6 research outputs found
Inclusive J/psi production in Upsilon decay via color-octet mechanisms
Calculations for inclusive J/psi production in Upsilon decay through the
color-octet mechanisms b\bar b[^3S_1, 1] -> c \bar c[^{2s+1}L_J, 8] + g are
presented. It is shown that these O(alpha^5_sv^4_c) contributions compete with
other color-octet mechanisms considered in the literature. A critical numerical
analysis of the color-octet contributions to Upsilon -> J/psi + X shows that
further work in this channel, both theoretical and experimental, is necessary
in order to clearly understand the significance of the color-octet component of
the c \bar c component inside the J/psi system.Comment: Minor changes, corrected typos, version to be published in PR
SUSY Higgs Boson Decays into Scalar Quarks: QCD Corrections
In supersymmetric theories, the decays of the neutral CP-even and CP-odd as
well as the charged Higgs bosons into scalar quarks, in particular into top and
bottom squarks, can be dominant if they are kinematically allowed. We calculate
the QCD corrections to these decay modes in the minimal supersymmetric
extension of the Standard Model, including all quark mass terms and squark
mixing. These corrections turn out to be rather large, altering the decay
widths by an amount which can be larger than 50%. The corrections can be either
positive or negative, and depend strongly on the mass of the gluino. We also
discuss the QCD corrections to the decays of heavy scalar quarks into light
scalar quarks and Higgs bosons.Comment: 17 pages, 6 figure
Structure of a <i>Clostridium botulinum</i> C143S Thiaminase I/Thiamin Complex Reveals Active Site Architecture,
Thiaminases are responsible for the
degradation of thiamin and
its metabolites. Two classes of thiaminases have been identified based
on their three-dimensional structures and their requirements for a
nucleophilic second substrate. Although the reactions of several thiaminases
have been characterized, the physiological role of thiamin degradation
is not fully understood. We have determined the three-dimensional
X-ray structure of an inactive C143S mutant of Clostridium
botulinum (Cb) thiaminase I with bound thiamin at
2.2 Ã… resolution. The C143S/thiamin complex provides atomic level
details of the orientation of thiamin upon binding to Cb-thiaminase
I and the identity of active site residues involved in substrate binding
and catalysis. The specific roles of active site residues were probed
by using site directed mutagenesis and kinetic analyses, leading to
a detailed mechanism for Cb-thiaminase I. The structure of Cb-thiaminase
I is also compared to the functionally similar but structurally distinct
thiaminase II
Structural basis for antibiotic action of the B1 antivitamin 2′-methoxy-thiamine
The natural antivitamin 2′-methoxy-thiamine (MTh) is implicated in the suppression of microbial growth. However, its mode of action and enzyme-selective inhibition mechanism have remained elusive. Intriguingly, MTh inhibits some thiamine diphosphate (ThDP) enzymes, while being coenzymatically active in others. Here we report the strong inhibition of Escherichia coli transketolase activity by MTh and unravel its mode of action and the structural basis thereof. The unique 2′-methoxy group of MTh diphosphate (MThDP) clashes with a canonical glutamate required for cofactor activation in ThDP-dependent enzymes. This glutamate is forced into a stable, anticatalytic low-barrier hydrogen bond with a neighboring glutamate, disrupting cofactor activation. Molecular dynamics simulations of transketolases and other ThDP enzymes identify active-site flexibility and the topology of the cofactor-binding locale as key determinants for enzyme-selective inhibition. Human enzymes either retain enzymatic activity with MThDP or preferentially bind authentic ThDP over MThDP, while core bacterial metabolic enzymes are inhibited, demonstrating therapeutic potential