Mechanistic basis for the emergence of catalytic competence against carbapenem antibiotics by the GES family of beta-lactamases

A major mechanism of bacterial resistance to beta-lactam antibiotics (penicillins, cephalosporins, carbapenems, etc.) is the production of beta-lactamases. A handful of class A beta-lactamases have been discovered that have acquired the ability to turn over carbapenem antibiotics. This is a disconcerting development, as carbapenems are often considered last resort antibiotics in the treatment of difficult infections. The GES family of beta-lactamases constitutes a group of extended spectrum resistance enzymes that hydrolyze penicillins and cephalosporins avidly. A single amino acid substitution at position 170 has expanded the breadth of activity to include carbapenems. The basis for this expansion of activity is investigated in this first report of detailed steady-state and pre-steady-state kinetics of carbapenem hydrolysis, performed with a class A carbapenemase. Monitoring the turnover of imipenem (a carbapenem) by GES-1 (Gly-170) revealed the acylation step as rate-limiting. GES-2 (Asn-170) has an enhanced rate of acylation, compared with GES-1, and no longer has a single rate-determining step. Both the acylation and deacylation steps are of equal magnitude. GES-5 (Ser-170) exhibits an enhancement of the rate constant for acylation by a remarkable 5000-fold, whereby the enzyme acylation event is no longer rate-limiting. This carbapenemase exhibits k(cat)/K(m) of 3 x 10(5) m(-1)s(-1), which is sufficient for manifestation of resistance against imipenem.

Regiospecific syntheses of 6alpha-(1R-Hydroxyoctyl)penicillanic acid and 6beta-(1R-hydroxyoctyl)penicillanic acid as mechanistic probes of class D beta-lactamases

The unique hydrophobic surface patches in class D beta-lactamases presented an opportunity for designing two compounds, 6alpha-(1R-hydroxyoctyl)penicillanic acid and 6beta-(1R-hydroxyoctyl)penicillanic acid, as mechanistic probes of these enzymes. In a sequence of three synthetic steps from benzhydryl 6,6-dibromopenicillanate, the targeted compounds were prepared in a stereospecific manner.

Use of two-year disease-free survival (DFS) as a primary endpoint in stage III adjuvant colon cancer trials with fluoropyrimidines with or without oxaliplatin or irinotecan: New data from 12,676 patients from MOSAIC, X-ACT, PETACC-3, NSAPB C-06 and C-07, and C89803

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Background: The ACCENT group previously validated DFS with 3 years (yr) median follow-up (f-up) based on 20,898 pts from trials testing 5-FU based regimens (rx) (3yr DFS) as an endpoint to predict overall survival with 5 yr median f-up (5yr OS) (Sargent, JCO 2005). ACCENT further proposed (1) 2yr DFS predicts 5yr OS, (2) a stronger relationship between DFS and OS in stage III pts (Sargent JCO 2007) and (3) 6 or 7 yrs are necessary to demonstrate DFS and OS association in future trials due to extended survival following recurrence (de Gramont ASCO 2008). The relationship between endpoints in more recent trials with oral fluoropyrimidines, oxaliplatin, and irinotecan is unknown. Methods: Concordance between 2 and 3yr DFS, and 5 and 6yr OS was examined in 6 randomized phase III trials from 1997–2002. Individual data for 12,676 pts was analyzed; 2 trials tested oxaliplatin, 2 irinotecan, and 2 oral rx vs 5-FU/LV control. Association between DFS and OS hazard ratios (HRs) via weighted least squares (WLS), and concordance between predicted and actual within-trial HRs, were calculated overall and for stage III pts. Results: Overall association between 3 yr DFS and 5 yr OS HRs was reduced compared to the prior ACCENT analysis (Table). In stage III pts, the association between DFS and OS HRs remained strong. Observed 5 and 6yr OS HRs were predicted accurately by 2yr DFS overall and in stage III pts (within 95% prediction limits in all trials). In all pts, DFS HRs were more highly associated with 6 vs 5yr OS HRs. Conclusions: In recent trials in stage III pts, DFS HRs based on 2yr median f-up are highly predictive of 5 and 6yr OS HRs. In all pts the association between DFS and OS HRs is stronger for 6yr OS, but 7yr follow-up may be required. These data support 3yr DFS as a primary endpoint for modern stage III trials, and indicate that 2yr DFS would also be an appropriate primary endpoint.

[Table: see text]

[Table: see text]

Symptom burden in cancer survivors: A report from the American Cancer Society's studies of cancer survivors

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Background: Cancer survivors may have persistent symptoms as a result of the cancer or the late effects of cancer treatment, although few studies have assessed symptom burden after completion of cancer therapy. We described symptom burden and risk factors for high symptom burden in a population-based sample of cancer survivors. Methods: Cancer survivors were drawn from 11 state cancer registries and surveyed by telephone or mail approximately 1 year after diagnosis; 3697 respondents completed the Modified Rotterdam Symptom Checklist (RSCL-M) and Profile of Mood States-37 (POMS-37). Factor analyses were used to generate composite scores from RSCL-M and POMS-37.Two-step cluster analyses using these composite scores were performed to identify cancer survivors with high symptoms. Results: Six composite scores (pain, nausea/vomiting, fatigue, poor bowel and bladder control, weight loss, and unspecified symptoms) were generated for physical symptoms (RSCL-M). Five composite scores (anxious/depressed, miserable, energetic, exhausted, and annoyed) were generated for mental symptoms (POMS-37). Cluster analyses revealed that 20% of survivors reported both high physical and high mental symptoms. Multivariate logistic regression revealed risk factors associated with high physical and high mental symptoms: younger than 55, household income lower than $40,000, high school or lower education level, unemployed when diagnosed with cancer, lack of health insurance, distant cancer, one or more comorbid conditions, diagnosis of lung cancer, and previous chemotherapy. Conclusions: One of five cancer survivors in this survey belonged to a group with high physical and mental symptoms, which were associated with certain risk factors. These findings will help clinicians and researchers identify subgroups of survivors most likely to report high symptoms and therefore most in need of symptom management. The high symptoms reported by survivors without insurance may have policy implications.

No significant financial relationships to disclose.

Health-related quality of life indicators and overall quality of life: Results from a cluster analysis on baseline EORTC QLQ-C30 data from 6,739 cancer patients

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Background: Increasingly randomized controlled trials in cancer research include Health-related Quality of Life (HRQoL) alongside traditional biomedical outcome measures. The majority of these trials focus on a general cancer HRQoL measure. The objective of this meta-analysis was to identify which HRQoL indicators influence a patient's overall quality of life, in order to better understand the changes in such a generic scale. Methods: Retrospective pooling of 29 European Organisation for Research and Treatment of Cancer (EORTC) clinical trials, among 10 cancer sites, yielded baseline EORTC QLQ-C30 data for a total of 6,739 patients. A cluster analysis, using Ward's method, was performed to determine how the 15 HRQoL indicators, and the Global Health scale (GH) in particular, cluster overall and by cancer characteristics. Cronbach's alpha coefficient (α) was used to measure internal consistency. Dendrograms of the HRQoL indicators were plotted for each cancer type. Results: Three main clusters emerged: a physical function related cluster (physical functioning, role functioning, fatigue and pain, α = 0.83), a psychological function related cluster (emotional functioning, cognitive functioning and insomnia, α = 0.64) and a gastrointestinal cluster (nausea and vomiting and appetite loss, α = 0.68). The GH scale was found to be part of the physical function cluster in the overall dataset (α = 0.85). This result was reproduced for both metastatic and non-metastatic patients. When looking across the 10 different cancer sites, the GH scale was mainly linked with a physical component in brain, head and neck, lung, melanoma, ovarian, pancreatic and prostate cancer. However, in breast and testicular cancer, GH was more strongly associated with the emotional scales. Conclusions: This study shows that the GH scale of the EORTC QLQ-C30 is most strongly linked with a patient's physical status. This result is consistent across stage of disease and most cancer sites. The different results seen in patients with breast and testicular cancer deserve additional investigation.

No significant financial relationships to disclose.

A potent gelatinase inhibitor with anti-tumor-invasive activity and its metabolic disposition

Metastatic tumors lead to more than 90% fatality. Despite the importance of invasiveness of tumors to poor disease outcome, no anti-invasive compounds have been commercialized. We describe herein the synthesis and evaluation of 4-(4-(thiiranylmethylsulfonyl)phenoxy)-phenyl methanesulfonate (compound 2) as a potent and selective inhibitor of gelatinases (matrix metalloproteinases-2 and -9), two enzymes implicated in invasiveness of tumors. It was demonstrated that compound 2 significantly attenuated the invasiveness of human fibrosarcoma cells (HT1080). The metabolism of compound 2 involved hydroxylation at the alpha-methylene, which generates sulfinic acid, thiirane ring-opening, followed by methylation and oxidation, and cysteine conjugation of both the thiirane and phenyl rings.

A sperm GPI-anchored protein elicits sperm-cumulus cross-talk leading to the acrosome reaction

The acrosome reaction has long been thought to be induced by the zona pellucida. Here we report the identification and function of a novel human sperm glycosylphosphatidylinositol (GPI)-anchored membrane protein, NYD-SP8. The release of the protein during sperm-egg interaction and its binding to the cumulus, the first layer of egg investment, elicits cross-talk between the gametes and produces calcium dependant release of progesterone, which lead to the acrosome reaction. An in vivo mouse model of NYD-SP8 immunization is also established showing a reduced fertility rate. Thus, contrary to accepted dogma, our study demonstrates for the first time that, prior to reaching the zona pellucida, sperm may release a surface protein that acts on the cumulus cells leading to the acrosome reaction, which may be important for determining the outcome of fertilization.

IGF-1 regulation of type II collagen and MMP-13 expression in rat endplate chondrocytes via distinct signaling pathways

OBJECTIVE

Abnormal maturation and ossification of the endplate chondrocytes play a central role in the pathogenesis of degenerative disorders of the cervical spine. It is widely held that insulin like growth factor-1 (IGF-1) stimulates chondrocyte proliferation and inhibits chondrocyte terminal differentiation both in vitro and in vivo. However, the mechanism underlying such regulation is not fully understood. The present study aimed to determine the role of IGF-1 on the mRNA expression of collagen type II, alpha 1 (Col2a1) and matrix metallopeptidase 13 (MMP-13) in rat endplate chondrocytes. The possible pathways that transduce IGF-1 effects such as phosphatidylinositol-3 (PI-3)-kinase (PI3K) and mitogen activated protein kinase (MAPK) were also investigated in these cells.

METHODS

Cultured endplate chondrocytes harvested from rat cervical spines were treated with IGF-1 (100ng/ml), and the changes in Col2a1 and MMP-13 mRNA were monitored with real-time polymerase chain reaction (PCR). MMP-13 activity was also assayed. Activation of signaling proteins was evaluated by western blot analysis. Cells were also treated with pharmacological agents that block PI3K and MAPK signaling pathways.

RESULTS

IGF-1 increased Col2a1 mRNA expression in rat endplate chondrocytes in a time- and dose-dependent manner. IGF-1 treatment resulted in a fourfold increase of Col2a1 mRNA with the effect maximizing at 24h. In contrast, IGF-1 treatment for 24h caused a roughly 50% reduction in MMP-13 mRNA. Similar effects were seen on the protein levels of type II collagen (col2) and MMP-13. Consistent with these results, IGF-1 also repressed MMP-13 activity. IGF-1 activated both the PI3K and the extracellular signal-regulated kinase (ERK) pathways as evidenced by phosphorylation of either Akt or ERK1/2 (respectively). The PI3K inhibitor Wartmannin significantly inhibited the IGF-1 effect on Col2a1 mRNA expression but did not affect IGF-1-induced repression of MMP-13 expression. In contrast, the ERK/MAPK inhibitor PD98059 significantly inhibited the effect of IGF-1 on MMP-13 mRNA repression and enhanced IGF-1-induced Col2a1 mRNA expression.

CONCLUSIONS

In rat endplate chondrocytes the PI3K pathway mainly transduces IGF-1 effect on col2 expression while the ERK pathway mediates IGF-1 effect on MMP-13 expression.

Clinical analysis of nervous system involvement in ANCA-associated systemic vasculitides

OBJECTIVE

To assess the clinical features of nervous system (NS) involvement in patients with ANCA-associated vasculitides (AAV), including microscopic polyangiitis (MPA), Wegener's granulomatosis (WG), and Churg-Strauss syndrome (CSS).

METHODS

One hundred and seventy-nine patients admitted to Peking Union Medical College Hospital from 1995 to 2008, including 93 cases of MPA, 61 cases of WG, and 25 cases of CSS, were enrolled in this study. Medical charts including demographic data, clinical features, laboratory findings, treatments and outcomes were systematically reviewed.

RESULTS

NS involvements were observed in 36.6% of MPA, 50.8% of WG, and 76.0% of CSS patients. Peripheral neuropathy predominated in each type of AAV. In CSS and MPA, the majority was mononeuritis multiplex and distal symmetrical polyneuropathy, whereas, differently, 64.5% of WG patients with NS involvement had cranial neuropathy. Central nervous system (CNS) involvement accounted for 21.1%, 29.4%, and 32.3% of neuropathy respectively in CSS, MPA and WG patients, including arachnoid hemorrhage, cerebrovascular neuro-pathy, meningitis, and diffuse brain damage. 157 (87.7%) AAV patients responded to treatment with high dose of prednisone plus immunosuppressants. Thirteen (14.0%) MPA and four (6.6%) WG patients died. The leading causes of death were diffuse alveolar hemorrhage (DAH) (6, 35.3%) and infection (6, 35.3%). No patient died directly of neuropathy.

CONCLUSION

NS involvement was common in AAVs and the characteristic of NS involvement was different among MPA, WG and CSS patients. DAH and infection instead of NS damage remained the leading causes of death in AAVs.

Taxonomy of the Streptomyces strain ZG0656 that produces acarviostatin alpha-amylase inhibitors and analysis of their effects on blood glucose levels in mammalian systems

AIMS

To clarify the taxonomic status of strain ZG0656 and analyse the effects of its acarviostatin products on blood glucose levels in mammalian systems.

METHODS AND RESULTS

Our program to screen for new alpha-amylase inhibitors led to the isolation of strain ZG0656. The polyphasic taxonomic study revealed that strain ZG0656 represents a novel variation of Streptomyces coelicoflavus, for which we propose the name S. coelicoflavus var. nankaiensis. Four chemically distinct alpha-amylase inhibitors, acarviostatins I03, II03, III03 and IV03, were isolated from strain ZG0656. Acarviostatins III03 and IV03 are both novel oligomers. All four acarviostatins are mixed noncompetitive porcine pancreas alpha-amylase inhibitors. Acarviostatin III03 is the most potent alpha-amylase inhibitor known to date. Moreover, in the in vitro and in vivo experiments, acarviostatins III03 showed significant inhibition of starch hydrolysis and glucose transfer to blood.

CONCLUSIONS

Strain ZG0656 is a novel variation of S. coelicoflavus, whose products are novel effective alpha-amylase inhibitors. Among the products, acarviostatins III03 could significantly depress blood glucose levels in mammalian systems and be developed towards a possible therapeutic agent for diabetes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Acarviostatin III03 is the most potent alpha-amylase inhibitor known to date. The oligomer will benefit the research on the relationship between alpha-amylase and various inhibitors and will offer more choices in diabetes treatments.

Identification and role of the homodimerization interface of the glycosylphosphatidylinositol-anchored membrane type 6 matrix metalloproteinase (MMP25)

The membrane type (MT) 6 matrix metalloproteinase (MMP) (MMP25) is a glycosylphosphatidylinositol-anchored matrix metalloproteinase (MMP) that is highly expressed in leukocytes and in some cancer tissues. We previously showed that natural MT6-MMP is expressed on the cell surface as a major reduction-sensitive form of M(r) 120, likely representing enzyme homodimers held by disulfide bridges. Among the membrane type-MMPs, the stem region of MT6-MMP contains three cysteine residues at positions 530, 532, and 534 which may contribute to dimerization. A systematic site-directed mutagenesis study of the Cys residues in the stem region shows that Cys(532) is involved in MT6-MMP dimerization by forming an intermolecular disulfide bond. The mutagenesis data also suggest that Cys(530) and Cys(534) form an intramolecular disulfide bond. The experimental observations on cysteines were also investigated by computational studies of the stem peptide, which validate these proposals. Dimerization is not essential for transport of MT6-MMP to the cell surface, partitioning into lipid rafts or cleavage of alpha-1-proteinase inhibitor. However, monomeric forms of MT6-MMP exhibited enhanced autolysis and metalloprotease-dependent degradation. Collectively, these studies establish the stem region of MT6-MMP as the dimerization interface, an event whose outcome imparts protease stability to the protein.

Investigation of the mechanism of the cell wall DD-carboxypeptidase reaction of penicillin-binding protein 5 of Escherichia coli by quantum mechanics/molecular mechanics calculations

Penicillin-binding protein 5 (PBP 5) of Escherichia coli hydrolyzes the terminal D-Ala-D-Ala peptide bond of the stem peptides of the cell wall peptidoglycan. The mechanism of PBP 5 catalysis of amide bond hydrolysis is initial acylation of an active site serine by the peptide substrate, followed by hydrolytic deacylation of this acyl-enzyme intermediate to complete the turnover. The microscopic events of both the acylation and deacylation half-reactions have not been studied. This absence is addressed here by the use of explicit-solvent molecular dynamics simulations and ONIOM quantum mechanics/molecular mechanics (QM/MM) calculations. The potential-energy surface for the acylation reaction, based on MP2/6-31+G(d) calculations, reveals that Lys47 acts as the general base for proton abstraction from Ser44 in the serine acylation step. A discrete potential-energy minimum for the tetrahedral species is not found. The absence of such a minimum implies a conformational change in the transition state, concomitant with serine addition to the amide carbonyl, so as to enable the nitrogen atom of the scissile bond to accept the proton that is necessary for progression to the acyl-enzyme intermediate. Molecular dynamics simulations indicate that transiently protonated Lys47 is the proton donor in tetrahedral intermediate collapse to the acyl-enzyme species. Two pathways for this proton transfer are observed. One is the direct migration of a proton from Lys47. The second pathway is proton transfer via an intermediary water molecule. Although the energy barriers for the two pathways are similar, more conformers sample the latter pathway. The same water molecule that mediates the Lys47 proton transfer to the nitrogen of the departing D-Ala is well positioned, with respect to the Lys47 amine, to act as the hydrolytic water in the deacylation step. Deacylation occurs with the formation of a tetrahedral intermediate over a 24 kcal x mol(-1) barrier. This barrier is approximately 2 kcal x mol(-1) greater than the barrier (22 kcal x mol(-1)) for the formation of the tetrahedral species in acylation. The potential-energy surface for the collapse of the deacylation tetrahedral species gives a 24 kcal x mol(-1) higher energy species for the product, signifying that the complex would readily reorganize and pave the way for the expulsion of the product of the reaction from the active site and the regeneration of the catalyst. These computational data dovetail with the knowledge on the reaction from experimental approaches.

Influence of mitochondrial enzyme deficiency on adult neurogenesis in mouse models of neurodegenerative diseases

Mitochondrial defects including reduction of a key mitochondrial tricarboxylic acid cycle enzyme alpha-ketoglutarate-dehydrogenase complex (KGDHC) are characteristic of many neurodegenerative diseases. KGDHC consists of alpha-ketoglutarate dehydrogenase, dihydrolipoyl succinyltransferase (E2k), and dihydrolipoamide dehydrogenase (Dld) subunits. We investigated whether Dld or E2k deficiency influences adult brain neurogenesis using immunohistochemistry for the immature neuron markers, doublecortin (Dcx) and polysialic acid-neural cell adhesion molecule, as well as a marker for proliferation, proliferating cell nuclear antigen (PCNA). Both Dld- and E2k-deficient mice showed reduced Dcx-positive neuroblasts in the subgranular zone (SGZ) of the hippocampal dentate gyrus compared with wild-type mice. In the E2k knockout mice, increased immunoreactivity for the lipid peroxidation marker, malondialdehyde occurred in the SGZ. These alterations did not occur in the subventricular zone (SVZ). PCNA staining revealed decreased proliferation in the SGZ of E2k-deficient mice. In a transgenic mouse model of Alzheimer's disease, Dcx-positive cells in the SGZ were also reduced compared with wild type, but Dld deficiency did not exacerbate the reduction. In the malonate lesion model of Huntington's disease, Dld deficiency did not alter the lesion-induced increase and migration of Dcx-positive cells from the SVZ into the ipsilateral striatum. Thus, the KGDHC subunit deficiencies associated with elevated lipid peroxidation selectively reduced the number of neuroblasts and proliferating cells in the hippocampal neurogenic zone. However, these mitochondrial defects neither exacerbated certain pathological conditions, such as amyloid precursor protein (APP) mutation-induced reduction of SGZ neuroblasts, nor inhibited malonate-induced migration of SVZ neuroblasts. Our findings support the view that mitochondrial dysfunction can influence the number of neural progenitor cells in the hippocampus of adult mice.

Aminoglycoside 2''-phosphotransferase type IIIa from Enterococcus

Aminoglycoside 2''-phosphotransferases mediate high level resistance to aminoglycoside antibiotics in Gram-positive microorganisms, thus posing a serious threat to the treatment of serious enterococcal infections. This work reports on cloning, purification, and detailed mechanistic characterization of aminoglycoside 2''-phosphotransferase, known as type Ic enzyme. In an unexpected finding, the enzyme exhibits strong preference for guanosine triphosphate over adenosine triphosphate as the phosphate donor, a unique observation among all characterized aminoglycoside phosphotransferases. The enzyme phosphorylates only certain 4,6-disubstituted aminoglycosides exclusively at the 2''-hydroxyl with k(cat) values of 0.5-1.0 s(-1) and K(m) values in the nanomolar range for all substrates but kanamycin A. Based on this unique substrate profile, the enzyme is renamed aminoglycoside 2''-phosphotransferase type IIIa. Product and dead-end inhibition patterns indicated a random sequential Bi Bi mechanism. Both the solvent viscosity effect and determination of the rate constant for dissociation of guanosine triphosphate indicated that at pH 7.5 the release of guanosine triphosphate is rate-limiting. A computational model for the enzyme is presented that sheds light on the structural aspects of interest in this family of enzymes.

Conformational analyses of thiirane-based gelatinase inhibitors

SB-3CT is a thiirane-containing inhibitor of the gelatinase class of matrix metalloprotease enzymes. In support of the mechanistic study of this inhibition, the conformational analyses of SB-3CT (and of two methyl-substituted derivatives) were undertaken using X-ray crystallography and molecular dynamics simulation.

Utility of QuantiFERON-TB Gold in-tube testing for latent TB infection in HIV-infected individuals

SETTING

Urban inner city human immunodeficiency virus (HIV) clinic.

OBJECTIVE

To evaluate tuberculin skin testing (TST) and QuantiFERON-TB Gold (QFT-G) testing in an HIV-infected population relative to the presence of risk factors for latent tuberculosis infection (LTBI).

DESIGN

Cross-sectional analysis of the response of a whole blood gamma interferon release assay to early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10) antigens and TST relative to known risk factors for LTBI.

RESULTS

Of 207 subjects enrolled, four were excluded due to missing data and three specimens yielded discordant results. Ten specimens were indeterminate due to inadequate response to mitogen. All indeterminate results occurred in subjects with CD(4) counts <200 cells/mm(3). Eleven QFT-G results and 13 TST results were positive. The concordance between TST and QFT-G was poor (kappa 0.38). QFT-G results were more likely than TST to be associated with risk factors for LTBI.

CONCLUSIONS

QFT-G, but not TST, showed a statistically significant association between the number of risk factors for LTBI and a positive result (OR 1.6). QFT-G testing may be more useful than TST in individuals with HIV infection.

Catalytic mechanism of penicillin-binding protein 5 of Escherichia coli

Penicillin-binding proteins (PBPs) and beta-lactamases are members of large families of bacterial enzymes. These enzymes undergo acylation at a serine residue with their respective substrates as the first step in their catalytic events. Penicillin-binding protein 5 (PBP 5) of Escherichia coli is known to perform a dd-carboxypeptidase reaction on the bacterial peptidoglycan, the major constituent of the cell wall. The roles of the active site residues Lys47 and Lys213 in the catalytic machinery of PBP 5 have been explored. By a sequence of site-directed mutagenesis and chemical modification, we individually introduced gamma-thialysine at each of these positions. The pH dependence of kcat/Km and of kcat for the wild-type PBP 5 and for the two gamma-thialysine mutant variants at positions 47 and 213 were evaluated. The pH optimum for the enzyme was at 9.5-10.5. The ascending limb to the pH optimum is due to Lys47; hence, this residue exists in the free-base form for catalysis. The descending limb from the pH optimum is contributed to by both Lys213 and a water molecule coordinated to Lys47. These results have been interpreted as Lys47 playing a key role in proton-transfer events in the course of catalysis during both the acylation and deacylation events. However, the findings for Lys213 argue for a protonated state at the pH optimum. Lys213 serves as an electrostatic anchor for the substrate.

Stereoselective epoxidation of 4-deoxypentenosides: a polarized-pi model

The high facioselectivity in the epoxidation of 4-deoxypentenosides (4-DPs) by dimethyldioxirane (DMDO) correlates with a stereoelectronic bias in the 4-DPs' ground-state conformations, as elucidated by polarized-pi frontier molecular orbital (PPFMO) analysis.