| Summary: | 博士 === 臺灣大學 === 生化科學研究所 === 98 === Cytokinins are important plant hormones, and their biosynthesis most begins with the transfer of isopentenyl group from dimethylallyl diphosphate (DMAPP) to the N6-amino group of adenine by either adenylate isopentenyltransferase (AIPT) or tRNA-IPT. Plant AIPTs use ATP/ADP as an isopentenyl acceptor and bacterial AIPTs prefer AMP, whereas tRNA-IPTs act on specific sites of tRNA. The crystal structure of an AIPT-ATP complex from Humulus lupulus (HlAIPT) presented here is similar to the previous structures of Agrobacterium AIPT and yeast tRNA-IPT. The enzyme is structurally homologous to the NTP-binding kinase family of proteins but forms a solvent-accessible channel that binds to the donor substrate DMAPP, which is directed toward the acceptor substrate ATP/ADP. When measured with isothermal titration calorimetry, some nucleotides displayed different binding affinities to HlAIPT with an order of ATP > dATP ~ ADP > GTP > CTP > UTP. However, further measurements of activity using these nucleotides showed that the order becomes ATP > ADP > dATP > CTP >> GTP ≈ UTP ≈ 0. In addition to DMAPP (C5), HlAIPT also showed remarkable activity for GPP (C10) as the donor substrate. Two basic residues Lys275 and Lys220 in HlAIPT interact with the β and γ-phosphate of ATP. By contrast, the interactions are absent in Agrobacterium AIPT because they are replaced by the acidic residues Asp221 and Asp171. Despite its structural similarity to the yeast tRNA-IPT, HlAIPT has evolved with a different binding strategy for adenylate. Finally, by screening a series of dinucleotide polyphosphates, it was found, surprisingly, that the binding affinities of some diadenosine polyphosphates (A(p)4A, A(p)5A and A(p)6A) are even higher than those of the original substrates (ATP and ADP). These results may imply that novel substrates of HlAIPT have been found in plant.
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