Hiroyoshi Esaki

Facilitated recruitment of Pdc2p, a yeast transcriptional activator, in response to thiamin starvation

In Saccharomyces cerevisiae, genes involved in thiamin pyrophosphate (TPP) synthesis (THI genes) and the pyruvate decarboxylase structural gene PDC5 are transcriptionally induced in response to thiamin starvation. Three positive regulatory factors (Thi2p, Thi3p, and Pdc2p) are involved in the expression of THI genes, whereas only Pdc2p is required for the expression of PDC5. Thi2p and Pdc2p serve as transcriptional activators and each factor can interact with Thi3p. The target consensus DNA sequence of Thi2p has been deduced. When TPP is not bound to Thi3p, the interactions between the regulatory factors are increased and THI gene expression is upregulated. In this study, we demonstrated that Pdc2p interacts with the upstream region of THI genes and PDC5. The association of Pdc2p or Thi2p with THI gene promoters was enhanced by thiamin starvation, suggesting that Pdc2p and Thi2p assist each other in their recruitment to the THI promoters via interaction with Thi3p. It is highly likely that, under thiamin-deprived conditions, a ternary Thi2p/Thi3p/Pdc2p complex is formed and transactivates THI genes in yeast cells. On the other hand, the association of Pdc2p with PDC5 was unaffected by thiamin. We also identified a DNA element in the upstream region of PDC5, which can bind to Pdc2p and is required for the expression of PDC5.

Enzymatic and structural characterization of an archaeal thiamin phosphate synthase.

Studies on thiamin biosynthesis have so far been achieved in eubacteria, yeast and plants, in which the thiamin structure is formed as thiamin phosphate from a thiazole and a pyrimidine moiety. This condensation reaction is catalyzed by thiamin phosphate synthase, which is encoded by the thiE gene or its orthologs. On the other hand, most archaea do not seem to have the thiE gene, but instead their thiD gene, coding for a 2-methyl-4-amino-5-hydroxymethylpyrimidine (HMP) kinase/HMP phosphate kinase, possesses an additional C-terminal domain designated thiN. These two proteins, ThiE and ThiN, do not share sequence similarity. In this study, using recombinant protein from the hyperthermophile archaea Pyrobaculum calidifontis, we demonstrated that the ThiN protein is an analog of the ThiE protein, catalyzing the formation of thiamin phosphate with the release of inorganic pyrophosphate from HMP pyrophosphate and 4-methyl-5-β-hydroxyethylthiazole phosphate (HET-P). In addition, we found that the ThiN protein can liberate an inorganic pyrophosphate from HMP pyrophosphate in the absence of HET-P. A structure model of the enzyme-product complex of P. calidifontis ThiN domain was proposed on the basis of the known three-dimensional structure of the ortholog of Pyrococcus furiosus. The significance of Arg320 and His341 residues for thiN-coded thiamin phosphate synthase activity was confirmed by site-directed mutagenesis. This is the first report of the experimental analysis of an archaeal thiamin synthesis enzyme.

Chemoselective Hydrogenation Catalyzed by Pd on Spherical Carbon

We have developed a highly chemoselective hydrogenation method using a novel palladium catalyst supported on spherical carbon (0.5 % Pd/SC). The 0.5 % Pd/SC exhibited a novel catalytic activity and could achieve the chemoselective hydrogenation of alkynes, alkenes, azides, nitro groups, and aliphatic O‐tert‐butyldimethylsilyl (TBS) ethers without hydrogenolysis of benzyl esters, benzyl ethers, nitriles, aromatic ketones, N‐carbobenzyloxy (N‐Cbz) protective groups, and aromatic O‐TBS ethers.

Amine‐Urea‐Mediated Asymmetric Cycloadditions between Nitrile Oxides and o‐Hydroxystyrenes by Dual Activation

The first example of asymmetric 1,3-dipolar cycloadditions between nitrile oxides and o-hydroxystyrenes, mediated by cinchona-alkaloid-based amine-ureas is reported. The method is based on a dual activation involving both LUMO and HOMO activations. In addition to the stoichiometric asymmetric induction, a catalytic amount of amine-urea enables the cycloadditions to proceed in an enantioselective manner. Computational studies strongly support the HOMO activation of o-hydroxystyrenes and LUMO activation of nitrile oxides by hydrogen-bonding interactions with the Brønsted acid/base bifunctional catalyst.

Pd on Spherical Carbon (Pd/SC)-Catalyzed Chemoselective Hydrogenation

It is well known that the activity of a supported catalyst is deeply dependent on the physicochemical properties of the support material that include fabric, size, shape, steric structure, and dispersion of the supported metal. We have developed a chemoselective hydrogenation method using a heterogeneous zero-valent palladium catalyst supported on spherical carbon (Pd/SC) with a large average diameter (0.36 mm). Pd/SC was found to be an efficient catalyst for the hydrogenation of alkyne, alkene, azido, nitro, and aliphatic O-tert-butyldimethylsilyl (TBS) functionalities without the hydrogenolysis of benzyl ester and ether, nitrile, aromatic ketone, aromatic O-TBS, and N-carbobenzyloxy (N-Cbz) functionalities. The present method is promising as a general, practical, and chemoselective hydrogenation process.