Annual Plant Reviews, Biochemistry of Plant Secondary Metabolism by Michael Wink

Author:Michael Wink
Language: eng
Format: epub
Publisher: Wiley
Published: 2011-06-09T16:00:00+00:00

4.4.2.2 Pinoresinol/lariciresinol reductase

Pinoresinol/lariciresinol reductase (PLR) catalyses the stereospecific NADPH-dependent reduction of first pinoresinol to lariciresinol and afterwards lariciresinol to secoisolariciresinol (Fig. 4.8). The first PLR forming (−)-secoisolariciresinol was characterized, purified and cloned from Forsythia intermedia (Chu et al., 1992; Dinkova-Kostova et al., 1996). This PLR reduces (+)-pinoresinol to (+)-lariciresinol and furtheron to (−)-secoisolariciresinol, thus conserving the conformation. Together with phenylcoumaran benzylic ether reductases (PCBERs) and isoflavonoid reductases, PLRs form the PIP family of reductases (Dinkova-Kostova et al., 1996; Gang et al., 1999b). The crystal structures of PLR as well as the related PCBER have been solved by Min et al. (2003) and thus helped in the understanding of the distinct enantioand regiospecificity of these dimeric enzymes.

PLRs with different enantioselectivities have been detected in and cloned from different Linum species (L. album, L. usitatissimum, L. perenne) by the group of Fuss (von Heimendahl et al., 2005; Hemmati et al., 2007a). Enzymes of opposite enantioselectivity occur within one species, e.g. in Thuja plicata (Fujita et al., 1999), in Linum usitatissimum (Hemmati et al., personal communication) and in Daphne species or Arctium lappa (Okunishi et al., 2001; Suzuki et al., 2002b). Moreover, the reductase can change its enantioselectivity as demonstrated for (+)-pinoresinol/(−)-lariciresinol reductase from L. perenne involved in the biosynthesis of justicidin B. Down-regulation of the corresponding gene by RNAi reduced the accumulation of justicidin B in hairy root cultures of L. perenne down to 24% (Hemmati et al., 2007a).

PLR was found to be expressed in the seed coats of maturing seeds of Linum usitatissimum where secoisolariciresinol diglucoside (as cell wallbound ‘polymer’) is the main lignan stored (Teoh et al., 2003; Hano et al., 2006b). The cell wall-localized lignan complex was further identified by Ford et al. (2001) as secoisolariciresinol diglucoside-hydroxymethyl glutaryl-ester linked oligomers.

4.4.2.3 Secoisolariciresinol dehydrogenase

(−)-Secoisolariciresinol is oxidized to (−)-matairesinol by the NAD(P)-dependent secosiolariciresinol dehydrogenase (SDH; Fig. 4.8) identified, characterized and cloned from Forsythia intermedia and Podophyllum peltatum (Xia et al., 2001). The enzyme showed similarities to NAD(H)-dependent short-chain dehydrogenases/reductases. The Podophyllum enzyme has been crystallized and its structure is solved (Youn et al., 2005). It is active as a homotetramer. A highly conserved catalytic triad (Ser153, Tyr167 and Lys171) was identified in which Tyr167 functions as a general base. During catalysis NAD binds first, followed by (−)-secoisolariciresinol. The hydride abstracted from the substrate takes the pro-S position in NADH (Moinuddin et al., 2006). The catalytic step catalysed by SDH is the last one to finally establish the configuration of the dibenzylbutyrolactone lignans which usually are accumulated optically pure in plants (Suzuki and Umezawa, 2007).

SDH in crude protein preparations from Daphne species (Thymelaeaceae) showed the preferential NADP-dependent formation of (−)-matairesinol, although the (+)-enantiomer is accumulated in the plant (Okunishi et al., 2004).

4.4.2.4 Methylation reactions

Methylation of matairesinol withcell-freeextracts of Forsythiaintermedialeads to the formation of arctigenin and isoarctigenin (Ozawa et al., 1993). SAM serves as methyl donor. With racemic matairesinol as a substrate, racemic arctigenin and isoarctigenin were formed, with a slight preference for the (−)-enantiomers. Only one methyl group was transferred to matairesinol. It

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