Cinnamoyl-coenzyme

This enzyme [EC 6.2.1.12] (also referred to as hydroxy-cinnamoyl-CoA ligase and 4-coumarate CoA hgase) catalyzes the reaction of 4-coumarate with ATP and coenzyme A to yield 4-coumaroyl-CoA, AMP, and pyrophosphate (or, diphosphate). 

The polyketide synthases responsible for chain extension of cinnamoyl-CoA starter units leading to flavonoids and stilbenes, and of anthraniloyl-CoA leading to quinoline and acridine alkaloids (see page 377) do not fall into either of the above categories and have now been termed Type TTT PKSs. These enzymes differ from the other examples in that they are homodimeric proteins, they utilize coenzyme A esters rather than acyl carrier proteins, and they employ a single active site to perform a series of decarboxylation, condensation, cyclization, and aromatization reactions. 

Cinnamic acids, as their coenzyme A esters, may also function as starter units for chain extension with malonyl-CoA units, thus combining elements of the shikimate and acetate pathways (see page 80). Most commonly, three C2 units are added via malonate giving rise to flavonoids and stilbenes, as described in the next section (page 149). However, there are several examples of products formed from a cinnamoyl-CoA starter... 

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.

From a quantitative point of view, the main phenolics are phenylpropanoid derivatives which accumulate as esters, flavonoids, or are polymerized as lignins (Ref. l). All these products may be synthesized in the same organ from cinnamic acids via their cinnamoyl-coenzyme A thioesters (Ref. 23). Consequently, the following reactions may indeed occur simultaneously ... 

Reduction of a coenzyme a ester to the corresponding aldehyde, catalyzed by cinnamoyl CoA reductases (CCRs), is an important reaction in lignin biosynthesis (Fig. 5). Plants contain small CCR gene families [91] encoding enzymes with differences in overall specificity for monolignol precursors with different aromatic ring substitution patterns. 

Genetic modification of the lignin biosynthesis by downregulation of the cinnamoyl-coenzyme A reductase in poplar P. tremula x Populus alba) decreased the lignin content and increased breakdown or remodeling of noncellulosic cell wall polymers as supported by FT-IR analysis [105]. 

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