␥-Glutamyltranspeptidase (GGT) is a heterodimic enzyme that is generated from the precursor protein through posttranslational processing and catalyzes the hydrolysis of ␥-glutamyl bonds in ␥-glutamyl compounds such as glutathione and͞or the transfer of the ␥-glutamyl group to other amino acids and peptides. We have determined the crystal structure of GGT from Escherichia coli K-12 at 1.95 Å resolution. GGT has a stacked ␣␤␤␣ fold comprising the large and small subunits, similar to the folds seen in members of the N-terminal nucleophile hydrolase superfamily. The active site Thr-391, the N-terminal residue of the small subunit, is located in the groove, from which the pocket for ␥-glutamyl moiety binding follows. We have further determined the structure of the ␥-glutamyl

Helicobacter pylorigamma-glutamyltranspeptidase (HpGT) is a glutathione-degrading enzyme that has been shown to be a virulence factor in infection. It is expressed as a 60-kDa inactive precursor that must undergo autocatalytic processing to generate a 40-kDa/20-kDa heterodimer with full gamma-glutamyl amide bond hydrolase activity. The new N terminus of the processed enzyme, Thr-380, is the catalytic nucleophile in both the autoprocessing and enzymatic reactions, indicating that HpGT is a member of the N-terminal nucleophile hydrolase superfamily. To further investigate activation as a result of autoprocessing, the structure of HpGT has been determined to a resolution of 1.9 A. The refined model contains two 40-kDa/20-kDa heterodimers in the asymmetric unit and has structural features comparable with other N-terminal nucleophile hydrolases. Autoprocessing of HpGT leads to a large conformational change, with the loop preceding the catalytic Thr-380 moving >35 A, thus relieving steric constraints that likely limit substrate binding. In addition, cleavage of the proenzyme results in the formation of a threonine-threonine dyad comprised of Thr-380 and a second conserved threonine residue, Thr-398. The hydroxyl group of Thr-398 is located equidistant from the alpha-amino group and hydroxyl side chain of Thr-380. Mutation of Thr-398 to an alanine results in an enzyme that is fully capable of autoprocessing but is devoid of enzymatic activity. Substrate docking studies in combination with homology modeling studies of the human homologue reveal additional mechanistic details of enzyme maturation and activation, substrate recognition, and catalysis.

We study the dispersion relation of a metamaterial composed of metallic disks and bars arranged to have kagomé symmetry and find that a plasmonic flat band is formed by the topological nature of the kagomé lattice. To confirm the flat-band formation, we fabricate the metamaterial and make transmission measurements in the terahertz regime. Two bands formed by transmission minima that depend on the polarization of the incident terahertz beams are observed. One of the bands corresponds to the flat band, as confirmed by the fact that the resonant frequency is almost independent of the incident angle.

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Technologies are being developed to manipulate electromagnetic waves using artificially structured materials such as photonic crystals and metamaterials, with the goal of creating primary optical devices. For example, artificial metallic periodic structures show potential for the construction of devices operating in the terahertz frequency regime. Here we demonstrate the fabrication of photo-designed terahertz devices that enable the real-time, wide-range frequency modulation of terahertz electromagnetic waves. These devices are comprised of a photo-induced, planar periodic-conductive structure formed by the irradiation of a silicon surface using a spatially modulated, femtosecond optical pulsed laser. We also show that the modulation frequency can be tuned by the structural periodicity, but is hardly affected by the excitation power of the optical pump pulse. We expect that our findings will pave the way for the construction of all-optical compact operating devices, such as optical integrated circuits, thereby eliminating the need for materials fabrication processes.

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