- Reversal of the substrate specificity of CMP N-glycosidase to dCMP
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MilB is a CMP hydrolase involved in the early steps of biosynthesis of the antifungal compound mildiomycin. An enzyme from the bacimethrin biosynthetic pathway, BcmB, is closely related to MilB in both sequence and function. These two enzymes belong to the nucleoside 2′-deoxyribosyltransferase (NDT) superfamily. NDTs catalyze N-glycosidic bond cleavage of 2′- deoxynucleosides via a covalent 2-deoxyribosyl-enzyme intermediate. Conservation of key active site residues suggests that members of the NDT superfamily share a common mechanism; however, the enzymes differ in their substrate preferences. Substrates vary in the type of nucleobase, the presence or absence of a 2′-hydroxyl group, and the presence or absence of a 5′-phosphate group. We have determined the structures of MilB and BcmB and compared them to previously determined structures of NDT superfamily members. The comparisons reveal how these enzymes differentiate between ribosyl and deoxyribosyl nucleotides or nucleosides and among different nucleobases. The 1.6 A structure of the MilB-CMP complex reveals an active site feature that is not obvious from comparisons of sequence alone. MilB and BcmB that prefer substrates containing 2′-ribosyl groups have a phenylalanine positioned in the active site, whereas NDT family members with a preference for 2′-deoxyribosyl groups have a tyrosine residue. Further studies show that the phenylalanine is critical for the specificity of MilB and BcmB toward CMP, and mutation of this phenylalanine residue to tyrosine results in a 1000-fold reversal of substrate specificity from CMP to dCMP.
- Sikowitz, Megan D.,Cooper, Lisa E.,Begley, Tadhg P.,Kaminski, Pierre Alexandre,Ealick, Steven E.
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p. 4037 - 4047
(2013/07/27)
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- Reversible and in Situ Formation of Organic Arsenates and Vanadates as Organic Phosphate Mimics in Enzymatic Reactions: Mechanistic Investigation of Aldol Reactions and Synthetic Applications
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A synthetic strategy is developed that uses organic phosphate utilizing enzymes as catalysts and a mixture of an organic alcohol and inorganic arsenate or vanadate to replace the organic phosphate substrate.In this process, inorganic arsenate or vanadate reacts with the alcohol reversibly in situ to form a mixture of esters, one of which is accepted by the enzyme as a substrate.Examples of the utility of this approach are demonstrated in enzymatic aldol condensations catalyzed by fructose-1,6-diphosphate aldolase, fuculose-1-phosphate aldolase, and rhamnulose-1-phosphate aldolase with a mixture of dihydroxyacetone and inorganic arsenate as substrate.Several uncommon sugars and deoxy sugars are prepared on 5-17-mmol scales.Mechanistic studies on an aldol reaction indicate that the redox reaction between dihydroxyacetone and inorganic vanadate prohibits the use of such a mixture to replace dihydroxyacetone phosphate in enzymatic aldol condensations.
- Drueckhammer, Dale G.,Durrwachter, J. Robert,Pederson, Richard L.,Crans, Debbie C.,Daniels, Lacy,Wong, Chi-Huey
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- FURANOSE RING ANOMERIZATION: A KINETIC STUDY OF THE 5-DEOXYPENTOSES AND 5-O-METHYLPENTOSES
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The anomerzation of 5-deoxy-L-pentoses (1-4) and 5-O-methyl-D-pentoses (5-8) in aqueous solution has been studied by (13)C saturation-transfer n.m.r. (s.t.-n.m.r.) spectroscopy, using compounds substituted with (13)C at the anomeric carbon atom.Unidirectional rate-constants of ring-opening (kopen) and ring closing (kclose) have been obtained for these compounds under identical solution conditions (50mM acetate buffer, pH 4.0 at 60 deg C), and have been compared to those measured for the D-tetroses (9 and 10) and four D-pentose 5-phosphates (11-14).Based on these comparisons, several correlations between furanose structure and reactivity have been revealed, and models have been proposed to explain the observed kinetic behavior of compounds 1-10.The effect of exocyclic structure on acid-catalyzed rate-constants was also examined by comparing the behavior of 5-deoxy-L-lyxose and 5-O-methyl-D-lyxose.Some consideration has been given to identifying the factors (enthalpic and entropic) that may play roles in determining the effect of structure on anomerization reactivity.
- Snyder, Joseph R.,Serianni, Anthony S.
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- Anomerization of Furanose Sugars and Sugar Phosphates
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Thermodynamic and kinetic parameters for the ring-opening and -closing reactions of several aldo- and ketofuanoses and their phosphate esters have been determined by NMR line-width and saturation-transfer methods.Cyclic forms interconvert via a single, acyclic carbonyl form under either acid or base catalysis.Ring-opening rates do not correlate with thermodynamic stability of the rings.For aldofuranose phosphates, α anomers open faster than β anomers; for ketofuranose phosphates the converse is observed.Intramolecular catalysis of anomerization by the phosphate group of sugar phosphates is documented.Biological and mechanistic implications of the observed kinetics are discussed.
- Pierce, John,Serianni, Anthony S.,Barker, Robert
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p. 2448 - 2456
(2007/10/02)
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