Studies on the active site of Escherichia coli adenylosuccinate synthetase by chemical modification and site-directed mutagenesis

Reaction of E. coli adenylosuccinate synthetase with thiol reagents leaded to modification of one cysteine residue per subunit without significant loss of enzyme activity. Modification of a second cysteine residue occurred under 3.4 M urea and resulted in complete loss of enzyme activity. The remaining two cystein residues were modified only after treatment with 8 M urea. The reactive cystein had been identified as Cys[superscript]291, and the thiol exposed with 3.5 M urea is Cys[superscript]344. When Cys[superscript]344 was replaced by either serine or alanine, the mutant enzymes were as active as the wild type enzyme. These findings point to the nonessential roles of sulfhydryl groups in the enzyme;Incubation of the enzyme with low concentrations of pyridoxal 5[superscript]\u27-phosphate (PLP) resulted in a rapid loss of activity. Stoichiometry and peptide isolation studies showed that two lysine residues were modified. Complete protection of the enzyme against inactivation can be observed only in the presence of GTP. One lysine residue was protected under these conditions. In contrast to GTP, addition of the other two substrates either alone or together to reaction mixtures did not render protection. Peptide mapping revealed that the lysine shielded by GTP is Lys[superscript]140. Replacing the Lys[superscript]140 with Ile[superscript]140 resulted in total loss of the activity. These results suggest that Lys[superscript]140 may play an important role in enzymatic activity;Chemical modification of the enzyme with phenylglyoxal resulted in an inhibition of enzyme acitivity. The substrates, GTP or IMP, can partially protect the enzyme against inactivation. The other substrate, aspartate, had no such effect. In the presence of both IMP and GTP, nearly complete protection of the enzyme against inactivation can be observed. Stoichiometry studies showed that only 1 reactive arginine residue was modified and this arginine residue could be shielded by GTP and IMP. Sequence analysis indicated that Arg[superscript]147 is the site of phenylglyoxal modification. The mutant enzyme (R147L) showed increased Michaelis constants for IMP and GTP relative to the wild type system, whereas the K[subscript] m for aspartate exhibits no significant change compared with the native enzyme. In addition, k[subscript] cat of the R147L mutant decreased by a factor of 1.3 x 10[superscript]4. These results indicate that Arg[superscript]147 is critical for enzyme catalysis

Shear Viscosity of a Gluon Plasma in Perturbative QCD

We calculate the shear viscosity $\eta$ to entropy density $s$ ratio $\eta /s$ of a gluon plasma in kinetic theory including the gg->gg and ggggg processes. Due to the suppressed contribution to $\eta$ in the gg->gg forward scattering, it is known that the gluon bremsstrahlung ggggg process also contributes at the same order $O(\alpha_{s}^{-2})$ in perturbative QCD. Using the Gunion-Bertsch formula for the ggggg matrix element which is valid for the limit of soft bremsstrahlung, we find that the result is sensitive to whether the same limit is taken for the phase space. Using the exact phase space, the ggggg contribution becomes more important to $\eta$ than gg->gg for $\alpha_{s}\gtrsim 2\times 10^{-3}$. Therefore, at $\alpha_{s}=0.1$, $\eta /s\simeq 1.0$, between 2.7 obtained by Arnold, Moore and Yaffe (AMY) and 0.5 obtained by Xu and Greiner. If the soft bremsstrahlung limit is imposed on the phase space such that the recoil effect from the bremsstrahlung gluon is neglected, then the correction from the ggggg process is about 10-30% of the total which is close to AMY's prediction. This shows that the soft bremsstrahlung approximation is not as good as previously expected.Comment: RevTex 4, 14 pages, 3 figures; The results for the soft bremsstrahlung limit for the phase space are added. The difference between AMY and XG approach is addressed with more clarificatio

How Perfect a Gluon Plasma Can Be in Perturbative QCD?

The shear viscosity to entropy density ratio, \eta /s, characterizes how perfect a fluid is. We calculate the leading order \eta /s of a gluon plasma in perturbation using the kinetic theory. The leading order contribution only involves the elastic gg -> gg (22) process and the inelastic ggggg (23) process. The Hard-Thermal-Loop (HTL) treatment is used for the 22 matrix element, while the exact matrix element in vacuum is supplemented by the gluon Debye mass insertion for the 23 process. Also, the asymptotic mass is used for the external gluons in the kinetic theory. The errors from not implementing HTL and the Landau-Pomeranchuk-Migdal effect in the 23 process, and from the uncalculated higher order corrections, are estimated. Our result for \eta /s lies between that of Arnold, Moore and Yaffe (AMY) and Xu and Greiner (XG). Our result shows that although the finite angle contributions are important at intermediate \alpha_s (\alpha_s \sim 0.01-0.1), the 22 process is still more important than 23 when \alpha_s < 0.1. This is in qualitative agreement with AMY's result. We find no indication that the proposed perfect fluid limit \eta /s \simeq 1/(4\pi) can be achieved by perturbative QCD alone.Comment: ReVTex 4, 11 pages, 5 figures. A coding error in the exact matrix element for the 23 process is corrected. Results in Fig. 2,3 and Table I are re-calculated, and relevant discussions are adjusted. Part of the conclusion is change

Quantum state transmission in a cavity array via two-photon exchange

The dynamical behavior of a coupled cavity array is investigated when each cavity contains a three-level atom. For the uniform and staggered intercavity hopping, the whole system Hamiltonian can be analytically diagonalized in the subspace of single-atom excitation. The quantum state transfer along the cavities is analyzed in detail for distinct regimes of parameters, and some interesting phenomena including binary transmission, selective localization of the excitation population are revealed. We demonstrate that the uniform coupling is more suitable for the quantum state transfer. It is shown that the initial state of polariton located in the first cavity is crucial to the transmission fidelity, and the local entanglement depresses the state transfer probability. Exploiting the metastable state, the distance of the quantum state transfer can be much longer than that of Jaynes-Cummings-Hubbard model. A higher transmission probability and longer distance can be achieved by employing a class of initial encodings and final decodings.Comment: 8 pages, 7 figures. to appear in Phys. Rev.

Pružanje lokalnih javnih službi u Kini – institucionalna analiza

The article reviews the institutional change of local public service provision in China. The emphasis is placed on the characteristics and reasons of the evolution process. The first part presents the characteristics and changes of local public service provision in five periods. The second part analyses public service provision through structural-instrumental, cultural and environmental perspectives. The third part explores whether China has experienced “pendulum-type” movements like some Europe countries. The institutional change of local public service in China is incremental. Although it shows some convergence with the trend of certain European countries, it retains special features.U radu se analizira institucionalna promjena u pružanju lokalnih javnih službi u Kini. Naglasak je na karakteristikama i razlozima procesa evolucije. U prvom se dijelu prezentiraju karakteristike i promjene u pružanju lokalnih javnih službi u pet faza. U drugom se dijelu pružanje lokalnih javnih službi analizira iz strukturalno- instrumentalne, kulturološke i okolišne perspektive. Treći dio istražuje ima li razvoj pružanja lokalnih javnih službi u Kini značajke krucijalnih promjena nalik na one u europskim zemljama (npr. remunicipalizacija). Institucionalna promjena lokalnih javnih službi u Kini je postupna. Premda postoje određene sličnosti s trendovima u nekim europskim zemljama, ona ipak zadržava svoje specifične osobine