4,548 research outputs found
alpha -Lactalbumin (LA) Stimulates Milk beta-1,4-Galactosyltransferase I (beta 4Gal-T1) to Transfer Glucose from UDP-glucose to N-Acetylglucosamine: CRYSTAL STRUCTURE OF beta 4Gal-T1·LA COMPLEX WITH UDP-Glc*
beta-1,4-Galactosyltransferase 1 (Gal-T1) transfers galactose (Gal) from UDP-Gal to N-acetylglucosamine (GlcNAc), which constitutes its normal galactosyltransferase (Gal-T) activity. In the presence of alpha -lactalbumin (LA), it transfers Gal to Glc, which is its lactose synthase (LS) activity. It also transfers glucose (Glc) from UDP-Glc to GlcNAc, constituting the glucosyltransferase (Glc-T) activity, albeit at an efficiency of only 0.3-0.4% of Gal-T activity. In the present study, we show that LA increases this activity almost 30-fold. It also enhances the Glc-T activity toward various N-acyl substituted glucosamine acceptors. Steady state kinetic studies of Glc-T reaction show that the Km for the donor and acceptor substrates are high in the absence of LA. In the presence of LA, the Km for the acceptor substrate is reduced 30-fold, whereas for UDP-Glc it is reduced only 5-fold. In order to understand this property, we have determined the crystal structures of the Gal-T1·LA complex with UDP-Glc·Mn2+ and with N-butanoyl-glucosamine (N-butanoyl-GlcN), a preferred sugar acceptor in the Glc-T activity. The crystal structures reveal that although the binding of UDP-Glc is quite similar to UDP-Gal, there are few significant differences observed in the hydrogen bonding interactions between UDP-Glc and Gal-T1. Based on the present kinetic and crystal structural studies, a possible explanation for the role of LA in the Glc-T activity has been proposed
Competition between Superconductivity and Charge Density Wave Ordering in the LuIr(SiGe) Alloy System
We have performed bulk measurements such as dc magnetic susceptibility,
electrical resistivity and heat capacity on the pseudo-ternary alloys
LuIr(SiGe) to study the interplay and competition
between superconductivity and the charge density wave (CDW) ordering
transition. We track the evolution of the superconducting transition
temperature T and the CDW ordering temperature T as a function
of x (concentration of Ge) (). We find that increasing x
(increasing disorder) suppresses the T rapidly with the concomitant
increase in T. We present a temperature-concentration (or volume) phase
diagram for this system and compare our results with earlier work on
substitution at the Lu or Ir site to show how dilution at the Si site presents
a different situation from these other works. The heat capacity data in the
vicinity of the CDW transition has been analyzed using a model of critical
fluctuations in addition to a mean-field contribution and a smooth lattice
background. We find that the critical exponents change appreciably with
increasing disorder. This analysis suggests that the strong-coupling and non
mean-field like CDW transition in the parent compound LuIrSi
changes to a mean-field like transition with increasing Ge concentration.Comment: 14 pages and 8 figures. Accepted for publication in Phys. Rev.
Phase transitions in LuIrSi
We report the results of our investigations on a polycrystalline sample of
LuIrSi which crystallizes in the UCoSi type structure
(Ibam). These investigations comprise powder X-ray diffraction, magnetic
susceptibility, electrical resistivity and high temperature (120-300 K) heat
capacity studies. Our results reveal that the sample undergoes a
superconducting transition below 3.5 K. It also undergoes a first order phase
transition between 150-250 K as revealed by an upturn in the resistivity, a
diasmagnetic drop in the magnetic susceptibility and a large anomaly (20-30
J/mol K) in the specific heat data. We observe a huge thermal hysteresis of
almost 45 K between the cooling and warming data across this high temperature
transition in all our measurements. Low temperature X-ray diffraction
measurements at 87 K reveals that the compound undergoes a structural change at
the high temperature transition. Resistivity data taken in repeated cooling and
warming cycles indicate that at the high temperature transition, the system
goes into a highly metastable state and successive heating/cooling curves are
found to lie above the previous one and the resistance keeps increasing with
every thermal cycle. The room temperature resistance of a thermaly cycled piece
of the sample decays exponentialy with time with a decay time constant
estimated to be about 10 secs. The anomaly (upturn) in the resistivity and
the large drop (almost 45%) in the susceptibility across the high temperature
transition suggest that the observed structural change is accompanied or
induced by an electronic transition.Comment: 7 figures, 1 table and 18 reference
Amorphization of Vortex Matter and Reentrant Peak Effect in YBaCuO
The peak effect (PE) has been observed in a twinned crystal of
YBaCuO for Hc in the low field range, close to
the zero field superconducting transition temperature (T(0)) . A sharp
depinning transition succeeds the peak temperature T of the PE. The PE
phenomenon broadens and its internal structure smoothens out as the field is
increased or decreased beyond the interval between 250 Oe and 1000 Oe.
Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus
of the T(H) values shows a reentrant characteristic with a nose like
feature located at T(H)/T(0)0.99 and H100 Oe (where
the FLL constant apenetration depth ). The upper part of
the PE curve (0.5 kOeH10 kOe) can be fitted to a melting scenario with
the Lindemann number c0.25. The vortex phase diagram near T(0)
determined from the characteristic features of the PE in
YBaCuO(Hc) bears close resemblance to that in
the 2H-NbSe system, in which a reentrant PE had been observed earlier.Comment: 15 pages and 7 figure
Vortex Phase Diagram of weakly pinned YBaCuO for H c
Vortex phase diagram in a weakly pinned crystal of YBCO for H c
is reviewed in the light of a recent elucidation of the process of `inverse
melting' in a Bismuth cuprate system and the imaging of an interface between
the ordered and the disordered regions across the peak effect in 2H-NbSe.
In the given YBCO crystal, a clear distinction can be made between the second
magnetization peak (SMP) and the peak effect (PE) between 65 K and 75 K. The
field region between the peak fields of the SMP (H) and the onset
fields of the PE (H)is not only continuously connected to the Bragg
glass phase at lower fields but it is also sandwiched between the higher
temperature vortex liquid phase and the lower temperature vortex glass phase.
Thus, an ordered vortex state between H and H can get
transformed to the (disordered) vortex liquid state on heating as well as to
the (disordered) vortex glass state on cooling, a situation analogous to the
thermal melting and the inverse melting phenomenon seen in a Bismuth cuprate.Comment: Presented in IWCC-200
Isoniazid-resistant mutants of Mycobacterium tuberculosis H 37 RV: uptake of isoniazid and the properties of NADase inhibitor
Six independent isoniazid-resistant mutants of Mycobacterium tuberculosis were isolated under conditions which largely ensured the selection of one-step mutants. The properties of these mutants with respect to the nicotinamide adenine dinucleotide nucleosidase (NADase) inhibitor, isoniazid uptake and peroxidase activity were studied. The uptake of isoniazid by the mutants and their sensitivity to isoniazid were increased by ethylenediaminetetra-acetic acid (EDTA) and sodium dodecylsulphate. It is suggested that EDTA and sodium dodecylsulphate bring about these effects by altering the cell permeability
Critical behavior at de-pinning of a driven disordered vortex matter in 2H-NbS2
We report unusual jamming in driven ordered vortex flow in 2H-NbS2.
Reinitiating movement in these jammed vortices with a higher driving force, and
halting it thereafter once again with a reduction in drive, unfolds a critical
behavior centered around the de-pinning threshold via divergences in the
lifetimes of transient states, validating the predictions of a recent
simulation study, which also pointed out a correspondence between plastic
de-pinning in vortex matter and the notion of random organization proposed in
the context of sheared colloids undergoing diffusive motion.Comment: Phys. Rev. B (in press, 2012). The paper has 14 pages of Text+ Refs.
with 4 figures. (Note as some of the figure files are large in size, to
enable faster download, the file size has been kept small and the figure
resolution are low. The online version of the paper to appear in PRB will
contain the higher resolution figures
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