Structural requirements for the modulatory effect of 6-substituted pterins on interleukin 2 receptor binding

Molecular cloning of a cDNA coding for GTP cyclohydrolase I from Dictyostelium discoideum

The GTP cyclohydrolase I (GTP-CH) gene of the cellular slime mould Dictyostelium discoideum has been cloned and sequenced. The 855 bp cDNA of this gene contains the open reading frame (ORF) encoding 232 amino acids with a predicted molecular mass of approx. 26 kDa. Southern blot analysis indicated the presence of a single gene for GTP-CH in Dictyostelium. PCR amplification of the ORF from chromosomal DNA and sequencing showed the existence of a 101 bp intron in the GTP-CH gene of Dictyostelium discoideum. The amino acid sequence has 47% and 49% positional identity to those of the human and yeast enzymes respectively. Most of the sequence variation between species is located in the N-terminal part of the protein. The overall identity with the E. coli protein is markedly lower. The enzyme was expressed in E. coli and purified as a 68 kDa fusion protein with the maltose-binding protein of E. coli. GTP-CH of Dictyostelium is heat-stable and showed maximal activity at 60 degrees C. The Km value for GTP is 50 microM.

Conformational investigation of the cofactor (6R,1'R,2'S-)-5,6,7,8-tetrahydrobiopterin

(6R,1'R,2'S)-5,6,7,8-Tetrahydrobiopterin is an essential cofactor for several enzymes. Different theoretical models (molecular mechanics, semiempirical quantum chemical calculations) investigating its stereostructure have yielded diverging answers. To clarify these issues, combined molecular mechanical and ab initio quantum chemical calculations were performed, investigating both the axial and the equatorial orientation of the dihydroxypropyl side-chain. After geometry optimization, the resulting most stable structures were subjected to systematic variation of two side-chain torsional angles in order to study the conformational flexibility. The axial side-chain orientation is slightly more stable than the equatorial form. Two weak intramolecular hydrogen bonds contribute to stabilization of the axial conformer, while in the equatorial conformer only one hydrogen bond is detected. An 8 ps molecular dynamical simulation at 310 K suggests that, at realistic temperatures, the molecule is flexible enough to undergo internal motions (rotations, vibrations), rendering questionable the biological significance of mere conformational properties.

The conformational flexibility of 5,6,7,8-tetrahydrobiopterin and 5,6,7,8-tetrahydroneopterin: a molecular dynamical simulation

5,6,7,8-Tetrahydrobiopterin is an essential cofactor of diverse enzymes. Of the eight possible stereoisomers, only the 6R,1'R,2'S-configuration is biologically active. Other stereoisomers, as well as other reduced pterins such as, e.g. 5,6,7,8-tetrahydroneopterin, fail to exhibit significant cofactor activity. Different theoretical models (molecular mechanics, semi-empirical quantum chemical calculations) investigating the stereostructure of tetrahydrobiopterin have yielded diverging answers. It has been claimed on the basis of semi-empirical quantum chemical calculations that conformational properties, and thus particular features in overall shape, might be responsible for the unique biological properties of natural tetrahydrobiopterin in contrast, e.g. to 6R,1'S,2'R-5,6,7,8-tetrahydroneopterin. Molecular dynamical simulations of both molecules at realistic temperatures demonstrate, however, that they possess sufficient conformational flexibility as to render questionable any biological significance of mere conformational properties.

Modulation of interleukin 2 high affinity binding to human T cells by a pyrimidodiazepine insect metabolite

An insect metabolite containing the little known pyrimido[4,5-b][1,4]diazepine ring system has been found to act as an effective mimic of tetrahydrobiopterin in its ability to modulate the affinity of interleukin 2 (IL-2) for its receptors on human T cells. Semi-empirical molecular orbital calculations reveal that while tetrahydrobiopterin has considerable flexibility, the pyrimidodiazepine has rather few conformational options and offers a useful model for exploring the nature of the pterin binding site.