Hydroxycinnamoyl coenzyme

Hoffmann, L., Besseau, S., Geoffroy, P., Ritzenthaler, Ch., Meyer, D., Lapierre, C., Pollet, B., and Legrand, M., 2004, Silencing of hydroxycinnamoyl-coenzyme A shikimate/quinate hydroxyltransferase affects phenylpropanoid biosynthesis, Plant Cell 16 1446-1465. 

Plant phenolics represent a very large group of defensive compounds defined here as having a phenol (hydroxybenzene) moiety. In some instances substances having a phenolic precursor (e.g. methoxybenzene derivatives) have conveniently also been included in this category. Phenolics derive biosynthetically from hydroxycinnamoyl coenzyme A (yielding a phenyl-propanoid moiety). 

Strack, D., Gross, W., Heilemann, J., Keller, H. and Ohm, S. (1988) Enzymic synthesis of hydroxycinnamic acid esters of glucaric acid and hydroaromatic acids from the respective 1-O-hydroxycinnamoylglucoside and hydroxycinnamoyl-coenzyme-A thioester as acyldonors with a protein preparation from Cestrum elegans leaves. Z. Naturforsch., 43c, 32-6. 

Scheme 1. Lignin biosynthesis and phenolic biosynthesis from hydroxycinnamoyl coenzyme A esters 17,100)...

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

Kamsteeg, J., Van Brederode, J., Hommels, C.H. and Van Nigtevecht, G. (1980) Identification, properties and genetic control of hydroxycinnamoy 1-coenzyme A antho-cyanidin 3-rhamnosyl (1 6) glucoside, 4 -hydroxycinnamoyl transferase isolated from petals of Silene dioica. Biochem. Physiol. Pfanzen, 175, 403-11. 

Figure 4 Current view of the phenylpropanoid pathway to the monolignols 19-23. 4CL, 4-hydroxycinnamate coenzyme Aligases pC3H , p-coumarate 3-hydroxylase C4H, cinnamate 4-hydroxylase CAD, cinnamyl alcohol dehydrogenases CCOMT, hydroxycinnamoyl CoA O-methyltransferases CCR, cinnamoyl-CoA oxidoreductases COMT, caffeic acid O-methyltransferases F5H , ferulate 5-hydroxylase HCT, hydroxycinnamoyl-CoA shikimate hydroxycinnamoyltransferase HOT, hydroxycinnamoyl-CoA quinate hydroxycinnamoyltransferase PAL, phenylalanine ammonia lyase TAL, tyrosine ammonia lyase.

The kinetic studies of the recombinant NtHCT also further provided evidence for the reverse reaction towards formation of the hydroxycinnamoyl CoAs 9 and 10 (C. L. Cardenas etal, manuscript in preparation). In the presence of coenzyme A, NtHCT catalyzed cleavage of the ester bond of the respective hydroxycinnamoyl shikimate esters 25 and 27. The reverse reaction kinetics demonstrated that NtHCT was able to convert -coumaroyl shikimate (25) to -coumaroyl CoA (9) and caffeoyl shikimate (27) to caffeoyl CoA (10), with cat/Am values of 31 000 and 19 llOmoF Is , respectively, at saturating concentrations of CoA. 

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