| Research Abstract |
(1) Recombinant chalcone synthase (CHS) and Stilbene synthase from various plants accepted CoA esters of long-chain fatty acid (CHS up to the C12 ester, while STS up to the C14 ester) as a starter substrate, and carried out sequential condensations with malonyl-CoA, leading to formation of triketide and tetraketide a-pyrones. The catalytic diversities of the enzymes provided further mechanistic insights into the type III PKS reactions, and suggested involvement of the CHS-superfamily enzymes in the biosynthesis of long-chain alkyl polyphenols such as urushiol and ginkgolic acid in plants.
A cDNA encoding a novel plant type III polyketide synthase (PKS) was cloned from rhubarb (Rheum palmatum). A recombinant enzyme expressed in E. coli accepted acetyl-CoA as a starter, carried out six successive condensations with malonyl-CoA and subsequent cyclization to yield an aromatic heptaketide, aloesone. The enzyme shares 60% amino acid sequence identity with chalcone synthases (CHSs), and mainta
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ins almost identical CoA binding site and catalytic residues conserved in the CHS superfamily enzymes. Further, homology modeling predicted that the 43-kDa protein has the same overall fold as CHS. This provides new insights into the catalytic functions of type III PKSs, and suggests further involvement in the biosynthesis of plant polyketides.
(2) 1-Methylidenesqualene and 25-methylidenesqualene were converted to 30-methylidenehop-22(29)-ene by squalene: hopene cyclase from Alicyclobacillus acidacaldarius. It was remarkable that both analogues generated the same product. The hopanyl intermediate cation, stabilized by the methylidene residue, enabled a rotation of the isobutenyl group at C-21 prior to the final proton elimination. In contrast, in the formation of hop-22(29)-ene, the final proton abstraction takes place regiospecifically from the Z-methyl group, which was verified by cyclization of (1,1,1,24,24,24-2H6) squalene into (23,23,23,30,30,30-2H6) hop-22(29)-ene. Squalene: hopene cyclase (SHC) from A, acidocaldarius accepted 26-methylidenesqualene and 27-methylidenesqualene as a substrate and converted to novel pentacyclic C31 polyprenoids; a dammarene derivative with a 6.6.6.5-6 ring system and 26-methylidene-hop-22(29)-ene, respectively. The broad substrate specificity of the enzyme provided important information on the structure and function of SHC. Interestingly, 27-MS was also found to be a potent inhibitor of the bacterial SHC (IC501/45 lM), while 26-MS just showed poor enzyme inhibition.
(3) A cDNA encoding adenylate isopentenyltransferase (AIPT) was cloned and sequenced from cones of hop (Humulus lupulus L.) by RT-PCR using oligonucleotide primers based on the conserved sequences of Arabidopsis thaliana AIPT isozymes (AtIPT1, AtIPT3, AtIPT4, AtIPT5, AtIPT6, AtIPT7 and AtIPT8). A full-length cDNA contained a 990-bp open reading frame encoding a molecular mass of 36,603 Da protein with 329 amino acids. Further, DNA sequencing of genomic DNA revealed absence of introns in the frame. On Southern blot analysis, a single AIPT gene was detected in H. lupulus, while RT-PCR analyses demonstrated that the gene was equally expressed in almost all tissues in the plant including roots, stems, leaves and cones. The deduced amino acid sequence shares 38-51% identity to those of A. thaliana AtIPTs. A recombinant enzyme expressed in Escherichia coli catalyzed isopentenyl transfer reaction from dimethylallyldiphosphate (DMAPP) to the N6 amino group of adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP), respectively. In contrast, other nucleotides; guanosine monophosphate (GMP), inosine monophosphate (IMP), cytosine monophosphate (CMP), uridine monophosphate (UMP), were not accepted as a substrate. Interestingly, steady-state kinetic analyses revealed that the isopentenylation of ADP and ATP were more efficient than that of AMP as previously reported for A. thaliana AtIPT4. Finally, H. lupulus AIPT contains the putative ATP/GTP binding motif at the N-terminal as in the case of other known isopentenyltransferases. Site-directed mutagenesis of a conserved Asp62, located right after the ATP/GTP binding motif, with Ala resulted in complete loss of enzyme activity. Less
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