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Biomedical subjects

W W Dai

Publications and source records attributed to W W Dai.

6 recordsLinked to original sources
PubMed1996

Aromatic amino acid transamination and methionine recycling in trypanosomatids.

Although trypanosomatids are known to rapidly transaminate exogenous aromatic amino acids in vitro and in vivo, the physiological significance of this reaction is not understood. In postmitochondrial supernatants prepared from Trypanosoma brucei brucei and Crithidia fasciculata, we have found that aromatic amino acids were the preferred amino donors for the transamination of alpha-ketomethiobutyrate to methionine. Intact C. fasciculata grown in the presence of [15N]tyrosine were found to contain detectable [15N]methionine, demonstrating that this reaction occurs in situ in viable cells. This process is the final step in the recycling of methionine from methylthioadenosine, a product of decarboxylated S-adenosylmethionine from the polyamine synthetic pathway. Mammalian liver, in contrast, preferentially used glutamine for this reaction and utilized a narrower range of amino donors than seen with the trypanosomatids. Studies with methylthioadenosine showed that this compound was readily converted to methionine, demonstrating a fully functional methionine-recycling pathway in trypanosomatids.

Amino Acids↗
PubMed1994

A direct sensitivity matrix approach for fast reconstruction in electrical impedance tomography.

In electrical impedance imaging, several proposed reconstruction algorithms have employed the concept of a sensitivity matrix, which can be used to relate the magnitude of a boundary voltage change of a 2D object to the change in conductivity inside the object that has given rise to it. The search for an appropriate inversion of the sensitivity matrix is the key to these algorithms. In this work, a method called the direct sensitivity matrix (DSM) approach for fast image reconstruction is proposed. Both theoretical and experimental results showing the efficiency of this proposed method are also presented.

Algorithms↗
PubMed1994

Using the Hilbert uniqueness method in a reconstruction algorithm for electrical impedance tomography.

This paper presents a new version of the layer stripping algorithm in the sense that it works essentially by repeatedly stripping away the outermost layer of the medium after having determined the conductivity value in this layer. In order to stabilize the ill posed boundary value problem related to each layer, we base our algorithm on the Hilbert uniqueness method (HUM) and implement it with the boundary element method (BEM).

Algorithms↗
PubMed1993

The active site structure of the calcium-containing quinoprotein methanol dehydrogenase.

Pyrroloquinoline quinone (PQQ), widely found in nature, serves as the redox cofactor in bacterial methanol dehydrogenase (MEDH), a heterotetrameric enzyme that oxidizes methanol to formaldehyde. The refined structure of MEDH at 2.4-A resolution, based on recently obtained amino acid sequence data, reveals that the PQQ, located in a central channel of the disk-shaped protein, is sandwiched between a Trp side chain and a very unusual vicinal disulfide. A Ca2+ ion forms a bridge between PQQ and the protein molecule, very close to a putative substrate binding pocket. The vicinal disulfide may form during PQQ incorporation and possibly act to hold the latter in place.

Alcohol Oxidoreductases↗
PubMed1992

The three-dimensional structures of methanol dehydrogenase from two methylotrophic bacteria at 2.6-A resolution.

The structures of methanol dehydrogenase (MEDH) from two closely related methylotrophic bacteria, Methylophilus methylotrophus and W3A1, have been determined at 2.6-A resolution. The molecule, a quinoprotein of molecular mass of about 138 kDa, contains two heavy (H) and two light (L) subunits of unknown sequence and two molecules of noncovalently associated pyrroloquinoline quinone. The two enzymes crystallize isomorphously in space group P2(1) with one H2L2 heterotetramer in the asymmetric unit. The electron density map of the M. methylophilus enzyme was obtained by multiple isomorphous replacement with anomalous scattering and improved by solvent leveling and electron density averaging. For model building, the amino acid sequence of MEDH from Paracoccus denitrificans for the H subunit and from Methylobacterium extorquens AM1 for the L subunit were used to represent the unknown amino acid sequence. At the present time, 579 and 57 amino acid residues for the large and small subunits, respectively, have been fitted into the map. The phases for MEDH from M. methylophilus were used directly to analyze the W3A1 structure, and both structures were refined to R-factors (where R = sigma[Fo-Fc[/sigma Fo) of 0.277 and 0.266, respectively. The L subunit contains a long alpha-helix and an extended N-terminal segment, both lying on the molecular surface of the H subunit. The H subunit contains eight antiparallel beta-sheets, each consisting of four strands arranged topologically like the letter W. The eight Ws are arranged circularly, forming the main disc-shaped body of the subunit, with some short helices and loops connecting the consecutive Ws, as well as some excursions within and between some of the Ws. The pyrroloquinoline quinone prosthetic group is located in the central channel of the large subunit near the surface of the molecule. The topology of the eight-W folding unit is similar to those of the six- and seven-W folding units previously reported for three other proteins, neuraminidase, methylamine dehydrogenase, and galactose oxidase.

Alcohol Oxidoreductases↗