2,507 research outputs found
Structural Studies on Flavin Reductase PheA2 Reveal Binding of NAD in an Unusual Folded Conformation and Support Novel Mechanism of Action
The catabolism of toxic phenols in the thermophilic organism Bacillus thermoglucosidasius A7 is initiated by a two-component enzyme system. The smaller flavin reductase PheA2 component catalyzes the NADH-dependent reduction of free FAD according to a ping-pong bisubstrate-biproduct mechanism. The reduced FAD is then used by the larger oxygenase component PheA1 to hydroxylate phenols to the corresponding catechols. We have determined the x-ray structure of PheA2 containing a bound FAD cofactor (2.2 Angstrom), which is the first structure of a member of this flavin reductase family. We have also determined the x-ray structure of reduced holo-PheA2 in complex with oxidized NAD (2.1 Angstrom). PheA2 is a single domain homodimeric protein with each FAD-containing subunit being organized around a six-stranded beta-sheet and a capping alpha-helix. The tightly bound FAD prosthetic group (K-d=10 nM) binds near the dimer interface, and the re face of the FAD isoalloxazine ring is fully exposed to solvent. The addition of NADH to crystalline PheA2 reduced the flavin cofactor, and the NAD product was bound in a wide solvent-accessible groove adopting an unusual folded conformation with ring stacking. This is the first observation of an enzyme that is very likely to react with a folded compact pyridine nucleotide. The PheA2 crystallographic models strongly suggest that reactive exogenous FAD substrate binds in the NADH cleft after release of NAD product. Nanoflow electrospray mass spectrometry data indeed showed that PheA2 is able to bind one FAD cofactor and one FAD substrate. In conclusion, the structural data provide evidence that PheA2 contains a dual binding cleft for NADH and FAD substrate, which alternate during catalysis
Evolution of Neutron-Star, Carbon-Oxygen White-Dwarf Binaries
At least one, but more likely two or more, eccentric neutron-star,
carbon-oxygen white-dwarf binaries with an unrecycled pulsar have been
observed. According to the standard scenario for evolving neutron stars which
are recycled in common envelope evolution we expect to observe \gsim 50 such
circular neutron star-carbon oxygen white dwarf binaries, since their formation
rate is roughly equal to that of the eccentric binaries and the time over which
they can be observed is two orders of magnitude longer, as we shall outline. We
observe at most one or two such circular binaries and from that we conclude
that the standard scenario must be revised. Introducing hypercritical accretion
into common envelope evolution removes the discrepancy by converting the
neutron star into a black hole which does not emit radio waves, and therefore
would not be observed.Comment: 25 pages, 1 figure, accepted in Ap
A Survey of Best Monotone Degree Conditions for Graph Properties
We survey sufficient degree conditions, for a variety of graph properties,
that are best possible in the same sense that Chvatal's well-known degree
condition for hamiltonicity is best possible.Comment: 25 page
Does mindful parenting mediate the association between maternal anxiety during pregnancy and child behavioral/emotional problems?
Objectives Maternal anxiety during pregnancy is a risk factor for child behavioral/emotional problems. Knowledge about explanatory factors mediating this link is scarce. Maternal anxiety during pregnancy may be an indicator of adverse postnatal environmental conditions, including maternal anxiety and poor parenting, possibly affecting child behavior. This study investigated whether maternal anxiety and mindful parenting in early childhood mediate the association between maternal anxiety during pregnancy and child behavioral/emotional problems. Methods This study was based on a sample of 118 mother-child dyads who participated in a cohort study that followed participants from pregnancy until 4 years after birth. At 21 weeksâ gestation and when the child was 4 years old, mothers completed questionnaires to assess state anxiety (State-Trait Anxiety Inventory) and general anxiety (Symptom Checklist-90 anxiety subscale). At age 4 years, mothers reported mindful parenting (Interpersonal Mindfulness in Parenting Scale) and child behavioral/emotional problems (Child Behavior Checklist). Results Maternal state anxiety during pregnancy was positively associated with child internalizing problems and negatively with mindful parenting. Maternal general anxiety during pregnancy was related with more child internalizing and externalizing problems but not with mindful parenting. The association between maternal state anxiety during pregnancy and child internalizing problems was sequentially mediated via concurrent maternal general anxiety and mindful parenting. Conclusions These findings suggest that maternal anxiety during pregnancy continues into early childhood and, because of this, it subsequently affects child internalizing behavior via poor mindful parenting. Replication studies are needed before developing interventions for tackling maternal anxiety during pregnancy and promoting mindful parenting
An Unusual Spectral State of an Ultra-luminous Very Soft X-ray Source during Outburst
We report the results of Chandra and XMM-Newton observations of a new
outburst of an ultra-luminous X-ray source in M101. During a Chandra monitoring
observation of M101, M101 ULX-1 was found to be in outburst in 2004 December,
the second outburst in 2004. The peak bolometric luminosity is about 3e40 erg/s
(7e39 erg/s in 0.3-7 keV). The outburst spectra are very soft and can generally
be fitted with a blackbody model with temperatures of 40-80 eV, similar to
supersoft X-ray sources in the Milky Way and in the Magellanic Clouds. In one
Chandra observation, the source spectrum appears to be harder with a
temperature of 150 eV. Such a spectral state is rarely seen in M101 ULX-1 and
no X-ray source in the Milky Way shows this kind of spectrum. However, such an
unusual spectral state very likely belongs to a new class of X-ray sources,
quasisoft X-ray sources, recently discovered in nearby galaxies. M101 ULX-1
returned to supersoft state in a subsequent XMM-Newton observation. Based on
the two outbursts in 2004, the extremely high luminosity (L_bol=1e40-1e41
erg/s), very soft X-ray spectra (kT=40-150 eV), transient behavior, and state
transition provide strong evidence that M101 ULX-1 harbors an intermediate-mass
black hole.Comment: 4 pages, 2 figures, accepted for publication in ApJ
Optical Detection of Two Intermediate Mass Binary Pulsar Companions
We report the detection of probable optical counterparts for two Intermediate
Mass Binary Pulsar (IMBP) systems, PSR J1528-3146 and PSR J1757-5322. Recent
radio pulsar surveys have uncovered a handful of these systems with putative
massive white dwarf companions, thought to have an evolutionary history
different from that of the more numerous class of Low Mass Binary Pulsars
(LMBPs) with He white dwarf companions. The study of IMBP companions via
optical observations offers us several new diagnostics: the evolution of main
sequence stars near the white-dwarf-neutron star boundary, the physics of white
dwarfs close to the Chandrasekhar limit, and insights into the recycling
process by which old pulsars are spun up to high rotation frequencies. We were
unsuccessful in our attempt to detect optical counterparts of PSR J1141-6545,
PSR J1157-5112, PSR J1435-6100, and PSR J1454-5846.Comment: 9 pages, 2 figures, accepted for publication in ApJ
The origin of the runaway high-mass X-ray binary HD153919/4U1700-37
Based on its Hipparcos proper motion, we propose that the high-mass X-ray
binary HD153919/4U1700-37 originates in the OB association Sco OB1. At a
distance of 1.9 kpc the space velocity of 4U1700-37 with respect to Sco OB1 is
75 km/s. This runaway velocity indicates that the progenitor of the compact
X-ray source lost about 7 Msun during the (assumed symmetric) supernova
explosion. The system's kinematical age is about 2 +/- 0.5 million years which
marks the date of the supernova explosion forming the compact object. The
present age of Sco OB1 is <8 Myr; its suggested core, NGC 6231, seems to be
somewhat younger (~5 Myr). If HD153919/4U1700-37 was born as a member of Sco
OB1, this implies that the initially most massive star in the system terminated
its evolution within 30
Msun. With these parameters the evolution of the binary system can be
constrained.Comment: 6 pages, latex, 3 embedded ps figures, to appear in A&
A Massive White Dwarf Companion to the Eccentric Binary Pulsar System PSR B2303+46
Pulsars in close, eccentric binary systems are usually assumed to have
another neutron star as a companion. These double neutron star binaries have
proven to be the best laboratories for experimental General Relativity and are
the most secure candidates for gravitational wave interferometers. We present
deep B, V, and R images of the field containing the eccentric binary pulsar
system PSR B2303+46. We find a faint, blue object (B=26.60+/-0.09;
(B-R)_0=-0.4+/-0.2) coincident with the timing position. We suggest this object
is the optical counterpart to the PSR B2303+46 system. The counterpart is too
bright to reflect emission from the pulsar or a neutron star companion. Most
likely, the companion of PSR B2303+46 is not a neutron star but a massive white
dwarf. We show that the observations are consistent with a hot white dwarf
companion (T_eff>~5*10^4 K) with cooling age equal the characteristic age of
the pulsar (t_cool~30 Myr) and mass within the range set by timing observations
and the Chandrasekhar mass (1.2<M_C<1.4 M_sun). Given the eccentric orbit, the
white dwarf must have formed before the neutron star, from what was originally
the more massive star in the binary. Due to mass transfer, the originally less
massive star could become sufficiently massive to end its life in a supernova
explosion and form the radio pulsar. We constrain the mass of the pulsar to be
in the range 1.24<M_psr<1.44 M_sun.Comment: Revised version, with only very small changes. 6 pages, 1 figure,
uses aas4pp2.sty, psfig.sty. Accepted for publication in Astrophysical
Journal Letter
Mutant mitochondrial elongation factor G1 and combined oxidative phosphorylation deficiency
Although most components of the mitochondrial translation apparatus are encoded by nuclear genes, all known molecular defects associated with impaired mitochondrial translation are due to mutations in mitochondrial DNA. We investigated two siblings with a severe defect in mitochondrial translation, reduced levels of oxidative phosphorylation complexes containing mitochondrial DNA (mtDNA)âencoded subunits, and progressive hepatoencephalopathy. We mapped the defective gene to a region on chromosome 3q containing elongation factor G1 (EFG1), which encodes a mitochondrial translation factor. Sequencing of EFG1 revealed a mutation affecting a conserved residue of the guanosine triphosphate (GTP)âbinding domain. These results define a new class of gene defects underlying disorders of oxidative phosphorylation
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