3v8e
Crystal structure of the yeast nicotinamidase Pnc1p bound to the inhibitor nicotinaldehyde
Structural highlights
FunctionPNC1_YEAST Catalyzes the deamidation of nicotinamide, an early step in the NAD(+) salvage pathway. Positively regulates SIR2-mediated silencing and longevity by preventing the accumulation of intracellular nicotinamide, an inhibitor of SIR2, during times of stress. Acts also on nicotinyl hydroxamate.[1] [2] [3] Publication Abstract from PubMedNicotinamidases catalyze the hydrolysis of nicotinamide to nicotinic acid and ammonia. Nicotinamidases are absent in mammals but function in NAD(+) salvage in many bacteria, yeast, plants, protozoa, and metazoans. We have performed structural and kinetic investigations of the nicotinamidase from Saccharomyces cerevisiae (Pnc1). Steady-state product inhibitor analysis revealed an irreversible reaction in which ammonia is the first product released, followed by nicotinic acid. A series of nicotinamide analogues acting as inhibitors or substrates were examined, revealing that the nicotinamide carbonyl oxygen and ring nitrogen are critical for binding and reactivity. X-ray structural analysis revealed a covalent adduct between nicotinaldehyde and Cys167 of Pnc1 and coordination of the nicotinamide ring nitrogen to the active-site zinc ion. Using this structure as a guide, the function of several residues was probed via mutagenesis and primary (15)N and (13)C kinetic isotope effects (KIEs) on V/K for amide bond hydrolysis. The KIE values of almost all variants were increased, indicating that C-N bond cleavage is at least partially rate limiting; however, a decreased KIE for D51N was indicative of a stronger commitment to catalysis. In addition, KIE values using slower alternate substrates indicated that C-N bond cleavage is at least partially rate limiting with nicotinamide to highly rate limiting with thionicotinamide. A detailed mechanism involving nucleophilic attack of Cys167, followed by elimination of ammonia and then hydrolysis to liberate nicotinic acid, is discussed. These results will aid in the design of mechanism-based inhibitors to target pathogens that rely on nicotinamidase activity. Structural and Kinetic Isotope Effect Studies of Nicotinamidase (Pnc1) from Saccharomyces cerevisiae.,Smith BC, Anderson MA, Hoadley KA, Keck JL, Cleland WW, Denu JM Biochemistry. 2012 Jan 10;51(1):243-56. Epub 2011 Dec 29. PMID:22229411[4] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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