A NEW FUNCTION FOR A COMMON FOLD - THE CRYSTAL-STRUCTURE OF QUINOLINIC ACID PHOSPHORIBOSYLTRANSFERASE

Background: Quinolinic acid (QA) is a neurotoxin and has been shown to be present at high levels in the central nervous system of patients w ith certain diseases, such as AIDS and meningitis. The enzyme quinolin ic acid phosphoribosyltransferase (QAPRTase) provides the only route f or QA metabolism and is also an essential step in de novo NAD biosynth esis. QAPRTase catalyzes the synthesis of nicotinic acid mononucleotid e (NAMN) from QA and 5-phosphoribosyl-1-pyrophosphate (PRPP), The stru ctures of several phosphoribosyltransferases (PRTases) have been repor ted, and all have shown a similar fold of a five-stranded beta sheet s urrounded by four cu helices, A conserved sequence motif of 13 residue s is common to these 'type I' PRTases but is not observed in the QAPRT ase sequence, suggestive of a different fold for this enzyme. Results: The crystal structure of QAPRTase from Salmonella typhimurium has bee n determined with bound QA to 2.8 Angstrom resolution, and with bound NAMN to 3.0 Angstrom resolution. Most significantly, the enzyme shows a completely novel fold for a PRTase enzyme comprising a two-domain st ructure: a mixed alpha/beta N-terminal domain and an alpha/beta barrel -like domain containing seven beta strands. The active site is located at the C-terminal ends of the beta strands of the alpha/beta barrel, and is bordered by the N-terminal domain of the second subunit of the dimer. The active site is largely composed of a number of conserved ch arged residues that appear to be important for substrate binding and c atalysis. Conclusions: The seven-stranded alpha/beta-barrel domain of QAPRTase is very similar in structure to the eight-stranded alpha/beta -barrel enzymes. The structure shows a phosphate-binding site that app ears to be conserved among many alpha/beta-barrel enzymes including in dole-3-glycerol phosphate synthase and flavocytochrome b2. The new fol d observed here demonstrates that the PRTase enzymes have evolved thei r similar chemistry from at least two completely different protein arc hitectures.