Mechanism of Acyl Transfer by the Class a Serine Beta-Lactamase of Streptomyces Albus G

in Biochemical Journal (1991), 279(Pt 1), 213-221

Optimization by energy minimization of stable complexes occurring along the pathway of hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase has highlighted a ... [more ▼]

Optimization by energy minimization of stable complexes occurring along the pathway of hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase has highlighted a proton shuttle that may explain the catalytic mechanism of the beta-lactamases of class A. Five residues, S70, S130, N132, T235 and A237, are involved in ligand binding. The gamma-OH group of T235 and, in the case of benzylpenicillin, the gamma-OH group of S130 interact with the carboxylate group, on one side of the ligand molecule. The side-chain NH2 group of N132 and the carbonyl backbone of A237 interact with the exocyclic CONH amide bond, on the other side of the ligand. The backbone NH groups of S70 and A237 polarize the carbonyl group of the scissile beta-lactam amide bond. Four residues, S70, K73, S130 and E166, and two water molecules, W1 and W2, perform hydrolysis of the bound beta-lactam compound. E166, via W1, abstracts the proton from the gamma-OH group of S70. While losing its proton, the O-gamma atom of S70 attacks the carbonyl carbon atom of the beta-lactam ring and, concomitantly, the proton is delivered back to the adjacent nitrogen atom via W2, K73 and S130, thus achieving formation of the acyl-enzyme. Subsequently, E166 abstracts a proton from W1. While losing its proton, W1 attacks the carbonyl carbon atom of the S70 ester-linked acyl-enzyme and, concomitantly, re-entry of a water molecule W'1 replacing W1 allows E166 to deliver the proton back to the same carbonyl carbon atom, thus achieving hydrolysis of the beta-lactam compound and enzyme recovery. The model well explains the differences found in the kcat. values for hydrolysis of benzylpenicillin and cephalosporin C by the Streptomyces albus G beta-lactamase. It also explains the effects caused by site-directed mutagenesis of the Bacillus cereus beta-lactamase I [Gibson, Christensen [less ▲]

Patron d'utilisatin des gîtes diurnes par deux loutres radiopistées dans le Marais Poitevin

in Artois, Marc (Ed.) XVème colloque francophone de mammalogie (1991, October)

Structure organisationnelle des PME : de la théorie à la pratique ou les dangers de l'entreprise familiale

in La Vie au Bureau (1991)

Cet article rappelle d'abord les principales caractéristiques organisationnelles de PME familiales et insiste notamment sur l'interaction constante entre la vie de l'entreprise et la vie de la famille ... [more ▼]

Cet article rappelle d'abord les principales caractéristiques organisationnelles de PME familiales et insiste notamment sur l'interaction constante entre la vie de l'entreprise et la vie de la famille dans la gestion des entreprises familiales. Il met ensuite en lumière divers comportements types, qui peuvent amener une PME familiale à traverser difficilement des situations de crise organisationnelle. [less ▲]

MG1131 + 0456 - Discovery of the optical Einstein ring with the NTT

in Astronomy and Astrophysics (1991), 250

Deep optical imaging and spectroscopy have been obtained for the well known radio Einstein ring MG1131 + 0456 with the ESO/NTT. The direct CCD frames show a red emission excess at the exact location of ... [more ▼]

Deep optical imaging and spectroscopy have been obtained for the well known radio Einstein ring MG1131 + 0456 with the ESO/NTT. The direct CCD frames show a red emission excess at the exact location of the radio ring. Subsequent subtraction of the lensing galaxy, assumed to be an elliptical galaxy, results in the detection of an optical ring which is strikingly similar to the radio ring. Gravitational lensing models, including source reconstruction, predicts that the source is likely a radio-galaxy for which both the optical and extended radio emissions are sufficiently distorted by the foreground lensing galaxy as to produce a nearly complete ring. The present spectroscopy and broad band photometry suggest that the lensing galaxy has a spectral energy distribution typical of a nonevolved elliptical, tentatively at z of 0.85. The radio-galaxy associated with the radio-ring is likely to be an elliptical at larger redshift, tentatively of 1.13. [less ▲]

L'immixtion de l'expertise humaine parmi les outils de gestion : l'apport des systèmes-experts

in La Vie au Bureau (1991)

Cet article décrit ce que sont les systèmes experts à la fin des années '80, leur intérêt en tant qu'outils de gestion additionnels et la manière dont ils permettent de valoriser l'expertise humaine en ... [more ▼]

Cet article décrit ce que sont les systèmes experts à la fin des années '80, leur intérêt en tant qu'outils de gestion additionnels et la manière dont ils permettent de valoriser l'expertise humaine en tant que capital intangible et immatériel d'une entreprise, notamment d'une PME. [less ▲]

Effect of treatment with octreotide on the morphology of growth hormone secreting pituitary adenomas : study of 24 cases.

in Endocrine Pathology (1991), 2(3), 123-131

Twenty-four acromegalic patients were treated with octreotide subcutaneously for periods of 3 to 6 weeks (group I, 12 cases) or 6 months (group II, 12 cases) before transsphenoidal surgery. Radiological ... [more ▼]

Twenty-four acromegalic patients were treated with octreotide subcutaneously for periods of 3 to 6 weeks (group I, 12 cases) or 6 months (group II, 12 cases) before transsphenoidal surgery. Radiological studies performed in 19 patients before and at the end of this treatment period revealed no changes in 8 cases. In 8 other cases, a slight reduction in tumorsize was observed, and in 3 cases an important shrinkage was documented. At surgery, the adenomatous tissue appeared softer than in nonpretreated patients, facilitating the operation. Pathological examination revealed widening of perivascular spaces with accumulation of fibrous tissue and more crinophagy than in nonpretreated patients but failed to reveal morphologically pronounced cell involution as observed in prolactin-producing adenomas treated with dopamine agonists. No significant difference in frequency or extent of cellular changes was noted between the two groups. These morphological findings seem to be more consistent with a functional inhibition of growth hormone release than with cellular alterations induced by octreotide. [less ▲]

HR Carinae - A luminous blue variable surrounded by an arc-shaped nebula

in Astronomy and Astrophysics (1991), 248

New high and medium dispersion spectroscopic observations of the luminous blue variable (LBV) star HR Car are presented, showing that this star has a multiple shell expanding atmosphere and undergoes ... [more ▼]

New high and medium dispersion spectroscopic observations of the luminous blue variable (LBV) star HR Car are presented, showing that this star has a multiple shell expanding atmosphere and undergoes spectral variations apparently correlated with the light variations similarly to the other LBVs. Most striking are the very broad emission wings (FWZI of about 3000 km/s) of the Balmer lines, reminiscent from the broad lines observed in the spectra of Of and WR stars. A kinematic distance of 5.4 kpc and a bolometric magnitude of -9.4 put HR Car among the most luminous stars of the Galaxy. The narrow band filter imagery also reveals an arc-shaped jet like nebula associated with HR Car. Spectroscopic observations of this nebula reveal an emission nature with very low excitation. Like the star itself, the nebula is characterized by a significant N/O overabundance, probably due to the presence of nuclear processed material ejected by the star. [less ▲]

Kainate and Nmda Toxicity for Cultured Developing and Adult Rat Spiral Ganglion Neurons: Further Evidence for a Glutamatergic Excitatory Neurotransmission at the Inner Hair Cell Synapse

in Brain Research (1991), 555(1), 75-83

In the inner ear, the excitatory amino acid glutamate is a proposed neurotransmitter acting at the synapse between hair cells and afferent auditory neurons. Using cultures of 5-day-old rat auditory ... [more ▼]

In the inner ear, the excitatory amino acid glutamate is a proposed neurotransmitter acting at the synapse between hair cells and afferent auditory neurons. Using cultures of 5-day-old rat auditory neurons, we show that the afferent auditory neuronal population can be divided, on the basis of its sensitivity to the neuronotoxic effect of glutamate and its analogs, in at least 3 subpopulations, one responding to N-methyl-D-aspartate (NMDA), one responding to kainate and a third minor one unresponsive to NMDA, kainic acid and glutamate. No toxic effect of quisqualate is observed. The use of specific antagonists (kynurenate and 2-amino-5-phosphonovalerate (DAP-5) demonstrates the specificity of the receptors to the excitatory amino acids on the afferent auditory neurons. Afferent auditory neurons from adult rats can also be cultured and in these preparations only the large neurons are sensitive to glutamate, kainate and NMDA while the small neurons are not responsive, suggesting that a glutamatergic neurotransmission occurs only at this synapse between the inner hair cells and the large radial afferent auditory neurons. We also show that, in vitro, the organ of Corti releases, in response to an increased potassium concentration and in the presence of calcium, a toxic activity for the afferent auditory neurons that is antagonized by kynurenate and DAP-5. Pathophysiological implications are discussed. [less ▲]

Calibration of the Alpha Centauri system - Metallicity and age

in Astronomy and Astrophysics (1991), 247

The binary system Alpha Cen A and B, with its well known parallax, its high quality orbit and the reasonable hypothesis of a common origin and age for both components, is a useful test of stellar ... [more ▼]

The binary system Alpha Cen A and B, with its well known parallax, its high quality orbit and the reasonable hypothesis of a common origin and age for both components, is a useful test of stellar evolution models. Evolutionary sequences are computed for 1.085 and 0.9 solar masses with different values of Z, Y and alpha (= l/Hp). Adopting luminosities and effective temperatures as derived from the observations, Z = 0.04, Y = 0.32, alpha = 1.6 and t = 5 Gyr. The Z-value derived from evolutionary sequences is very sensitive to the difference in effective temperature between the two components. [less ▲]

"Mystification et démystification chez Alphonse Allais"

in French Studies (1991), XLV(3), 279-294