No abstract

Three-coordinate bipyridyl complexes of gold, [(κ-bipy)Au(η-CH)][NTf], are readily accessed by direct reaction of 2,2'-bipyridine (bipy), or its derivatives, with the homoleptic gold ethylene complex [Au(CH)][NTf]. The cheap and readily available bipyridyl ligands facilitate oxidative addition of aryl iodides to the Au(I) center to give [(κ-bipy)Au(Ar)I][NTf], which undergo first aryl-zinc transmetalation and second C-C reductive elimination to produce biaryl products. The products of each distinct step have been characterized. Computational techniques are used to probe the mechanism of the oxidative addition step, offering insight into both the origin of the reversibility of this process and the observation that electron-rich aryl iodides add faster than electron-poor substrates. Thus, for the first time, all steps that are characteristic of a conventional intermolecular Pd(0)-catalyzed biaryl synthesis are demonstrated from a common monometallic Au complex and in the absence of directing groups.

The synthases that produce fatty acids in mammals (FASs) are arranged as large multidomain polypeptides. The growing fatty acid chain is bound covalently during chain elongation and reduction to the acyl carrier protein (ACP) domain that is then able to access each catalytic site. In this work we report the highresolution nuclear magnetic resonance (NMR) solution structure of the isolated rat fatty acid synthase apoACP domain. Fatty acid biosynthesis in mammals is important not only for energy homeostasis and development but also as a potential target for the treatment of obesity (1) and cancer (2). Type I fatty acid synthases (FASs) 3 that catalyze the biosynthesis of fatty acids in mammals, utilize simple acyl units, bound to the phosphopantetheine arm of a holo acyl carrier protein (ACP) domain, for chain initiation and elongation. The recent elucidation of the low resolution structure of the mammalian FAS by x-ray crystallography (3) has provided key structural and mechanistic insights into this important enzyme. Although the resolution of the crystal is insufficient to discern the backbone and side chains, the electron density has permitted the authors to propose a model based on the structures of individual domains and homologous enzymes. The authors have proposed a "head to head" dimeric model that comprises a central core consisting of the enol-reductase, dehydratase (DH), and ketosynthase with the malonyl transferase and ketoreductase domains being located peripherally. Dimerization occurs through association of the KS domains. Notable absences in the crystal structure are the locations of the peripheral ACP and thioesterase domains, suggesting that the positions of the ACP and thioesterase domains are relatively mobile compared with the core of the FAS. In comparison, bacterial Type II FASs consist of discrete monofunctional proteins (4). Structural studies of the Type II FAS ACP components are particularly well developed and have revealed the structural basis of acyl chain binding and the phenomenon of conformational switching. The crystal structures of Escherichia coli FAS butyryl (5), hexanoyl-, heptanoyl-, and decanoyl-ACPs (6) and nuclear magnetic resonance (NMR) structures of spinach FAS decanoyl-and stearoyl-ACPs have been reported (7). These structures reveal that during fatty acid biosynthesis, fully saturated acyl chains are sequestered within a central cavity in the ACP formed through conformational changes in the protein. The fatty acid chain is sequestered within the hydrophobic core of the ACP perhaps to protect the thioester moiety from hydrolysis. Binding of the acyl chain also influences the dynamics of the ACPs. Spinach FAS holo-ACP exists in equilibrium between a folded and largely disordered form, however, upon acylation this equilibrium is shifted toward the folded form. At present the physiological role of switching of this, and other ACPs, is unknown (8, 9). It has been suggested that switching confers allosteric regulation of the ACP, whereby its interaction with other enzymes...

While X-ray and NMR structures are now available for most components of the Type II fatty acid synthase (FAS), there are no structures for Type I FAS domains. A region from the mammalian (rat) FAS, including the putative acyl carrier protein (ACP), has been cloned and over-expressed. Here we report multinuclear, multidimensional NMR studies which show that this isolated ACP domain contains four alpha-helices (residues 8-16 [1]; 41-51 [2]; 58-63 [3] and 66-74 [4]) and an overall global fold characteristic of ACPs from both Type II FAS and polyketide synthases (PKSs). The overall length of the structured ACP domain (67 residues) is smaller than the structured regions of the Eschericia coli FAS ACP (75 residues), the actinorhodin PKS ACP (78 residues) and the Bacillus subtilis FAS ACP (76 residues). We further show that the rat FAS ACP is recognised as an efficient substrate by enzymes known to modify Type II ACPs including phosphopantetheinyl and malonyl transferases, but not by the heterologous S. coelicolor minimal polyketide synthase.

The actinorhodin (act) synthase acyl carrier protein (ACP) from Streptomyces coelicolor plays a central role in polyketide biosynthesis. Polyketide intermediates are bound to the free sulfhydryl group of a phosphopantetheine arm that is covalently linked to a conserved serine residue in the holo form of the ACP. The solution NMR structures of both the apo and holo forms of the ACP are reported, which represents the first high resolution comparison of these two forms of an ACP. Ensembles of twenty apo and holo structures were calculated and yielded atomic root mean square deviations of well-ordered backbone atoms to the average coordinates of 0.37 and 0.42 A, respectively. Three restraints defining the protein to the phosphopantetheine interface were identified. Comparison of the apo and holo forms revealed previously undetected conformational changes. Helix III moved towards helix II (contraction of the ACP), and Leu43 on helix II subtly switched from being solvent exposed to forming intramolecular interactions with the newly added phosphopantetheine side chain. Tryptophan fluorescence and S. coelicolor fatty acid synthase (FAS) holo-synthase (ACPS) assays indicated that apo-ACP has a twofold higher affinity (K(d) of 1.1 muM) than holo-ACP (K(d) of 2.1 muM) for ACPS. Site-directed mutagenesis of Leu43 and Asp62 revealed that both mutations affect binding, but have differential affects on modification by ACPS. Leu43 mutations in particular strongly modulate binding affinity for ACPS. Comparison of apo- and holo-ACP structures with known models of the Bacillus subtilis FAS ACP-holo-acyl carrier protein synthase (ACPS) complex suggests that conformational modulation of helix II and III between apo- and holo-ACP could play a role in dissociation of the ACP-ACPS complex.