TyrA proteins belong to a family of dehydrogenases that are dedicated to L-tyrosine biosynthesis. The three TyrA subclasses are distinguished by their substrate specificities, namely the prephenate dehydrogenases, the arogenate dehydrogenases, and the cyclohexadienyl dehydrogenases, which utilize prephenate, L-arogenate, or both substrates, respectively. The molecular mechanism responsible for TyrA substrate selectivity and regulation is unknown. To further our understanding of TyrA-catalyzed reactions, we have determined the crystal structures of Aquifex aeolicus prephenate dehydrogenase bound with NAD ؉ plus either 4-hydroxyphenylpyuvate, 4-hydroxyphenylpropionate, or L-tyrosine and have used these structures as guides to target active site residues for site-directed mutagenesis. From a combination of mutational and structural analyses, we have demonstrated that His-147 and Arg-250 are key catalytic and binding groups, respectively, and Ser-126 participates in both catalysis and substrate binding through the ligand 4-hydroxyl group. The crystal structure revealed that tyrosine, a known inhibitor, binds directly to the active site of the enzyme and not to an allosteric site. The most interesting finding though, is that mutating His-217 relieved the inhibitory effect of tyrosine on A. aeolicus prephenate dehydrogenase. The identification of a tyrosine-insensitive mutant provides a novel avenue for designing an unregulated enzyme for application in metabolic engineering.Tyrosine serves as a precursor for the synthesis of proteins and secondary metabolites such as quinones (1-3), alkaloids (4), flavonoids (5), and phenolic compounds (5, 6). In prokaryotes and plants, these compounds are important for viability and normal development (7).The TyrA protein family consists of dehydrogenase homologues that are dedicated to the biosynthesis of L-tyrosine. These enzymes participate in two independent metabolic branches that result in the conversion of prephenate to L-tyrosine, namely the arogenate route and the 4-hydroxyphenylpyruvate (HPP) 3 routes. Although both of these pathways utilize a common precursor and converge to produce a common end-product, they differ in the sequential order of enzymatic steps. Through the HPP route, prephenate is first decarboxylated by prephenate dehydrogenase (PD) to yield HPP, which is subsequently transaminated to L-tyrosine via a TyrB homologue (8). Alternatively, through the arogenate route, prephenate is first transaminated to L-arogenate by prephenate aminotransferase and then decarboxylated by arogenate dehydrogenase (AD) to yield L-tyrosine (9 -11) (see Fig. 1A).There are three classes of TyrA enzymes that catalyze the oxidative decarboxylation reactions in these two pathways. The enzymes are distinguished by the affinity for cyclohexadienyl substrates. PD and AD accept prephenate or L-arogenate, respectively, whereas the cyclohexadienyl dehydrogenases can catalyze the reaction using either substrate (12).To ensure efficient metabolite distribution of the pathway intermediates, T...

Under acidic conditions, aluminum (Al) toxicity is an important factor limiting plant productivity; however, the application of phosphorus (P) might alleviate the toxic effects of Al. In this study, seedlings of two vegetatively propagated Eucalyptus clones, E. grandis × E. urophylla ‘G9’ and E. grandis × E. urophylla ‘DH32-29’were subjected to six treatments (two levels of Al stress and three levels of P). Under excessive Al stress, root Al content was higher, whereas shoot and leaf Al contents were lower with P application than those without P application. Further, Al accumulation was higher in the roots, but lower in the shoots and leaves of G9 than in those of DH32-29. The secretion of organic acids was higher under Al stress than under no Al stress. Further, under Al stress, the roots of G9 secreted more organic acids than those of DH32-29. With an increase in P supply, Al-induced secretion of organic acids from roots decreased. Under Al stress, some enzymes, including PEPC, CS, and IDH, played important roles in organic acid biosynthesis and degradation. Thus, our results indicate that P can reduce Al toxicity via the fixation of elemental Al in roots and restriction of its transport to stems and leaves, although P application cannot promote the secretion of organic acid anions. Further, the higher Al-resistance of G9 might be attributed to the higher Al accumulation in and organic acid anion secretion from roots and the lower levels of Al in leaves.

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Fe and its oxides have been made in the fonn of nanocrystalline powders. The chemical as well as the vacuum deposition technologies were used to produce a great variety of microstructure, such as with different grain size, internal strain and inhomogeneity. The whole X -ray diffi·action patterns of these smnples were processed by Rietveld analysis using DBWS-9411 code supplied by Prof. R.A. Yotmg. On the other side, the interfaces of these nanocrystals were simulated by molecular dynamics with several theoretical structural models. After sufficient atomic relaxation the interface structrn·e reached equilibrium. Then the theoretical effect of interfaces on the whole X -ray diffraction pattern can be evaluated in detail. The comparison between the theoretical and experimental diffraction patterns gave a possibility to select the correct structural models as well as the interatomic potentials and related parllil1eters. Using the correct interface structure one can predict a series of physical and mechanical properties of nanocrystals on a much sounder basis as compm·ed to usual methods. This is important especially for those properties which are very difficult to measure experimentally. In this work we present a mathematical model capable of describing in detail the atomic structure of arbitrary grain boundaries (GB) in metals and other materials and constitutes the first step towards a complete crystallographic description of GBs. Due to its evident technological implications, GB have been intensely studied in the past with limited success, due, mostly, to the absence of a complete model accounting for the structure of arbitrary GB. This has hllil1pered the establishment of a correlation between bmmdary structrn·e and properties.Using tools closely related to those used in Quasi crystals, the present model describes in detail the atomic structure of arbitrwy GB, not being restricted to those known as "special" and establishes a link between the fields of quasicrystals and GB, permitting a better understanding of both. The model replaces the notion of "coincidence sites" by "best possible fit" and it is continuous (as the 0-lattice) over all the angular range.Om results show that the value of Sigma alone is not enough to characterize special boundaries. In particular, it has been found that some low Sigma GB in FCC metals have a very poor fit (Sigma 17) while others with higher Sigma are SUiprisingly good (Sigma 41 ), thus explaining previous experimental results indicating that Sigma 41 is a special boundary. TEM and AFM studies have been carried out to demonstrate the influence of growth parllil1eters and substr11te orientation on the structure of SiC films deposited simultanously and alternately from two sources by solid-somce molecular be= epitaxy at low substrate temperatures (750 C-900 C). At 750 C and high growth rates (>2nm/min) polycrystalline columnar films were grown which show a weak orientation relationship to the substrate. While growing the films under conditions for Si stabilization at the surface a...

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