Cleavage of nicotinamide adenine dinucleotide by the ribosome-inactivating protein from Momordica charantia.

The interaction of momordin, a type 1 ribosome-inactivating protein from Momordica charantia, with NADP(+) and NADPH has been investigated by X-ray diffraction analysis of complexes generated by co-crystallization and crystal soaking. It is known that the proteins of this family readily cleave the adenine-ribose bond of adenosine and related nucleotides in the crystal, leaving the product, adenine, bound to the enzyme active site. Surprisingly, the nicotinamide-ribose bond of oxidized NADP(+) is cleaved, leaving nicotinamide bound in the active site in the same position but in a slightly different orientation to that of the five-membered ring of adenine. No binding or cleavage of NADPH was observed at pH 7.4 in these experiments. These observations are in accord with current views of the enzyme mechanism and may contribute to ongoing searches for effective inhibitors.

Crystal structure of Escherichia coli ketopantoate reductase at 1.7 A resolution and insight into the enzyme mechanism.

Ketopantoate reductase (KPR, EC 1.1.1.169) catalyzes the NADPH-dependent reduction of ketopantoate to pantoate on the pantothenate (vitamin B(5)) biosynthetic pathway. The Escherichia coli panE gene encoding KPR was cloned and expressed at high levels as the native and selenomethionine-substituted (SeMet) proteins. Both native and SeMet recombinant proteins were purified by three chromatographic steps, to yield pure proteins. The wild-type enzyme was found to have a K(M)(NADPH) of 20 microM, a K(M)(ketopantoate) of 60 microM, and a k(cat) of 40 s(-1). Regular prismatic KPR crystals were prepared using the hanging drop technique. They belonged to the tetragonal space group P4(2)2(1)2, with cell parameters: a = b = 103.7 A and c = 55.7 A, accommodating one enzyme molecule per asymmetric unit. The structure of KPR was determined by the multiwavelength anomalous dispersion method using the SeMet protein, for which data were collected to 2.3 A resolution. The native data were collected to 1.7 A resolution and used to refine the final structure. The secondary structure comprises 12 alpha-helices, three 3(10)-helices, and 11 beta-strands. The enzyme is monomeric and has two domains separated by a cleft. The N-terminal domain has an alphabeta-fold of the Rossmann type. The C-terminal domain (residues 170-291) is composed of eight alpha-helices. KPR is shown to be a member of the 6-phosphogluconate dehydrogenase C-terminal domain-like superfamily. A model for the ternary enzyme-NADPH-ketopantoate ternary complex provides a rationale for kinetic data reported for specific site-directed mutants.

Antihyperglycemic N-sulfonyl-1a,2,6,6a-tetrahydro-1H,4H- [1,3]dioxepino[5,6-b]azirines: synthesis, X-ray structure analysis, conformational behavior, quantitative structure-property relationships, and quantitative structure-activity relationships.

A series of 1-sulfonyl-1a,2,6,6a-tetrahydro-1H,4H- [1,3]dioxepino[5,6-b]azirines, 4, has been synthesized and evaluated for its effects on blood glucose-decreasing activity. These derivatives were prepared from 4,7-dihydro-1,3-dioxepins 1 via vic(acylamino)halogenodioxepanes 2 and dioxepinoazirines 3. Quantitative structure--property relationship and quantitative structure--activity relationship models, based on X-ray and molecular mechanics analyses, to our knowledge the first in the field of antihyperglycemics, were developed. They allow the prediction of properties (RP-HPLC attention times) and activities (hypoglycemic activity ratio) by the Connolly's molecular surface areas. The lead compound in these models, sulfonyldioxepinoazirine 4i, expressed superior antihyperglycemic activity in comparison to metformin in alloxanized mice, irrespective of route of application. It significantly reduced blood glucose levels in glucose-primed mice, but it did not cause a dose dependent decrease of blood glucose level in healthy (nondiabetic, control) animals.