Crystal structure of quinolinic acid phosphoribosyltransferase from Mycobacterium tuberculosis: a potential TB drug target

Background: Mycobacterium tuberculosis is the single most deadly human path ogen and is responsible for nearly three million deaths every year. Recent elucidation of the mode of action of isoniazid, a frontline antimycobacteri al drug, suggests that NAD metabolism is extremely critical for this microo rganism. M. tuberculosis depends solely on the de novo pathway to meet its NAD demand. quinolinic acid phosphoribosyltransferase (QAPRTase), a key enz yme in the de novo biosynthesis of NAD, provides an attractive target for d esigning novel antitubercular drugs. Results: The X-ray crystal structure of the M, tuberculosis QAPRTase apoenz yme has been determined by multiple isomorphous replacement at 2.4 Angstrom resolution. Structures of the enzyme have also been solved in complex with the substrate quinolinic acid (QA), the inhibitory QA analog phthalic acid (PA), the product nicotinate mononucleotide (NAMN), and as a ternary compl ex with PA and a substrate analog, 5-phosphoribosyl-1-(beta-methylene)pyrop hosphate (PRPCP). The structure of the nonproductive QAPRTase-PA-PRPCP Mich aelis complex reveals a 5-phosphoribosyl-1-pyrophosphate-binding site that is different from the one observed in type 1 phosphoribosyltransferases (PR Tases). The type II PRTase active site of QAPRTase undergoes conformational changes that appear to be important in determining substrate specificity a nd eliciting productive catalysis, Conclusions: QAPRTase is the only known representative of the type II PRTas e fold, an unusual alpha/beta barrel, and appears to represent convergent e volution for PRTase catalysis, The active site of type II PRTase bears litt le resemblance to the better known type I enzymes.