzyme to which an inhibitor can bind, and hope to embark on high-throughput screening to identify other inhibitors soon.
Matthias Wilmanns, head of the European Molecular Biology Laboratory (EMBL) in Hamburg, Germany, told BioWorld International: "We think that our discovery opens up an entirely new direction for those working in drug discovery on Mycobacterium tuberculosis. We hope that this will facilitate work on developing new drugs to treat this disease, particularly the multidrug-resistant strains of the bacteria."
Wilmanns, together with his collaborators, reported the study in a paper in the Proceedings of the National Academy of Sciences, published online May 29, 2006. Its title is "The Mycobacterium tuberculosis LipB enzyme functions as a cysteine/lysine dyad acyltransferase."
About one-third of the world's population is infected with Mycobacterium tuberculosis, and every year 2 million people die of the disease. Strains of the bacterium that are resistant to all known antibiotics are becoming more and more common. Although tuberculosis is most common in Africa and Asia, its association with HIV and AIDS and increasing incidence in developed countries have put the need to tackle this problem firmly on the political agenda.
Wilmanns' group, which specializes in structural biology, collaborated with Stefan Kaufmann and colleagues at the Max Planck Institute for Infection Biology in Berlin, Germany, to identify proteins of M. tuberculosis that could be useful drug targets.
The genome of M. tuberculosis has more than 4,000 genes, so the Hamburg team focused first on genes encoding proteins that Kaufmann's group flagged - ones that appear to be important to the bacterium's survival. So far, the researchers have purified 110 proteins selected in that way, and research groups from EMBL and a neighboring structural biology group from the Max-Planck Society have crystallized 33 of them.
The enzyme LipB is on that short list. Studies have shown that, when M. tuberculosis infects human cells, LipB is made in large quantities. That's particularly true for multidrug-resistant varieties of M. tuberculosis. In addition, despite repeated attempts, researchers have failed to make a viable knockout mutant of M. tuberculosis lacking the gene for LipB, suggesting that the enzyme is vital to the survival of the bacterium.
Wilmanns said: "When our colleagues in Berlin analyzed protein expression patterns in human macrophages infected with M. tuberculosis, they found that LipB is up-regulated by a factor of 70, which is amazing. This was why we decided to determine its structure."
Qingjun Ma, from Wilmanns' group, purified LipB and obtained crystals of the molecule. Using the high-energy synchrotron radiation beamlines at EMBL Hamburg, on the campus of the German Electron Synchrotron Radiation Facility, he created an atom-by-atom map of the protein's structure.
To the researchers' surprise, the enzyme turned out to have a large cavity - its active site - that already was occupied by an inhibitor, a molecule called decanoic acid.
The presence of decanoic acid was an artefact, but a very useful one, as it provided the researchers with much valuable information about the function of LipB, and the structure of other compounds that might inhibit LipB.
Wilmanns said: "The next step is to go into large-scale compound screening and convince the pharmaceutical industry that this may be an interesting target. We believe that it is, because it belongs to a vital pathway, and M. tuberculosis has no backup mechanism that could take over LipB's role. This means that an inhibitor blocking its active site could shut down key processes the bacterium needs to survive and replicate."
Both groups will continue to look for other proteins of M. tuberculosis that could form ideal drug targets, including molecules that could help M. tuberculosis to persist in its dormant state, rather than becoming active.
"Structure-based drug discovery has been a big success in the battle against many other diseases, including cancer and AIDS," Wilmanns concluded. "Although this kind of approach has not been used in the field of tuberculosis up till now, there is no reason why it should not work just as well."