Novel type III polyketide synthases from Aloe arborescens.

Aloe arborescens is a medicinal plant rich in aromatic polyketides, such as pharmaceutically important aloenin (hexaketide), aloesin (heptaketide) and barbaloin (octaketide). Three novel type III polyketide synthases (PKS3, PKS4 and PKS5) were cloned and sequenced from the aloe plant by cDNA library screening. The enzymes share 85-96% amino acid sequence identity with the previously reported pentaketide chromone synthase and octaketide synthase. Recombinant PKS4 and PKS5 expressed in Escherichia coli were functionally identical to octaketide synthase, catalyzing the sequential condensations of eight molecules of malonyl-CoA to produce octaketides SEK4/SEK4b. As in the case of octaketide synthase, the enzymes are possibly involved in the biosynthesis of the octaketide barbaloin. On the other hand, PKS3 is a multifunctional enzyme that produces a heptaketide aloesone (i.e. the aglycone of aloesin) as a major product from seven molecules of malonyl-CoA. In addition, PKS3 also afforded a hexaketide pyrone (i.e. the precursor of aloenin), a heptaketide 6-(2-acetyl-3,5-dihydroxybenzyl)-4-hydroxy-2-pyrone, a novel heptaketide 6-(2-(2,4-dihydroxy-6-methylphenyl)-2-oxoethyl)-4-hydroxy-2-pyrone and octaketides SEK4/SEK4b. This is the first demonstration of the enzymatic formation of the precursors of the pharmaceutically important aloesin and aloenin by a wild-type PKS obtained from A. arborescens. Interestingly, the aloesone-forming activity was maximum at 50 degrees C, and the novel heptaketide pyrone was non-enzymatically converted to aloesone. In PKS3, the active-site residue 207, which is crucial for controlling the polyketide chain length depending on the steric bulk of the side chain, is uniquely substituted with Ala. Site-directed mutagenesis demonstrated that the A207G mutant dominantly produced the octaketides SEK4/SEK4b, whereas the A207M mutant yielded a pentaketide 5,7-dihydroxy-2-methylchromone.

Structure function analysis of novel type III polyketide synthases from Arabidopsis thaliana.

The genome sequencing project revealed presence of two active chalcone synthase (CHS) homologues (At1g02050 and At4g34850) in the model plant Arabidopsis thaliana. We report herein the two genes encode closely related novel plant-specific type III polyketide synthases (PKSs) that produces long-chain alkyl alpha-pyrones. PKS-A (At1g02050) and PKS-B (At4g34850) share significantly low amino acid sequence identity (20-40%) with other type III PKSs, and the phylogenetic tree analysis revealed that they form a separate cluster located closely to those of bacterial type III PKSs. When expressed in Escherichia coli, both PKS-A and PKS-B accepted unusually long (up to the C(20) chain-length) fatty acyl CoAs as a starter substrate, and carried out sequential condensations with malonyl-CoA to produce triketide and tetraketide alpha-pyrones. Interestingly, despite the low sequence identity, homology modeling revealed that the active-site architecture of PKS-A and PKS-B showed similarity to that of a bacterial type III PKS from Mycobacterium tuberculosis.

Cloning and functional analysis of a novel aldo-keto reductase from Aloe arborescens.

A novel aldo-keto reductase (AKR) was cloned and sequenced from roots of Aloe arborescens by a combination of RT-PCR using degenerate primers based on the conserved sequences of plant polyketide reductases (PKRs) and cDNA library screening by oligonucleotide hybridization. A. arborescens AKR share similarities with known plant AKRs (40-66% amino acid sequence identity), maintaining most of the active-site residues conserved in the AKR superfamily enzymes. Interestingly, despite the sequence similarity with PKRs, recombinant enzyme expressed in Escherichia coli did not exhibit any detectable PKR activities. Instead, A. arborescens AKR catalyzed NADPH-dependent reduction of various carbonyl compounds including benzaldehyde and DL-glyceraldehyde. Finally, a homology model on the basis of the crystal structure of Hordeum vulgare AKR predicted the active-site architecture of the enzyme.