Cloning and characterization of NADPH-dependent double-bond reductases from Alnus sieboldiana that recognize linear diarylheptanoids as substrates

Abstract

Diarylheptanoids are secondary metabolites of plants that comprise a C₆-C₇-C₆ scaffold. They can be broadly classified into linear-type and cyclic-type diarylheptanoids based on their chemical structures. Actinorhizal trees, such as Casuarina, Alnus, and Myrica, which form nodule symbiosis with actinomycetes Frankia, produce cyclic diarylheptanoids (CDHs); in Alnus sieboldiana Matsum. in particular, we have reported that the addition of CDHs leads to an increase in the number of nodules. However, the information available on the biosynthesis of CDHs is scarce. A greater number of plants CDHs (including those isolated from actinorhizal trees) with a saturated heptane chain have been isolated compared with linear, non-cyclic diarylheptanoids. To identify the genes involved in the synthesis of these compounds, genes with significant sequence similarity to existing plant double-bond reductases were screened in A. sieboldiana. This report describes the isolation and characterization of two A. sieboldiana double-bond reductases (AsDBR1 and AsDBR2) that catalyze the NADPH-dependent reduction of bisdemethoxycurcumin and curcumin. The optimum pH for the two enzymes was 5.0. The apparent K_m values for bisdemethoxycurcumin and NADPH were 4.24 and 3.53 μM in the case of AsDBR1, and 2.55 and 2.13 μM for AsDBR2. The k_cat value was 9.4-fold higher for AsDBR1 vs. AsDBR2 when using the bisdemethoxycurcumin substrate. Interestingly, the two AsDBRs failed to reduce the phenylpropanoid monomer.

Keywords: Actinorhizal symbiosis; Alnus sieboldiana; Alnusonol; Betulaceae; Bisdemethoxycurcumin; Curcumin; Cyclic diarylheptanoids; Diarylheptanoids; Double-bond reductase.