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The FucO protein, a member of the group III "iron-activated" dehydrogenases, catalyzes the interconversion between L-lactaldehyde and L-1,2-propanediol in Escherichia coli. The three-dimensional structure of FucO in a complex with NAD ؉ was solved, and the presence of iron in the crystals was confirmed by X-ray fluorescence. The FucO structure presented here is the first structure for a member of the group III bacterial dehydrogenases shown experimentally to contain iron. FucO forms a dimer, in which each monomer folds into an ␣/␤ dinucleotide-binding N-terminal domain and an all-␣-helix C-terminal domain that are separated by a deep cleft. The dimer is formed by the swapping (between monomers) of the first chain of the ␤-sheet. The binding site for Fe 2؉ is located at the face of the cleft formed by the C-terminal domain, where the metal ion is tetrahedrally coordinated by three histidine residues (His200, His263, and His277) and an aspartate residue (Asp196). The glycine-rich turn formed by residues 96 to 98 and the following ␣-helix is part of the NAD ؉ recognition locus common in dehydrogenases. Site-directed mutagenesis and enzyme kinetic assays were performed to assess the role of different residues in metal, cofactor, and substrate binding. In contrast to previous assumptions, the essential His267 residue does not interact with the metal ion. Asp39 appears to be the key residue for discriminating against NADP ؉ . Modeling L-1,2-propanediol in the active center resulted in a close approach of the C-1 hydroxyl of the substrate to C-4 of the nicotinamide ring, implying that there is a typical metal-dependent dehydrogenation catalytic mechanism.In Escherichia coli and other enterobacteria the anaerobic metabolism of L-fucose and L-rhamnose requires the enzyme lactaldehyde:propanediol oxidoreductase (FucO), which is encoded by the fucO gene of the fucose regulon (6,14,16,24,32). The breakdown of these methylpentoses generates the intermediate metabolite L-lactaldehyde, which under anaerobic conditions, with NADH as a cofactor, is reduced by FucO to L-1,2-propanediol, which is excreted as a fermentation product (14). In mutant strains of E. coli adapted to grow on L-1,2-propanediol, FucO catalyzes the oxidation of the polyol to L-lactaldehyde, which is subsequently oxidized to L-lactate by a specific aldehyde dehydrogenase (41) and introduced into the general metabolism. FucO, which is induced regardless of the respiratory conditions of the culture, remains fully active in the absence of oxygen (11). In the presence of oxygen, this enzyme becomes oxidatively inactivated by a metal-catalyzed oxidation mechanism (10).FucO is an iron-dependent metalloenzyme that is inactivated by other metals, such as zinc, copper, or cadmium (40), and has been reported to be a homodimer formed by monomers consisting of 383 amino acids and having a molecular mass of 40,644 Da. The iron in the active center accounts for the oxidative inactivation of FucO mentioned above (10). A putative iron-binding motif encompassing a 15-amino-ac...

Although the SREBPs are structurally similar to the E-box-binding helix-loop-helix proteins, the Arg-->Tyr substitution yields dramatically different DNA-binding properties that explain how they recognize StREs and regulate expression of genes important for membrane biosynthesis.

The X-ray structure of the C-terminal region of human eukaryotic translation initiation factor 4G (eIF4G) has been determined at 2.2 A resolution, revealing two atypical HEAT-repeat domains. eIF4G recruits various translation factors and the 40S ribosomal subunit to the mRNA 5' end. In higher eukaryotes, the C terminus of eIF4G (4G/C) supports translational regulation by recruiting eIF4A, an RNA helicase, and Mnk1, the kinase responsible for phosphorylating eIF4E. Structure-guided surface mutagenesis and protein-protein interaction assays were used to identify binding sites for eIF4A and Mnk1 within the HEAT-repeats of 4G/C. p97/DAP5, a translational modulator homologous to eIF4G, lacks an eIF4A binding site in the corresponding region. The second atypical HEAT domain of the 4G/C binds Mnk1 using two conserved aromatic/acidic-box (AA-box) motifs. Within the first AA-box, the aromatic residues contribute to the hydrophobic core of the domain, while the acidic residues form a negatively charged surface feature suitable for electrostatic interactions with basic residues in Mnk1.

Alternating d(GA)n DNA sequences form antiparallel stranded homoduplexes which are stabilized by the formation of G.A pairs. Three base pairings are known to occur between adenine and guanine: AH+ (anti).G(syn), A(anti).G(anti) and A(syn).G(anti). Protonation of the adenine residues is not involved in the stabilization of this structure, since it is observed at any pH value from 8.3 to 4.5; at pH < or = 4.0 antiparallel stranded d(GA.GA) DNA is destabilized. The results reported in this paper strongly suggest that antiparallel stranded d(GA.GA) homoduplexes are stabilized by the formation of alternating A(anti).G(anti) and G(anti).A(syn) pairs. In this structure, all guanine residues are in the anti conformation with their N7 position freely accessible to DMS methylation. On the other hand, adenines in one strand adopt the anti conformation, with their N7 position also free for reaction, while those of the opposite strand are in the syn conformation, with their N7 position hydrogen bonded to the guanine N1 group of the opposite strand. A regular right‐handed helix can be generated using alternating G(anti).A(syn) and A(anti).G(anti) pairs.

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