to play a critical role in determining substrate specificity. Mechanism The mechanism for the reduction of the CoA thioester to the aldehyde involves a hydride transfer to the carbonyl carbon from NADPH, forming a tetrahedral intermediate with a formal negative charge on the oxygen atom. This negative charge is thought to be stabilized partially via hydrogen bonding with the hydrogen atoms of nearby tyrosine and serine side chains. The serine and tyrosine residues are conserved across all CCRs as part of a catalytic triad along with lysine, which is thought to control the pKa of the tyrosine via electrostatic interactions with the ribose group of the NADPH. The tetrahedral intermediate then collapses, kicking out the CoA and forming an aldehyde as the final product. The thiolate of the CoA is protonated either as it leaves by a nearby residue or after it is free from the binding pocket and out of the enzyme all-together; the exact mechanism is currently unclear, but evidence suggests that a cysteine residue may play the role of thiolate proton donor. Biological Function Phylogenomic analysis indicates that enzymes with true CCR activity first evolved in the of land plants. Most if not all modern land plants and all vascular plants are believed to have at least one functional CCR, an absolute requirement for any plant species with lignified tissues. Most CCR homologs are highly expressed during development, especially in stem, root, and xylem cells which require the enhanced structural support provided by lignin. However, certain CCRs are not constitutively expressed throughout development and are only up-regulated during enhanced lignification in response to stressors such as pathogen attack. CCR is especially important because it acts as a final control point for regulation of metabolic flux toward the monolignols and therefore toward lignin as well; prior to this reduction step, the cinnamoyl-CoA's can still enter into other expansive specialized metabolic pathways. For example, feruloyl-CoA is a precursor of the coumarin scopoletin, a compound believed to play an important role in plant pathogen response. CCR also plays a role in determining lignin composition by regulating levels of the different monomers according to its specific activity toward particular cinnamoyl-CoA's. Monocots and dicots, for example, tend to have very different lignin patterns: lignin found in monocots typically has a higher percentage of p-coumaroyl alcohol-derived subunits, while lignin found in dicots is typically composed of almost entirely coniferyl alcohol and sinapyl alcohol