Prior evidence supporting the direct observation of phosphorane intermediates in enzymatic phosphoryl transfer reactions was based on the interpretation of electron density corresponding to trigonal species bridging the donor and acceptor atoms. Close examination of the crystalline state of β-phosphoglucomutase, the archetypal phosphorane intermediate-containing enzyme, reveals that the trigonal species is not PO − 3 , but is MgF − 3 (trifluoromagnesate). Although MgF − 3 complexes are transition state analogues rather than phosphoryl group transfer reaction intermediates, the presence of fluorine nuclei in near-transition state conformations offers new opportunities to explore the nature of the interactions, in particular the independent measures of local electrostatic and hydrogen-bonding distributions using 19 F NMR. Measurements on three β-PGM-MgF − 3 -sugar phosphate complexes show a remarkable relationship between NMR chemical shifts, primary isotope shifts, NOEs, cross hydrogen bond F⋯H-N scalar couplings, and the atomic positions determined from the highresolution crystal structure of the β-PGM-MgF − 3 -G6P complex. The measurements provide independent validation of the structural and isoelectronic MgF − 3 model of near-transition state conformations.19F NMR | phosphoryl transfer enzyme | transition state analogue | trifluoromagnesate T he mono-and diesters of phosphoric acid have commanding and ubiquitous roles in all species of life. As structural components they show remarkable stability to spontaneous hydrolysis under near physiological conditions (25°C), with half-lives for P-O bond cleavage in phosphate diesters estimated at ca. 10 7 years and for monoesters ca. 10 12 years (1, 2). Yet, they are susceptible to enzyme-catalyzed hydrolysis and phosphoryl group transfer reactions either between two oxygens, or between oxygen and nitrogen or sulfur, with turnover numbers adequate to support a vast array of biological processes, e.g. Serratia nuclease k cat ca. 2; 500 s −1 (3), E. coli alkaline phosphatase k cat ≥ 45 s −1 (4), and human protein tyrosine phosphatase β k cat ca.

Etoposide is one of the most successful chemotherapeutic agents used for the treatment of human cancers. The drug kills cells by inhibiting the ability of topoisomerase II to ligate nucleic acids that it cleaves during the double-stranded DNA passage reaction. Etoposide is composed of a polycyclic ring system (rings A-D), a glycosidic moiety at the C4 position, and a pendant ring (E-ring) at the C1 position. Although drug-enzyme contacts, as opposed to drug-DNA interactions, mediate the entry of etoposide into the topoisomerase II-drug-DNA complex, the substituents on etoposide that interact with the enzyme have not been identified. Therefore, saturation transfer difference [ 1 H]-nuclear magnetic resonance spectroscopy and protein-drug competition binding assays were employed to define the groups on etoposide that associate with yeast topoisomerase II and human topoisomerase IIα. Results indicate that the geminal protons of the A-ring, the H5 and H8 protons of the B-ring, as well as the H2' and H6' protons and the 3'-and 5'-methoxyl protons of the pendent E-ring interact with both enzymes in the binary protein-ligand complexes. In contrast, no significant nuclear Overhauser enhancement signals arising from the C-ring, the D-ring, or the C4 glycosidic moiety were observed with either enzyme, suggesting that there is limited or no contact between these portions of etoposide and topoisomerase II in the binary complex. The functional importance of E-ring substituents was confirmed by topoisomerase II-mediated DNA cleavage assays.Etoposide is one of the most successful chemotherapeutic agents used for the treatment of human cancers (1-4). The drug currently is in its third decade of clinical use and is front line

Reported is a systematic study of the "fitness" (in terms of kcat/Km) of a series of phosphonate mimics of glucose 6-phosphate (G6P) as unnatural substrates for G6P dehydrogenase from Leuconostoc mesenteroides. The four G6P analogues (9, 10, 15a, and 15b) differ only in the degree of fluorination at the "bridging" phosphonate carbon. All have been synthesized from benzyl 6-O-trifluoromethanesulfonyl-2,3,4-tri-O-benzyl beta-D-glucopyranoside (6). The phosphonates with bridging CH2 (9) and CF2 (10) groups are cleanly obtained by direct displacements with the appropriate LiX2CP(O)(OEt)2 reagents (X = H, F) in 15 min at -78 degrees C. For the (alpha-monofluoro)alkylphosphonates (15a/b), homologation of 6 is achieved via lithiodithiane-mediated triflate displacement, followed by aldehyde unmasking [CaCO3, Hg(ClO4)2, H2O]. Addition of diethyl phosphite anion produces diastereomeric, (alpha-hydroxy)phosphonates 13a/b (1.4:1 ratio) which may be readily separated by chromatography. The stereochemistry of the minor diastereomer was established as 7(S) via X-ray crystallographic structure determination of its p-bromobenzoate derivative, 16b. Treatment of the major 7(R) diastereomer with DAST produces alpha-fluorinated phosphonate 14a, in modest yield, with inversion of configuration, as established, again, by X-ray crystallography. To our knowledge, this is first example of DAST-mediated fluorination of a (nonbenzylic, nonpropargylic) secondary (alpha-hydroxy)phosphonate and thus establishes the stereochemical course of this transformation. alpha-Deprotonation/kinetic quenching of 14a provides access to the 7(R)-epimer (14b). For all four protected phosphonates (7, 8, 14a, and 14b), diethyl phosphonate ester deprotection was carried out with TMSBr, followed by global hydrogenolytic debenzylation to produce the free phosphonates, as alpha/beta anomeric mixtures. Titrations of G6P itself and the free phosphonic acids provides second pKa values of 6.5 (1, bridging-O), 5.4 (10, bridging-CF2), 6.2 (14a, bridging-CHF), and 7.6 (9, bridging-CH2). Leuconostoc mesenteroides G6PDH-mediated oxidation and Lineweaver-Burk analysis yields normalized kcat/Km values of 0.043 (14b, bridging-7(R)-CHF), 0.11 (10, bridging-CF2), 0.23 (14b, bridging-CH2), and 0.46 (14a, bridging-7(S)-CHF) relative to G6P itself, largely reflecting differences in Km. The fact that kcat/Km increases by more than an order of magnitude in going from the 7(R)-alpha-monofluoroalkyl phosphonate (worst substrate) to the 7(S)-diastereomer (best substrate) is especially notable and is discussed in the context of the known phosphate binding pocket of this enzyme as revealed by X-ray crystallography (Adams, M. J. et al. Structure 1994, 2, 1073-1087).

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How changes in enzyme structure and dynamics facilitate passage along the reaction coordinate is a fundamental unanswered question. Here, we use time-resolved mix-and-inject serial crystallography (MISC) at an X-ray free electron laser (XFEL), ambient-temperature X-ray crystallography, computer simulations, and enzyme kinetics to characterize how covalent catalysis modulates isocyanide hydratase (ICH) conformational dynamics throughout its catalytic cycle. We visualize this previously hypothetical reaction mechanism, directly observing formation of a thioimidate covalent intermediate in ICH microcrystals during catalysis. ICH exhibits a concerted helical displacement upon active-site cysteine modification that is gated by changes in hydrogen bond strength between the cysteine thiolate and the backbone amide of the highly strained Ile152 residue. These catalysis-activated motions permit water entry into the ICH active site for intermediate hydrolysis. Mutations at a Gly residue (Gly150) that modulate helical mobility reduce ICH catalytic turnover and alter its pre-steady-state kinetic behavior, establishing that helical mobility is important for ICH catalytic efficiency. These results demonstrate that MISC can capture otherwise elusive aspects of enzyme mechanism and dynamics in microcrystalline samples, resolving long-standing questions about the connection between nonequilibrium protein motions and enzyme catalysis.

Sodium gammahydroxybutyrate (GHB) is an endogenous compound that has been under investigation in the management of narcolepsy for about two decades. The data confirm that GHB treatment decreases daytime sleepiness and episodes of cataplexy, sleep paralysis, and hypnagogic hallucinations. The current study evaluated the pharmacokinetics of GHB, given twice in one night to six narcoleptic patients who had been chronically taking GHB nightly on a similar basis. Results confirmed earlier reports and showed nonlinear pharmacokinetics. Maximum concentrations were reached in 40 +/- 6.2 and 35.7 +/- 7 minutes after the first and second dose respectively. Mean AUCinf was 17731.6 +/- 4867 mg/mL/m. Mean GHB T1/2 was 53 +/- 19 minutes. GHB elimination appears to be capacity-limited in some patients when administered at a fixed dose of 3 g twice nightly at a 4-hour interval.

This report presents an overview of the family of naturally occurring 'vinylic' amino acids, namely those that feature a C-C double bond directly attached to the α-carbon, along the side chain. Strategies that have been brought to bear on the stereocontrolled synthesis of these olefinic amino acids are surveyed. The mechanistic diversity by which such 'vinylic triggers' can be actuated in a PLP (pyridoxal phosphate) enzyme active site is then highlighted by discussions of vinylglycine (VG), its substituted congeners, particularly AVG [4E-(2'-aminoethoxy)vinylglycine], and a naturally occurring VG-progenitor, SMM (S-methylmethionine).

A method has been devised for the rapid identification of nonsense mutations (UAG, UAA, UGA codons) in Salmonella . The mutations to be tested are reverted, and the revertants are replica-printed onto lactose plates spread with lawns of tester strains. These tester strains contain F′ lac episomes with nonsense mutations in the episomal Z gene. The revertants are infected with the episome from the tester strain lawn. Because S. typhimurium is unable to ferment lactose, only those revertants which have nonsense suppressors are able to grow on lactose. If colonies appear on the lactose plate, it may be concluded that the original strain carries a nonsense mutation, since nonsense suppressors suppress the mutant phenotype.