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J Biol Chem, Vol. 275, Issue 7, 4871-4879, February 18, 2000

The Molecular Structure of Hyperthermostable Aromatic Aminotransferase with Novel Substrate Specificity from Pyrococcus horikoshii^*

Ikuo Matsui^§, Eriko Matsui, Yukihiro Sakai, Hisasi Kikuchi^¶, Yutaka Kawarabayasi^¶, Hideaki Ura, Shin-ichi Kawaguchi, Seiki Kuramitsu, and Kazuaki Harata^§

From the  National Institute of Bioscience and Human Technology, Tsukuba, Ibaraki 305, the ^¶ National Institute of Technology and Evaluation, Ministry of International Trade and Industry, Nishihara, Shibuyaku, Tokyo, and the  Department of Biology, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

Aromatic amino acid aminotransferase (ArATPh), which has a melting temperature of 120 °C, is one of the most thermostable aminotransferases yet to be discovered. The crystal structure of this aminotransferase from the hyperthermophilic archaeon Pyrococcus horikoshii was determined to a resolution of 2.1 Å. ArATPh has a homodimer structure in which each subunit is composed of two domains, in a manner similar to other well characterized aminotransferases. By the least square fit after superposing on a mesophilic ArAT, the ArATPh molecule exhibits a large deviation of the main chain coordinates, three shortened -helices, an elongated loop connecting two domains, and a long loop transformed from an -helix, which are all factors that are likely to contribute to its hyperthermostability. The pyridine ring of the cofactor pyridoxal 5'-phosphate covalently binding to Lys²³³ is stacked parallel to F121 on one side and interacts with the geminal dimethyl-CH/ groups of Val²⁰¹ on the other side. This tight stacking against the pyridine ring probably contributes to the hyperthermostability of ArATPh. Compared with other ArATs, ArATPh has a novel substrate specificity, the order of preference being Tyr > Phe > Glu > Trp > His Met > Leu > Asp > Asn. Its relatively weak activity against Asp is due to lack of an arginine residue corresponding to Arg²⁹²* (where the asterisk indicates that this is a residues supplied by the other subunit of the dimer) in pig cytosolic aspartate aminotransferase. The enzyme recognizes the aromatic substrate by hydrophobic interaction with aromatic rings (Phe¹²¹ and Tyr⁵⁹*) and probably recognizes acidic substrates by a hydrophilic interaction involving a hydrogen bond network with Thr²⁶⁴*.

The atomic coordinates and structure factors (code 1DJU) have been deposited in the Protein Data Bank, Brookhaven National Laboratory, Upton, NY.

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