Asymmetric total synthesis of ent-cyclooroidin

An enantiospecific total synthesis of the pyrrole-imidazole natural product cyclooroidin from histidine is described. The key N1-C9 bond is constructed through an intramolecular SN2-type of reaction of a chloro ester. Subsequent imidazole azidation at the 2-position, pyrrole bromination, azide reduction, and deprotection leads to the completion of the synthesis.

Stereo-selective affinity labelling of sheep liver sorbitol dehydrogenase by chloro-substituted analogues of 2-bromo-3-(5-imidazolyl)propionic acid

The role of configuration for the affinity labelling of sheep liver sorbitol dehydrogenase by chloro-substituted analogues of 2-bromo-3-(5-imidazolyl)propionate (BrImPpOH) has been studied. A saturation kinetics mechanism applies which includes formation of a reversible complex with the enzyme prior to alkylation of Cys-43. The pseudo first-order inactivation rate-constant, k2, and the dissociation constant for the reversible enzyme-affinity label complex. KEI, were determined at pH 7.4 and 23.5 degrees C. The stereo isomers of each affinity label exhibit different kinetic characteristics but, unlike with horse liver alcohol dehydrogenase, the discrimination between them is not absolute. For the different affinity labels, k2 varies with 2-chloro-3-(5-imidazolyl)methylpropionate (Me-ClImPpOH) > 2-chloro-3-(5-imidazolyl)propionate (ClImPpOH) > 2-chloro-3-(5-imidazolyl)propanol (ClImPOH), consistent with their order of inherent reactivity, and the specificity constant k2/KEI varies with (S)-Me-ClImPpOH > (S)-ClImPpOH > (S)-ClImPpOH > (R)-Me-ClImPpOH > (R)-ClImPpOH. Models of the affinity labels were built into the active site of the predicted subunit structure of the enzyme by using a computer-controlled display system. In each binary complex, the imidazole moiety of the affinity label was liganded to the catalytic zinc atom, and the angle Scys-C alpha-Cl was linear, in accordance with an SN2 mechanism. Both enantiomers of each label could form plausible complexes with the enzyme model, in agreement with the kinetic data. The enantiomeric selectivity, rather than absolute specificity, of the reaction appears due to the anion-binding site in sorbitol dehydrogenase being less developed than in horse liver alcohol dehydrogenase.

Salmonella typhimurium histidinol dehydrogenase: complete reaction stereochemistry and active site mapping

The stereochemistry of the L-histidinol dehydrogenase reaction was determined to be R at NAD for both steps, confirming previous results with a fungal extract [Davies, D., Teixeira, A., & Kenworthy, P. (1972) Biochem. J. 127, 335-343]. NMR analysis of monodeuteriohistidinols produced by histidinol/NADH exchange reactions arising via reversal of the alcohol oxidation reaction indicated a single stereochemistry at histidinol for that step. Comparison of vicinal coupling values of the exchange products with those of L-alaninol and a series of (S)-2-amino-1-alcohols allowed identification of the absolute stereochemistry of monodeuteriohistidinols and showed that histidinol dehydrogenase removes first the pro-S then the pro-R hydrogens of substrate histidinol. The enzyme stereochemistry was confirmed by isotope effects for monodeuteriohistidinols as substrates for the pro-R-specific dehydrogenation catalyzed by liver alcohol dehydrogenase. Active site mapping was undertaken to investigate substrate-protein interactions elsewhere in the histidinol binding site. Critical binding regions are the side-chain amino group and the imidazole ring, whose methylation at the 1- or 2-position caused severe decreases in binding affinity. Use of alternative substrates further clarified active site interactions with the substrate. Compounds in which the alpha-amino group was replaced by chloro, bromo, or hydrogen substituents were not substrates of the overall reaction at 1/10,000 the normal rate.(ABSTRACT TRUNCATED AT 250 WORDS)