Why is Red Wine Colored Red

Have you ever wondered why red wine is colored red? I wrote about it in a blogpost called Anthocyanins Give Red Wine Their Color. The red color in grapes is attributable to the anthocyanins located in the grape berry skin.¹
I'm going to delve a little deeper in this blog about how anthocyanins are formed and why they end up being accumulated in the berry skin. The pathway for the formation of anthocyanins begins with the amino acid phenylalanine:²

In the diagram shown above, phenylalanine can be shunted into two pathways:

• General Phenylpropanoid Metabolic Pathway: Phenylalanine is first converted to 4-coumaroyl-CoA and this compound may be used in the production of lignins, coumarins and stilbenes
• Flavonoid Pathway: Conversion of 4-coumaroyl-CoA results in the production of a range of flavonoid compounds, the most common being aurones, flavones, flavonols, isoflavonoids, proanthocyanidins and anthocyanins.

The main point of control of anthocyanin production in the grape berry is after leucoanthocyanidin dioxygenase or LDOX. Expression of the enzyme that converts the precursor of anthocyanins, UDP glucose flavonoid 3-o-glucosyl transferase or UFGT was only detected in the berry skin and was always associated with anthocyanin accumulation. Red wines therefore, are the result of the extraction of anthocyanins from the grape berry skins and oh so much more chemistry! (Maybe in another blog)

References:
1. Douglas Adams, VI257, Lesson 8, pg. 23. All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.
2. Boss P, Davies C, Robinson S (1996b), "Expression of anthocyanin biosynthesis pathway genes in red and white grapes", Plant Mol Biol, 32:565 - 569.
3. Lijavetzky et al., 2006, Mol. Genet. Genomics, 2006: 427-435.
4. José Tomás Matus, Felipe Aquea and Patricio Arce-Johnson, BMC Plant Biology, 2008, 8:83.
5. 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.