Of 17 genes annotated in the
Arabidopsis
genome database as cinnamyl alcohol dehydrogenase (CAD) homologues, an
in silico
analysis revealed that 8 genes were misannotated. Of the remaining nine, six were catalytically competent for NADPH-dependent reduction of
p
-coumaryl, caffeyl, coniferyl, 5-hydroxyconiferyl, and sinapyl aldehydes, whereas three displayed very low activity and only at very high substrate concentrations. Of the nine putative CADs, two (AtCAD5 and AtCAD4) had the highest activity and homology (≈83% similarity) relative to bona fide CADs from other species. AtCAD5 used all five substrates effectively, whereas AtCAD4 (of lower overall catalytic capacity) poorly used sinapyl aldehyde; the corresponding 270-fold decrease in
k
enz
resulted from higher
K
m
and lower
k
cat
values, respectively. No CAD homologue displayed a specific requirement for sinapyl aldehyde, which was in direct contrast with unfounded claims for a so-called sinapyl alcohol dehydrogenase in angiosperms. AtCAD2, 3, as well as AtCAD7 and 8 (highest homology to sinapyl alcohol dehydrogenase) were catalytically less active overall by at least an order of magnitude, due to increased
K
m
and lower
k
cat
values. Accordingly, alternative and/or bifunctional metabolic roles of these proteins in plant defense cannot be ruled out. Comprehensive analyses of lignified tissues of various
Arabidopsis
knockout mutants (for AtCAD5, 6, and 9) at different stages of growth/development indicated the presence of functionally redundant CAD metabolic networks. Moreover, disruption of AtCAD5 expression had only a small effect on either overall lignin amounts deposited, or on syringyl-guaiacyl compositions, despite being the most catalytically active form
in vitro
.
在
拟南芥基因组数据库中注释为
肉桂醇脱氢酶(CAD)同源
基因的17个
基因中,
in silico分析发现8个
基因被错误注释。剩下的9个
基因中,有6个对
p-
香豆素醛、
咖啡醛、
木质素醛、5-羟基
木质素醛和芥子醛的
NADPH依赖性还原具有催化能力,而另外三个仅在非常高的底物浓度下表现出极低的活性。在这9个假定的CAD中,两个(AtCAD5和AtCA
D4)的活性和同源性(约83%相似度)相对于其他物种的真正CAD最高。AtCAD5有效地使用了所有五种底物,而AtCA
D4(总催化能力较低)使用芥子醛效果不佳;相应的270倍的
kenz减少是由于更高的
Km和更低的
kcat值所导致的。没有CAD同源物表现出对芥子醛的特殊要求,这与关于被称为被称为
芥子醇脱氢酶的角果植物的不成立的说法直接相反。AtCAD2、3以及AtCAD7和8(与
芥子醇脱氢酶的同源性最高)总体上催化活性至少降低一个数量级,由于增加的
Km和较低的
kcat值。因此,这些蛋白质在植物防御中的替代和/或双功能代谢作用不能被排除。对不同生长/发育阶段的各种
拟南芥敲除突变体(对于AtCAD5、6和9)的木质化组织进行全面分析表明存在功能冗余的CAD代谢网络。此外,尽管是
in vitro中最具催化活性的形式,破坏AtCAD5表达仅对总体沉积的
木质素量或丁香基-
愈创木基组成产生了很小的影响。