title: Mononuclear and dinuclear mechanisms for catalysis of phosphodiester cleavage by alkaline earth metal ions in aqueous solution.
abstract: In biological systems, enzymes often use metal ions, especially Mg(2+), to catalyze phosphodiesterolysis, and model aqueous studies represent an important avenue of examining the contributions of these ions to catalysis. We have examined Mg(2+) and Ca(2+) catalyzed hydrolysis of the model phosphodiester thymidine-5'-p-nitrophenyl phosphate (T5PNP). At 25 degrees C, we find that, despite their different Lewis acidities, these ions have similar catalytic ability with second-order rate constants for attack of T5PNP by hydroxide (k(OH)) of 4.1x10(-4)M(-1)s(-1) and 3.7x10(-4)M(-1)s(-1) in the presence of 0.30M Mg(2+) and Ca(2+), respectively, compared to 8.3x10(-7)M(-1)s(-1) in the absence of divalent metal ion. Examining the dependence of k(OH) on [M(2+)] at 50 degrees C indicates different kinetic mechanisms with Mg(2+) utilizing a single ion mechanism and Ca(2+) operating by parallel single and double ion mechanisms. Association of the metal ion(s) occurs prior to nucleophilic attack by hydroxide. Comparing the k(OH) values reveals a single Mg(2+) catalyzes the reaction by 1800-fold whereas a single Ca(2+) ion catalyzes the reaction by only 90-fold. The second Ca(2+) provides an additional 10-fold catalysis, significantly reducing the catalytic disparity between Mg(2+) and Ca(2+).
authors.affiliation: Department of Chemistry, Drew University, 36 Madison Ave., Madison, NJ 07940, USA.
authors.forename: Brittany A
authors.initials: BA
authors.lastname: Kirk
authors.forename: Corey L
authors.initials: CL
authors.lastname: Cusack
authors.forename: Eric
authors.initials: E
authors.lastname: Laager
authors.forename: Evan
authors.initials: E
authors.lastname: Rochlis
authors.forename: Terace
authors.initials: T
authors.lastname: Thomas
authors.forename: Adam G
authors.initials: AG
authors.lastname: Cassano
mesh_terms: D002118:Calcium / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D002384:Catalysis / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis / Q000187:drug effects; D006878:Hydroxides / Q000737:chemistry; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D008956:Models, Chemical; D009596:Nitrophenols / Q000737:chemistry / Q000378:metabolism; D010755:Organophosphates / Q000737:chemistry* / Q000378:metabolism; D009943:Organophosphorus Compounds / Q000737:chemistry / Q000378:metabolism; D012996:Solutions / Q000737:chemistry*; D013942:Thymine Nucleotides / Q000737:chemistry / Q000378:metabolism; D014867:Water / Q000737:chemistry
chemical_list: D006878:Hydroxides; D009596:Nitrophenols; D010755:Organophosphates; D009943:Organophosphorus Compounds; D012996:Solutions; D013942:Thymine Nucleotides; D014867:Water; C008644:nitrophenylphosphate; D008274:Magnesium; D002118:Calcium