Evolution of plant defense mechanisms - Relationships of phenylcoumaran benzylic ether reductases to pinoresinol-lariciresinol and isoflavone reductases

Pinoresinol-lariciresinol and isoflavone reductase classes are phylogenetic ally related, as is a third, the so-called "isoflavone reductase homologs," This study establishes the first known catalytic function for the latter, as being able to engender the NADPH-dependent reduction of phenylcoumaran b enzylic ethers, Accordingly, all three reductase classes are involved in th e biosynthesis of important and related phenylpropanoid-derived plant defen se compounds. In this investigation, the phenylcoumaran benzylic ether redu ctase from the gymnosperm, Pinus taeda, was cloned, with the recombinant pr otein heterologously expressed in Escherichia coli, The purified enzyme red uces the benzylic ether functionalities of both dehydrodiconiferyl alcohol and dihydrodehydrodiconiferyl alcohol, with a higher affinity for the forme r, as measured by apparent K-m and V-max values and observed kinetic H-3-is otope effects. It abstracts the 4R-hydride of the required NADPH cofactor i n a manner analogous to that of the pinoresinol-lariciresinol reductases an d isoflavone reductases. A similar catalytic function was observed for the corresponding recombinant reductase whose gene was cloned from the angiospe rm, Populus trichocarpa. Interestingly, both pinoresinol-lariciresinol redu ctases and isoflavone reductases catalyze enantiospecific conversions, wher eas the phenylcoumaran benzylic ether reductase only shows regiospecific di scrimination. A possible evolutionary relationship among the three reductas e classes is proposed, based on the supposition that phenylcoumaran benzyli c ether reductases represent the progenitors of pinoresinol-lariciresinol a nd isoflavone reductases.