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J Steyaert

Publications and source records attributed to J Steyaert.

At least 19 recordsLinked to original sources
PubMed2006

Transition-state complex of the purine-specific nucleoside hydrolase of T. vivax: enzyme conformational changes and implications for catalysis.

Nucleoside hydrolases cleave the N-glycosidic bond of ribonucleosides. Crystal structures of the purine-specific nucleoside hydrolase from Trypanosoma vivax have previously been solved in complex with inhibitors or a substrate. All these structures show the dimeric T. vivax nucleoside hydrolase with an "open" active site with a highly flexible loop (loop 2) in its vicinity. Here, we present the crystal structures of the T. vivax nucleoside hydrolase with both soaked (TvNH-ImmH(soak)) and co-crystallised (TvNH-ImmH(co)) transition-state inhibitor immucillin H (ImmH or (1S)-1-(9-deazahypoxanthin-9-yl)-1,4-dideoxy-1,4-imino-D-ribitol) to 2.1 A and 2.2 A resolution, respectively. In the co-crystallised structure, loop 2 is ordered and folds over the active site, establishing previously unobserved enzyme-inhibitor interactions. As such this structure presents the first complete picture of a purine-specific NH active site, including leaving group interactions. In the closed active site, a water channel of highly ordered water molecules leads out from the N7 of the nucleoside toward bulk solvent, while Trp260 approaches the nucleobase in a tight parallel stacking interaction. Together with mutagenesis results, this structure rules out a mechanism of leaving group activation by general acid catalysis, as proposed for base-aspecific nucleoside hydrolases. Instead, the structure is consistent with the previously proposed mechanism of leaving group protonation in the T. vivax nucleoside hydrolase where aromatic stacking with Trp260 and an intramolecular O5'-H8C hydrogen bond increase the pKa of the N7 sufficiently to allow protonation by solvent. A mechanism that couples loop closure to the positioning of active site residues is proposed based on a comparison of the soaked structure with the co-crystallized structure. Interestingly, the dimer interface area increases by 40% upon closure of loop 2, with loop 1 of one subunit interacting with loop 2 of the other subunit, suggesting a relationship between the dimeric form of the enzyme and its catalytic activity.

Animals↗
PubMed2006

Semantic category fluency versus initial letter fluency over 60 seconds as a measure of automatic and controlled processing in healthy school-aged children.

Verbal fluency was operationalized as the number of words produced in a restricted category (i.e., semantic category [SCF] and words beginning with a given letter [ILF]) in 60 seconds. Word production in the first 15 seconds of either type of fluency task was defined as a measure of automatic information processing, whereas word production in the remaining 45 seconds (in 15-second periods) was taken as a measure of controlled information processing. Data revealed that over 60 seconds healthy children aged 8.4-9.7 years (n = 91) produced significantly more words and less incorrect responses on the SCF task than on the ILF task. Although word production was a function of both type of task and time, it was highest in the initial time slice of either type of fluency and decreased as time on task increased. Finally, no sex differences were found for any measure of performance on either type of fluency task. In contrast, the level of occupational achievement of the caregiver (LOA) appeared to be a determinant of the child's performance on either type of fluency task, indicating that LOA affects higher-order processes, such as the automation of newly learned verbal skills and effortful processing.

Child↗
PubMed2006

Expanded motor and psychiatric phenotype in autosomal dominant Segawa syndrome due to GTP cyclohydrolase deficiency.

BACKGROUND: Segawa syndrome due to GTP cyclohydrolase deficiency is an autosomal dominant disorder with variable expression, that is clinically characterised by l-dopa responsive, diurnally fluctuating dystonia and parkinsonian symptoms. OBJECTIVE: To delineate the neurological and psychiatric phenotype in all affected individuals of three extended families. METHODS: GTP cyclohydrolase deficiency was documented by biochemical analyses, enzymatic measurements in fibroblasts, and molecular investigations. All affected individuals were examined neurologically, and psychiatric data were systematically reviewed. RESULTS: Eighteen affected patients from three families with proven GTP cyclohydrolase deficiency were identified. Eight patients presenting at less than 20 years of age had typical motor symptoms of dystonia with diurnal variation. Five family members had late-presenting mild dopa-responsive symptoms of rigidity, frequent falls, and tendonitis. Among mutation carriers older than 20 years of age, major depressive disorder, often recurrent, and obsessive-compulsive disorder were strikingly more frequent than observed in the general population. Patients responded well to medication increasing serotonergic neurotransmission and to l-dopa substitution. Sleep disorders including difficulty in sleep onset and maintenance, excessive sleepiness, and frequent disturbing nightmares were present in 55% of patients. CONCLUSION: Physicians should be aware of this expanded phenotype in affected members of families with GTP cyclohydrolase deficiency.

Adolescent↗
PubMed2005

Controlled visuomotor preparation deficits in attention-deficit/hyperactivity disorder.

To the best of the authors' knowledge, there are no published reports on visuomotor preparation in attention-deficit/hyperactivity disorder (ADHD). This is unfortunate, because research suggests that ADHD is an output-related deficit, and suboptimal execution of tasks may be the result of incomplete visuomotor preparation. The authors compared 19 children with ADHD with 124 healthy and 120 pathological controls in terms of their performance (speed, speed variability, and accuracy) on the finger precuing test, a test measuring (automatic and controlled) visuomotor preparation. The data implied that children with ADHD have an impaired ability to engage in effortful, controlled visuomotor preparation activities. Fast, automatic response preparation was not affected by ADHD. In addition, children with ADHD showed more variability in overall test performance than other children. No group differences were found in response accuracy.

Attention Deficit Disorder with Hyperactivity↗
PubMed2004

Verbal fluency over time as a measure of automatic and controlled processing in children with ADHD.

The performance of ADHD children on semantic category fluency (SCF) versus initial letter fluency (ILF) tasks was examined. For each participant, word production was recorded for each 15-s time slice on each task. Performance on both fluency tasks was compared to test the hypothesis that children with ADHD are characterized by a performance deficit on the ILF task because performance on this task is less automated than performance on the SCF. Children classified with ADHD (N = 20) were compared to children with other psychopathology (N = 118) and healthy controls (N = 130). Results indicated that the groups could not be differentiated by the total number of words produced in 60 s in either fluency task. As hypothesized, a significant interaction of group by productivity over time by type of fluency task was found: ADHD children had more problems finding words in the first 15 s of the IFL than did children in the other two groups, and as compared with their performance on the SCF. Results were taken to indicate that children with ADHD symptoms show a delay in the development of automating skills for processing abstract verbal information.

Attention Deficit Disorder with Hyperactivity↗
PubMed2004

Hereditary motor and sensory neuropathy (HMSN) IA, developmental delay and autism related disorder in a boy with duplication (17)(p11.2p12).

We present a 6-year-old boy with moderate developmental delay, gait disturbance, autism related disorder and mild dysmorphic features. He was seen for evaluation of his retardation since the age of 2.8 years. At first sight, a cytogenetic analysis showed a normal 46,XY karyotype. Neurological examination at the age of 5.5 years revealed a motor and sensory polyneuropathy. A quantitative Southern blot with probes PMP22 and VAW409 specific for Charcot-Marie-Tooth type 1 (CMT1) disclosed a duplication which confirmed the diagnosis HMSN Ia. Subsequently, GTG banded metaphases were re-evaluated and a small duplication 17p was seen on retrospect. Additional FISH with probe LSISMS (Vysis) specific for the Smith-Magenis region at 17p11.2 again showed a duplication. Both parents had a normal karyotype and the duplication test for CMT1 showed normal results for both of them. The boy had a de novo 46,XY,dup(17)(p11.2p12) karyotype. The present observation confirms previous findings of mild psychomotor delay, neurobehavioural features and minor craniofacial anomalies as the major phenotypic features of dup(17)(p11.2) and dup(17)(p11.2p12); in cases of duplications comprising the PMP22 locus HMSN1 is associated. A recognizable facial phenotype emerges characterized by a broad forehead, hypertelorism, downslant of palpebral fissures, smooth philtrum, thin upper lip and ear anomalies.

Autistic Disorder↗
PubMed2003

The XYY syndrome: a follow-up study on 38 boys.

In the last decade there has been a significant increase in the proportion of XYY males detected prenatally, mostly as a fortuitous finding. It is of utmost importance to obtain a clear idea of the developmental profile of boys with karyotype 47,XYY and of possible problem areas during further development in order to inform the parents correctly during pregnancy and to provide an adequate surveillance later on. In this study we observed 38 XYY males, of which 12 were diagnosed prenatally. We found that these patients are at considerably increased risk for delayed language--and/or motor development. From birth on, weight, height and head circumference are above average values. The majority attends kindergarten in the normal education circuit although in 50% of the cases psychosocial problems are documented. From primary school age on, there is an increased risk for child psychiatric disorders such as autism. Moreover, although normally intelligent, many of these boys are referred to special education programmes.

Adolescent↗
PubMed2001

Hydrophobic core manipulations in ribonuclease T1.

Differential scanning calorimetry, urea denaturation, and X-ray crystallography were combined to study the structural and energetic consequences of refilling an engineered cavity in the hydrophobic core of RNase T1 with CH(3), SH, and OH groups. Three valines that cluster together in the major hydrophobic core of T1 were each replaced with Ala, Ser, Thr, and Cys. Compared to the wild-type protein, all these mutants reduce the thermodynamic stability of the enzyme considerably. The relative order of stability at all three positions is as follows: Val > Ala approximately equal to Thr > Ser. The effect of introducing a sulfhydryl group is more variable. Surprisingly, a Val --> Cys mutation in a hydrophobic environment can be as or even more destabilizing than a Val --> Ser mutation. Furthermore, our results reveal that the penalty for introducing an OH group into a hydrophobic cavity is roughly the same as the gain obtained from filling the cavity with a CH(3) group. The inverse equivalence of the behavior of hydroxyl and methyl groups seems to be crucial for the unique three-dimensional structure of the proteins. The importance of negative design elements in this context is highlighted.

Amino Acid Sequence↗
PubMed2001

Structure and function of a novel purine specific nucleoside hydrolase from Trypanosoma vivax.

The purine salvage pathway of parasitic protozoa is currently considered as a target for drug development because these organisms cannot synthesize purines de novo. Insight into the structure and mechanism of the involved enzymes can aid in the development of potent inhibitors, leading to new curative drugs. Nucleoside hydrolases are key enzymes in the purine salvage pathway of Trypanosomatidae, and they are especially attractive because they have no equivalent in mammalian cells. We cloned, expressed and purified a nucleoside hydrolase from Trypanosoma vivax. The substrate activity profile establishes the enzyme to be a member of the inosine-adenosine-guanosine-preferring nucleoside hydrolases (IAG-NH). We solved the crystal structure of the enzyme at 1.6 A resolution using MAD techniques. The complex of the enzyme with the substrate analogue 3-deaza-adenosine is presented. These are the first structures of an IAG-NH reported in the literature. The T. vivax IAG-NH is a homodimer, with each subunit consisting of ten beta-strands, 12 alpha-helices and three small 3(10)-helices. Six of the eight strands of the central beta-sheet form a motif resembling the Rossmann fold. Superposition of the active sites of this IAG-NH and the inosine-uridine-preferring nucleoside hydrolase (IU-NH) of Crithidia fasciculata shows the molecular basis of the different substrate specificity distinguishing these two classes of nucleoside hydrolases. An "aromatic stacking network" in the active site of the IAG-NH, absent from the IU-NH, imposes the purine specificity. Asp10 is the proposed general base in the reaction mechanism, abstracting a proton from a nucleophilic water molecule. Asp40 (replaced by Asn39 in the IU-NH) is positioned appropriately to act as a general acid and to protonate the purine leaving group. The second general acid, needed for full enzymatic activity, is probably part of a flexible loop located in the vicinity of the active site.

Amino Acid Sequence↗
PubMed2001

Factors affecting the relation between parental education as well as occupation and problem behaviour in Dutch 5- to 6-year-old children.

BACKGROUND: This study investigated whether problem behaviour in 5- to 6-year-old children is related to parental education and occupation. It also analysed the contribution of correlating factors to explain this association. METHOD: The Child Behaviour Checklist was administered to a large community sample of 1317 children who were in the 1st year of normal primary school in the Netherlands. Outcome measures were total problem score, and externalising and internalising scale scores. RESULTS: Results of the logistic regression analyses indicated that higher rates of reported behaviour problems were significantly associated with low parental education and occupation. These associations were mediated by low maternal age at delivery and single-parent families. The number of children in a family and physical illness of the parents did not contribute to these associations. CONCLUSIONS: Parental education and occupation have a large impact on the mental health of young children. Psychosocial and biological factors are possible explanations for this phenomenon.

Attention Deficit and Disruptive Behavior Disorder↗
PubMed2001

The contribution of metal ions to the conformational stability of ribonuclease T1: crystal versus solution.

In the crystalline state, ribonuclease T1 binds calcium ions at different lattice-dependent positions. In solution, its conformational stability is also remarkably increased in the presence of divalent metal ions. Combining urea unfolding studies and X-ray crystallography, we compared the presence of several metal ions at specific sites in the protein to their contribution to the overall stabilizing effect in solution. We constructed thermodynamic cycles involving particular metal ions and specific carboxylate functions. The resulting coupling energies indicate that some (but not all) metal ions found at lattice contacts in crystal structures may indeed significantly contribute to stability enhancement in the presence of metal ions in solution.

Aspartic Acid↗
PubMed2001

Child psychiatric diagnoses in a population of Dutch schoolchildren aged 6 to 8 years.

OBJECTIVE: To determine the prevalence rates of child psychiatric diagnoses in a school-based population of children aged 6 to 8 years in the south of the province of Limburg (The Netherlands). METHOD: In a two-stage design 1,317 children were screened with the Child Behavior Checklist. From 403 of these children, child psychiatric information was obtained with the Amsterdam Diagnostic Interview for Children and Adolescents (ADIKA, DSM-III-R/IV). Data were generalized to the responder group (n = 1,317) and to the entire cohort (N = 2,290). For the latter procedure, a prediction model was used to generalize ADIKA results to the nonresponders (n = 973). RESULTS: Estimates of the prevalence of different ADIKA diagnoses in the responder group were quite comparable with those for the entire cohort. Twenty-four percent of the entire cohort met criteria for a single disorder, and 21.0% met criteria for two or more disorders. However, in only 5.7% of the cases parents did report a need for help. CONCLUSIONS: Where other studies generalize psychiatric diagnoses to the responder group only, this report adds new information by generalizing the prevalence to a school-based cohort of children aged 6 to 8 years. These prevalence estimates are of importance with regard to the demand for care for child psychopathology.

Child↗
PubMed2001

Neighbourhood level and individual level SES effects on child problem behaviour: a multilevel analysis.

OBJECTIVE: This study examined whether neighbourhood level socioeconomic variables have an independent effect on reported child behaviour problems over and above the effect of individual level measures of socioeconomic status. DESIGN AND SETTING: Multilevel analysis of cross sectional survey data relating individual level child behavioural problems and parental measures of socioeconomic status with neighbourhood level measures of socioeconomic deprivation in the city of Maastricht, the Netherlands. PARTICIPANTS: Children born in the years 1990-1991 attending the second grade of normal kindergarten schools in the city of Maastricht, the Netherlands. Out of 1417 eligible 5-7 year olds, the parents of 734 children (51.8%) agreed to participate. MAIN RESULTS: Child behaviour problems were more frequent in families of low parental occupation and education (F=14.51, df 3, 721, p<0.001; F=12.20, df 3, 721, p<0.001, respectively) and in families living in deprived neighbourhoods (F=13.26, df 2, 722, p<0.001). Multilevel random effects regression analysis showed that the effect of neighbourhood level deprivation remained after adjustment for individual level socioeconomic status (B over three levels of deprivation: 1.36; 95%CI=0.28, 2.45). CONCLUSIONS: Living in a more deprived neighbourhood is associated with higher levels of child problem behaviour, irrespective of individual level socioeconomic status. The additional effect of the neighbourhood may be attributable to contextual variables such as the level of social cohesion among residents.

Child↗
PubMed2000

Analysis of a water mediated protein-protein interactions within RNase T1.

Buried and well-ordered solvent molecules are an integral part of each folded protein. For a few individual water molecules, the exchange kinetics with solvent have been described in great detail. So far, little is known about the energetics of this exchange process. Here, we present an experimental approach to investigate water-mediated intramolecular protein-protein interactions by use of double mutant cycles. As a first example, we analyzed the interdependence of the contribution of two side chains (Asn9 and Thr93) to the conformational stability of RNase T1. In the folded state, both side chains are involved in the "solvation of the same water molecule WAT1. The coupling of the contributions of Asn9 and Thr93 to the conformational stability of RNase T1 was measured by urea unfolding and differential scanning calorimetry. The structural integrity of each mutant was analyzed by X-ray crystallography. We find that the effects of the Asn9Ala and the Thr93Ala mutations on the conformational stability are additive in the corresponding double mutant. We conclude that the free energy of the WAT1 mediated intramolecular protein-protein interaction in the folded state is very similar to solvent mediated protein-protein interaction in the unfolded state.

Calorimetry, Differential Scanning↗