General Phenylpropanoid/Flavonoid Pathway

On February 16th, I blogged about Why is Red Wine Colored Red and in that blog, I included this schematic of the pathway to anthocyanins that began with the amino acid phenylalanine:¹

I then found a diagram that showed a more complete schematic of the flavonoid pathway after it diverges at the point of the reaction involving 4-coumaroyl-CoA. By looking at the schematic above and comparing it to the schematic below, the point of synthesis of the three main flavonoid classes flavonols, anthocyanins, and flavan-3-ols can be discerned.²

Flavan-3-ol monomers include (+)-catechin, (-)-epicatechin, (-)-epigallocatechin, (-)-epicatechin-3-O-gallate, and traces of (+)-gallocatechin. The oligomers and polymers of these flavan-3-ol monomers, are the most abundant flavonoids found in berries.
Why is this important:

• the most abundant flavonoids are proanthocyanidins (PAs)
• the major quality determinants for fruit and wine are the proanthocyanidins
• the total content of PAs is usually higher in seeds
• large quantities of (-)-epicatechin-3-O-gallate are located in the seeds
• the polymer size of PAs is much larger in the skin with (-)-epigallocatechin being a major skin PA subunit

References:
1. Boss PK, Davies C, Robinson SP, Expression of anthocyanin biosynthesis pathway genes in red and white grapes, Plant Mol Biol., 1996, Nov32(3):565-9.
2. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.