H-Asp-Asp-Asp-OH

The following structural and conformationally constrained analogues of Ac-Asp-Glu-OH (1) were synthesized: Ac-Glu-Glu-OH (2), Ac-D-Asp-Glu-OH (3), Ac-Glu-Asp-OH (4), Ac-Asp-Asp-OH (5), Ac-Asp-3-aminohexanedioic acid (6), Ac-3-amino-3-(carboxymethyl)propanoyl-Glu-OH (7), N-succinyl-Glu-OH (8), N-maleyl-Glu-OH (9), N-fumaryl-Glu-OH (10), and Ac-delta ZAsp-Glu-OH (11). These analogues were evaluated for their ability to inhibit the hydrolysis of Ac-Asp-[3,4-3H]-Glu-OH by N-acetylated alpha-linked acidic dipeptidase (NAALA dipeptidase) in order to gain some insight into the structural requirements for the inhibition of this enzyme. Analogues 4-6 and 9 were very weak inhibitors of NAALA dipeptidase (Ki greater than 40 microM), while 2, 3, and 7 with Ki values ranging from 3.2-8.5 microM showed intermediate inhibitory activity. The most active inhibitors of NAALA dipeptidase were compounds 8, 10, and 11 with Ki values of 0.9, 0.4, and 1.4 microM, respectively. These results suggest that the relative spacing between the side chain carboxyl and the alpha-carboxyl group of the C-terminal residue may be important for binding to the active site of the enzyme. They also indicate that the chi 1 torsional angle for the aspartyl residue is in the vicinity of 0 degrees.

Diverting aminoacyl-transfer RNAs (tRNAs) from protein synthesis is a well-known process used by a wide range of bacteria to aminoacylate membrane constituents. By tRNA-dependently adding amino acids to glycerolipids, bacteria change their cell surface properties, which intensifies antimicrobial drug resistance, pathogenicity, and virulence. No equivalent aminoacylated lipids have been uncovered in any eukaryotic species thus far, suggesting that tRNA-dependent lipid remodeling is a process restricted to prokaryotes. We report here the discovery of ergosteryl-3β-O-l-aspartate (Erg-Asp), a conjugated sterol that is produced by the tRNA-dependent addition of aspartate to the 3β-OH group of ergosterol, the major sterol found in fungal membranes. In fact, Erg-Asp exists in the majority of "higher" fungi, including species of biotechnological interest, and, more importantly, in human pathogens like Aspergillus fumigatus We show that a bifunctional enzyme, ergosteryl-3β-O-l-aspartate synthase (ErdS), is responsible for Erg-Asp synthesis. ErdS corresponds to a unique fusion of an aspartyl-tRNA synthetase-that produces aspartyl-tRNAAsp (Asp-tRNAAsp)-and of a Domain of Unknown Function 2156, which actually transfers aspartate from Asp-tRNAAsp onto ergosterol. We also uncovered that removal of the Asp modifier from Erg-Asp is catalyzed by a second enzyme, ErdH, that is a genuine Erg-Asp hydrolase participating in the turnover of the conjugated sterol in vivo. Phylogenomics highlights that the entire Erg-Asp synthesis/degradation pathway is conserved across "higher" fungi. Given the central roles of sterols and conjugated sterols in fungi, we propose that this tRNA-dependent ergosterol modification and homeostasis system might have broader implications in membrane remodeling, trafficking, antimicrobial resistance, or pathogenicity.

Paramagnetic molecular centers produced by gamma irradiation at 77 K and at room temperature in the novel compound Al5(OH)15(Asp)3.3H2O were studied by ESR spectroscopy. The g value of 2.0034 and the lack of such lines in pure aluminum hydroxide suggested that all the paramagnetic centers observed are related to the aspartic acid molecule. However, none of the paramagnetic centers gave an ESR spectrum characteristic for gamma-irradiated pure aspartic acid powder. The influence of the oxygen on the formation of the paramagnetic centers was noticed. The extreme stability of the paramagnetic molecular centers formed in Al5(OH)15(Asp)3.3H2O suggests that aspartic acid complexed in aluminum hydroxide is a good trap for gamma-radiation energy.