β-Xylosidases from filamentous fungi: an overview

β-Xylosidases are hydrolytic enzymes which play an important role in xylan degradation, hydrolyzing xylobiose and xylooligosaccharides to xylose from the non-reducing end. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they secrete β-xylosidases into the medium. Besides, fungal β-xylosidases are highly advantageous for their elevated activity levels and specificity. Interest in xylanolytic enzymes has been increasing, for their possible application in many biotechnological processes. This fact has driven the isolation, purification and characterization of several β-xylosidases. In this review, the mechanisms of action, substrate specificities, physicochemical characteristics, regulation at molecular level, molecular cloning and classification of filamentous fungal β-xylosidases are described. The potential industrial applications of fungal β-xylosidases will also be presented.     

Effects of EEG of the osmotic opening of the blood-brain barrier in rats

EEG activity was recorded in rats submitted to osmotic opening of the BBB by intracarotid mannitol infusion.This procedure produced an immediate short-lasting depression of the EEG and a tardive paroxysmal EEG activity. Both these phenomena were more relevant on the ipsilateral hemisphere. In some instances a tonico-clonic seizure was recorded.Pre-treatment with diazepam abolished the occurrence of the tardive EEG and behavioral modifications.In accord with previous findings, focal seizure activity is likely to be responsible for the metabolic abnormalities associated with osmotic opening of the BBB. This preparation therefore produces in the brain unphysiological states in respect to local metabolism and electrical function.     

Contribution to catalysis and stability of the five cysteines in Escherichia coli aspartate aminotransferase. Preparation and properties of a cysteine-free enzyme.

The five cysteines, at positions 82, 191, 192, 270, and 401, of Escherichia coli aspartate aminotransferase (AATase) were, individually and in some combinations, converted to alanine by site-directed mutagenesis (C82A, C191A, C192A, C270A, C401A). Cys-191, which is conserved in all AATase isozymes, was mutated to serine as well (C191S). A quintuple mutant, with all cysteines converted to alanines (Quint), was also constructed. The effects of these single and multiple mutations were examined by steady-state kinetics and urea denaturation. The thermal stabilities of Quint and of the wild-type enzyme (WT) were determined by differential scanning calorimetry. The mutants had kcat values up to 50% greater than that of WT and KMAsp and KM alpha-KG values up to 1.5- and 3.3-fold higher than that of WT. The mutants C82A and C191A exhibit nearly the same CM in urea denaturation experiments as WT, while the other single mutants and Quint are less stable, with CM differences of up to 0.7 M urea. Quint is also less thermostable than WT, with a delta TM of 3.3-4.4 degrees C. Thus the five cysteine replacements yield small, but significant, changes in catalytic and denaturation parameters, but none of the cysteines was found to be essential. The changes manifested in the mutation of the conserved Cys-191 to alanine are no greater than those observed with the four nonconserved cysteines. We consider the evolutionary implications of these findings.     

Influence of exogenous NO donator and variations in the Ca<Superscript>2+</Superscript> content on transport activity related to tonoplast proton pumps in ontogenesis and under hyperosmotic stress

The changes in transport activity of tonoplast proton pumps under the influence of exogenous NO donator and modulation of Ca²⁺ concentration jointly and separately were investigated at different stages of ontogenesis and under hyperosmotic stress. The results suggest that both exogenous NO donator and Ca²⁺ ions can influence the activity of transport processes related to tonoplast and this influence is especially evident in the period of growth and accumulation of metabolites. Under hyperosmotic stress, H⁺-pyrophosphatase plays a more important role than H⁺-ATPase: the activity of the former increases 2.3-fold compared to the control osmotic conditions, whereas the activity of H⁺-ATPase is practically unchanged. H⁺-pyrophosphatase was more responsive to the presence of exogenous NO donator and to variations in Ca²⁺ concentration. The effects of exogenous NO donator on tonoplast proton pumps depended on the concentration of Ca²⁺, which apparently can mediate NO action.     

Structure–antimicrobial activity of some natural isocoumarins and their analogues

Three naturally occurring isocoumarins (paepalantine, paepalantine 9-O-beta-D-glucopyranoside and paepalantine 9-O-beta-D-allopyranosyl(1 --> 6) glucopyranoside) and two semi-synthetic analogues, 9,10-acylated compound and 9-OH-10-methylated compound, structurally similar to paepalantine, were evaluated for antimicrobial activity using a spectrophotometric microdilution technique. The paepalantine was active against S. aureus, S. epidermidis, and E. faecalis while the other four compounds proved ineffective against all microorganisms tested at concentrations of 500 microg/ml. Variations in phenolic substitution at OH-9 and/or OH-10 in the paepalantine molecule resulted in compounds without antimicrobial activity. A combination of structural features, two phenolic groups as cathecolic system, forms an oxygenated system arrangement that may reflect the potentially antimicrobial properties of paepalantine.     

Penetration of analogues of H2O and CO2 in proteins studied by room temperature phosphorescence of tryptophan.

The influence of the protein matrix on the reactivity of external molecules with a species buried within the protein interior is considered in two general ways: (1) there may be structural fluctuations that allow for the diffusive penetration of the small molecules and/or (2) the external molecule may react over a distance. As a means to study the protein matrix, a reactive species within the protein can be formed by exciting tryptophan to the triplet state, and then the reaction of the triplet-state molecule with an external molecule can be monitored by a decrease in phosphorescence. In this work, the quenching ability (i.e., reactivity) was examined for H2S, CS2, and NO2- acting on tryptophan phosphorescence in parvalbumin, azurin, horse liver alcohol dehydrogenase, and alkaline phosphatase. A comparison of charged versus uncharged quenchers (H2S vs SH- and CS2 vs NO2-) reveals that the uncharged molecules are much more effective than charged species in quenching the phosphorescence of fully buried tryptophan, whereas the quenching for exposed tryptophan is relatively independent of the charge of the quencher. This is consistent with the view that uncharged triatomic molecules can penetrate the protein matrix to some extent. The energies of activation of the quenching reaction are low for the charged quenchers and higher for the uncharged CS2. A model is presented in which the quenchability of a buried tryptophan is inversely related to the distance from the surface when diffusion through the protein is the rate-limiting step.(ABSTRACT TRUNCATED AT 250 WORDS)     

Becoming a face expert.

Young children do not form representations of newly encountered faces as efficiently as do adults. A first step in explaining this difference, like any age-related change, is locating its source. A major source of the improvement is acquisition of knowledge of faces per se, as opposed to age-related changes in general pattern encoding or memorial skills. Two consequences of expertise at individualizing members of classes that share a basic configuration are known: a large inversion effect and a caricature advantage. It is possible that both of these effects reflect increased reliance, with expertise, on configuration distinguishing features. Several phenomena that indicate that inversion interferes with the encoding of configural aspects of faces are reviewed. Finally, developmental data are presented that confirm the suspicion that there are at least two distinct sources of the vulnerability of face encoding to inversion, perhaps reflecting two distinct senses of 'configural encoding' of faces, only one of which is implicated in adult expertise at face encoding.     

Antagonism by theophylline of respiratory inhibition induced by adenosine

The effects on respiration of an analogue of adenosine, L-2-N6-(phenylisopropyl)adenosine (PIA), and of the methylxanthine, theophylline, were determined in 19 vagotomized glomectomized cats whose end-tidal PCO2 was kept constant by means of a servo-controlled ventilator. Integrated phrenic nerve activity was used to represent respiratory output. Our results show that PIA, whether given systemically or into the third cerebral ventricle, depressed respiration. Systemically administered theophylline stimulated respiration. Theophylline given intravenously, or into the third ventricle not only reversed the depressive effects of previously administered PIA but caused further increases of respiration above the control level. Prior systemic administration of theophylline blocked both respiratory and hypotensive effects of subsequently administered PIA. Effects of either agent on medullary extracellular fluid pH did not explain the results. We conclude that the adenosine analogue PIA, acts to inhibit neurons in the brain that are involved in the control of respiration and that its effects are blocked by theophylline. We suggest that adenosine acts as a tonic modulator of respiration and that theophylline stimulates breathing by competitive antagonism of adenosine at neuronal receptor sites.