title: One substrate, five products: reactions catalyzed by the dihydroneopterin aldolase from Mycobacterium tuberculosis.
abstract: Tetrahydrofolate cofactors are required for one carbon transfer reaction involved in the synthesis of purines, amino acids, and thymidine. Inhibition of tetrahydrofolate biosynthesis is a powerful therapeutic strategy in the treatment of several diseases, and the possibility of using antifolates to inhibit enzymes from  Mycobacterium tuberculosis  has been explored. This work focuses on the study of the first enzyme in tetrahydrofolate biosynthesis that is unique to bacteria, dihydroneopterin aldolase (MtDHNA). This enzyme requires no metals or cofactors and does not form a protein-mediated Schiff base with the substrate, unlike most aldolases. Here, we were able to demonstrate that the reaction catalyzed by MtDHNA generates three different pterin products, one of which is not produced by other wild-type DHNAs. The enzyme-substrate complex partitions 51% in the first turnover to form the aldolase products, 24% to the epimerase product and 25% to the oxygenase products. The aldolase reaction is strongly pH dependent, and apparent pK(a) values were obtained for the first time for this class of enzyme. Furthermore, chemistry is rate limiting for the aldolase reaction, and the analysis of solvent kinetic isotope effects in steady-state and pre-steady-state conditions, combined with proton inventory studies, revealed that two protons and a likely solvent contribution are involved in formation and breakage of a common intermediate. This study provides information about the plasticity required from a catalyst that possesses high substrate specificity while being capable of utilizing two distinct epimers with the same efficiency to generate five distinct products.
authors.affiliation: Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, United States.
authors.forename: Clarissa M
authors.initials: CM
authors.lastname: Czekster
authors.forename: John S
authors.initials: JS
authors.lastname: Blanchard
mesh_terms: D000446:Aldehyde-Lyases / Q000737:chemistry* / Q000378:metabolism; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D003001:Cloning, Molecular; D004926:Escherichia coli / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009169:Mycobacterium tuberculosis / Q000201:enzymology*; D011485:Protein Binding; D013379:Substrate Specificity
chemical_list: D000446:Aldehyde-Lyases; C020044:dihydroneopterin aldolase