Both cinnamoyl CoA reductases (CCRs) and cinnamyl alcohol dehydrogenases (CADs) are substrate versatile NADPH-dependent enzymes. These proteins were detected and purified in the laboratories of Zenk and coworkers (118, 120, 123) in the early 1970s; CCR is a type-B reductase abstracting the 4-pro-S-hydrogen of NADPH (124), whereas CAD is a type-A reductase (4-pro-R-hydrogen abstraction) (123). As for 4CLs, all known CCRs and CADs are quite substrate versatile: CCR can catalyze the in vitro reduction of p-coumaroyl (14), caffeoyl (15), feruloyl (16), 5-hydroxyferuloyl (17), and sinapoyl (18) CoAs into the resulting aldehydes 19-23, and the first gene encoding a CCR was described in 1997 (125). CADs are able to catalyze the conversion of aldehydes 19-23 into the monolignols 1-5, with the first bona fide gene encoding a CAD reported by Knight et al. (126); earlier reports of CAD being cloned (127) later turned out to be malic enzyme (128, 129).

In the Arabidopsis Information Resource (TAIR), there were some 11 genes annotated as CCR/CCR-like, and another 17 as CAD/CAD-like (93). To date, only two CCRs have been demonstrated to have both this activity and physiological function (130-132). A similar situation exists for CADs: two of these (AtCAD4 and AtCAD5) are the most catalytically active, and another four (AtCAD2, 3, 7, and 8) have very low overall activities making their involvement in lignification/monolignol formation suspect (56); additionally, AtCAD1, 6, and 9 did not have any detectable CAD activities proper in vitro, and thus their biochemical function currently remains unknown (56).

The biochemical mechanism of CAD has also been the subject of considerable interest, and the X-ray crystal structure of AtCAD5 was recently described (133). Site-directed mu­tagenesis has implicated Glu70 in catalysis, the latter being coordinated with the catalytic Zn2+ through its side chain (Figure 7.9A). A proton shuttle mechanism for CAD has also been provisionally proposed (Figure 7.9B) but remains to be experimentally proven.