If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Polyketides Biosynthesis|
Some taxa known to possess this pathway include : Curcuma longa
Expected Taxonomic Range: Curcuma
Curcuminoids are a class of diarylheptanoid secondary metabolites produced in turmeric (Curcuma longa) and other related plant species. These yellow pigment compounds appear to offer numerous health benefits based on their anti-inflammatory, antioxidant, anti-tumor, and anti-amyloid properties [Maheshwari06, Ringman05, Weber05, Strimpakos08]. The rhizome of tumeric has been used both for medicine and as a source of spices for cooking, particularly in Asian societies [Sharma05a]. The biosynthesis of three of the most prominent curcuminoids, curcumin, demethoxycurcumin, and bisdemethoxycurcumin is described in this pathway.
About This Pathway
Curcuminoid biosynthesis has been a topic of research for many years. Radiotracer studies have suggested several potential routes for curcuminoid synthesis [Roughley73, Kita08] and complementary research on type III polyketide synthases in several species has provided further insights into potential curcuminoid biosynthetic mechanisms [Schroder97a, Kamo00, Brand06]. In general, these studies suggested that two phenylpropanoid units derived from phenylalanine condense to form the curcumin scaffold. But, it remained unknown whether the hydroxy and methoxy groups were added before or after scaffold formation. A set of labeling experiments suggested that cinnamate (and likely cinnamoyl-CoA) was the primary source of a putative curcumin skeleton intermediate that lacks any functional modifications on either ring (referred to as bisdeshydroxybisdesmethoxycurcumin in [Kita08]). But, the discovery and characterization of four enzymes involved in this pathway, DKS, and CURS1, 2, and 3, led to the proposal that 4-coumaryl-CoA and feruloyl-CoA are likely to be the starter CoA ester substrates used by these enzymes. In this proposed pathway, the hydroxy and methoxy groups are introduced as part of the starter substrates in the first and second condensation reactions to produce a modified diketide intermediate and the final curcuminoid products [Katsuyama09a, Katsuyama09]. It remains to be seen whether the pathway proposed by [Kita08] that calls for later modifications on the curcuminoid skeleton is performed through the activity of any enzymes from Curcuma longa that have not yet been characterized. Although it is not depicted on this pathway, the diketide-CoA esters are predicted to form a β-keto acid intermediate prior to the addition of the second CoA ester [Katsuyama09].
Regulation of curcuminoid biosynthesis will require further study, but a few hints have emerged from the characterization of these enzymes. DCS, the first enzyme in the pathway, appears to be subject to allosteric regulation. Because both DCS and the downstream CURS enzymes use feruloyl-CoA as a substrate, it may be important to ensure that not all of this compound is consumed by DCS. In comparison to the CURS enzymes, the homotropic activation of DCS may cause a relatively greater decrease in activity in response to reduced levels of feruloyl-CoA and thus promote an adequate flow of both the diketide-CoA esters and feruloyl-CoA to the downstream enzymes [Katsuyama09]. In addition, differential control of the three CURS (polyketide synthases) may help to account for differences in the levels and distribution of curcuminoids found within different organs and cultivars of tumeric. Although these enzymes are similar in sequence, they fall into two groups, with CURS1 and CURS2 showing similarities in substrate preference and transcript expression patterns that differ from CURS3 [Katsuyama09a, Katsuyama09].
Based on the medicinal importance of these compounds and other structurally related compounds found in other plant species, research on curcuminoid biosynthesis is likely to continue.
Subpathways: trans-cinnamoyl-CoA biosynthesis
Unification Links: PlantCyc:6432
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