Masaru Goto

Structural determinants for branched-chain aminotransferase isozyme-specific inhibition by the anticonvulsant drug gabapentin.

This study presents the first three-dimensional structures of human cytosolic branched-chain aminotransferase (hBCATc) isozyme complexed with the neuroactive drug gabapentin, the hBCATc Michaelis complex with the substrate analog, 4-methylvalerate, and the mitochondrial isozyme (hBCATm) complexed with gabapentin. The branched-chain aminotransferases (BCAT) reversibly catalyze transamination of the essential branched-chain amino acids (leucine, isoleucine, valine) to α-ketoglutarate to form the respective branched-chain α-keto acids and glutamate. The cytosolic isozyme is the predominant BCAT found in the nervous system, and only hBCATc is inhibited by gabapentin. Pre-steady state kinetics show that 1.3 mm gabapentin can completely inhibit the binding of leucine to reduced hBCATc, whereas 65.4 mm gabapentin is required to inhibit leucine binding to hBCATm. Structural analysis shows that the bulky gabapentin is enclosed in the active-site cavity by the shift of a flexible loop that enlarges the active-site cavity. The specificity of gabapentin for the cytosolic isozyme is ascribed at least in part to the location of the interdomain loop and the relative orientation between the small and large domain which is different from these relationships in the mitochondrial isozyme. Both isozymes contain a CXXC center and form a disulfide bond under oxidizing conditions. The structure of reduced hBCATc was obtained by soaking the oxidized hBCATc crystals with dithiothreitol. The close similarity in active-site structures between cytosolic enzyme complexes in the oxidized and reduced states is consistent with the small effect of oxidation on pre-steady state kinetics of the hBCATc first half-reaction. However, these kinetic data do not explain the inactivation of hBCATm by oxidation of the CXXC center. The structural data suggest that there is a larger effect of oxidation on the interdomain loop and residues surrounding the CXXC center in hBCATm than in hBCATc.

Small-signal and noise model of fully depleted silicon-on-insulator metal-oxide-semiconductor devices for low-noise amplifier

An RF small-signal and noise model of fully depleted silicon-on-insulator (FD-SOI) metal–oxide–semiconductor field effect transistor (MOSFET) is presented. The model together with its intrinsic model parameters extracted from de-embedding extrinsic parameters reproduces the frequency and noise response of FD-SOI MOSFETs. We have applied the proposed model to a low-noise amplifier (LNA) operating at 5.5 GHz, which is implemented in a 0.15 µm FD-SOI complementary metal–oxide–semiconductor (CMOS) technology. The simulated small-signal and noise performance of the LNA are in good agreement with the measured data of the fabricated LNA.

Analytical Expression Based Design of a Low-Voltage FD-SOI CMOS Low-Noise Amplifier

We propose a design methodology of a low-voltage CMOS low-noise amplifier (LNA) consisting of a common-source and a common-gate stages. We first derive equations of power gain, noise figure (NF) and input third-order intercept point (IIP3) of the two-stage LNA. A design methodology of the LNA is presented by using graphs based on analytical equations. A 1-V 5.4-GHz LNA was implemented in 0.15-μm fully-depleted silicon-on-insulator (FD-SOI) CMOS technology. Measurement results show a power gain of 23 dB, NF of 1.7 dB and IIP3 of -6.1 dBm with a power consumption of 8.3 mW. These measured results are consistent with calculated results, which ensures the validity of the derived equations and the proposed design methodology.

Accurate Small-Signal Modeling of FD-SOI MOSFETs

A new small-signal model for fully depleted silicon-on-insulator (FD-SOI) MOSFETs operating at RF frequencies is presented. The model accounts for the non-quasi-static effect by determining model parameters using a curve fitting procedure to reproduce the frequency response of FD-SOI MOSFETs. The accuracy of the model is validated by comparison of S parameters with measured results in the range from 0.2 GHz to 20 GHz.

Expression, purification and preliminary X-ray characterization of N-acetyl-γ-glutamyl-phosphate reductase from Thermus thermophilus HB8

N-Acetyl-gamma-glutamyl-phosphate reductase (AGPR) catalyses the NADPH-dependent reduction of N-acetyl-gamma-glutamyl phosphate to give the N-acetylglutamic semialdehyde. A recombinant form of AGPR from Thermus thermophilus HB8 has been crystallized by the hanging-drop vapour-diffusion technique using PEG 4000 as a precipitating agent. The crystals grew as colourless prisms, with unit-cell parameters a = b = 90.9, c = 139.5 A, alpha = beta = 90, gamma = 120 degrees. The crystals belong to the hexagonal space group P6(2)22 or P6(4)22 and are most likely to contain a monomer in the asymmetric unit, with a V(M) value of 2.19 A(3) Da(-1). The crystals diffract to a resolution of 2.2 A at beamline BL44B2 of SPring-8.

Structure, Induced Fit and Substrate Recognition of E. coli Branched-Chain Amino Acid Aminotransferase

Structures of the branched-chain amino acid aminotransferase fromE. coliand its complexes with substrate analogues have been solved by X-ray crystallographic method. BCAT is in a hexameric form, and catalyzes the transamination of branched-chain amino acids. The hexamer can be regarded as the assembly of three dimer units around a 3-fold axis. On binding of the substrate analogue, the flexible loop, which is disordered in the unliganded enzyme, moves toward the active-site entrance and shields the substrate from the solvent region. a-Carboxylate of the substrate directly interacts with an OH group of Tyr and two NH groups of β-turn and indirectly with Arg. The side chain of the branched-chain amino acid is in the cavity formed by aromatic rings and an isopropyl group of the active site residues.