# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 5 Plasticity and Promiscuity of the Plant Specialized Metabolism  
Compared to enzymes of the primary metabolism, enzymes of the specialized metabolism are, paradoxically, less specialized. They have lower \(k_{\text{cat}}\) values (the turnover number) and a broader substrate specificity, meaning that they catalyze the same reaction at similar, relatively low, rates on multiple substrates [147; 15]. If the different substrates for low substrate specific enzymes are available, this may result in pathways with a grid-like structure, as is commonly seen in the biosynthesis pathways of the core skeletons of specialized metabolites, including monolignols, flavonoids, oxylipins and brassinosteroids [148; 149]. Poor substrate specificity of modifying, rather than the core-pathway enzymes, may contribute extensively to structural diversity, depending on the availability and compartmentation of the substrates. The broad substrate specificity of BAHD acyltransferases is well documented [150; 151]. Across the Solanaceae family, considering 12 aliphatic CoA donors with only a few BAHD enzymes as the catalysts, more than 6000 theoretical structures for tetra-acylsugars are possible [152]. Broad specificity also leaves the door open for substrate promiscuity of the enzymes, referring to their ability to catalyze slow, secondary, physiologically irrelevant reactions on alternative substrates that become available under particular conditions [153; 15]. Promiscuous activity becomes apparent in mutants and transgenic plants with perturbed biosynthesis pathways. For example, _Arabidopsis coumarin synthase (cosy)_ mutants accumulate a large array of \(o\)-hydroxyphenylpropanoids that are not detected in wild type plants [5]. The combination of a mutation in the gene encoding caffeic acid \(O\)-methyltransferasease (comt) with over-expression of ferulate 5-hydroxylase in Arabidopsis resulted in shifts in phenolic metabolism toward the formation of 5-hydroxy-substituted phenylpropanoids. In the latter example, it was proposed that these compoundsoriginated from existing biochemical routes that are normally used for 5-methoxylated and 5-unsubstituted phenylpropanoids [76]. In general, perturbation of an enzymatic step in the phenylproponoid metabolism leads to diversion of the metabolic flux into upstream and branch pathways, and sometimes the newly accumulating phenolic molecules oxidatively couple into small oligomers, further extending the structural diversity [69]. In poplar, down-regulation of the gene encoding CINNAMYL ALCOHOL DEHYDROGENASE1 (CAD1), an enzyme that catalyzes the last step of monolignol biosynthesis, led to, besides the accumulation of a range of hydroxycinnamate derivates that were not observed in wild type plants, the apparition of a newly identified coupling product of two sinapaldehyde radicals [7]. In Arabidopsis _atr2_ mutants (_Arabidopsis thaliana cytochrome p450 reductase2_), radical coupling lead to the formation of a series of new, H-unit enriched oligoliglions [77]. Also in the primary metabolism, substrate promiscuity and catalytic promiscuity, i.e., an aberrant reaction catalyzed on the enzyme's normal substrate, are a common phenomenon [126; 154]. Recent studies show that products formed by mis-catalysis in the plant primary metabolism are detoxified by repair enzymes that reconvert the abnormal metabolites to normal ones [154; 155; 156; 157]. In plants with perturbed phenylpropanoid biosynthesis, modifying enzymes transfer hexose, acetylhexose, sulfate, hexuronate, malate, glutamate, aspartate, shikimate or quinate to abnormally accumulating phenolics, altering their solubility, toxic potential and compartmentalization [69]. The combination of oxidative radical coupling and several enzymatic decorations can result in highly conjugated molecules, such as, for example, a radical coupling product of coniferyl alcohol and ferulic acid, conjugated with a glucose and a malate moiety, identified in _ccr1_ (_cinnamoyl-ca reductase1_) Arabidopsis mutants [158].