Malic Acid in Grapes: Part 4 The Role of Malic Acid

I've been blogging about tartaric and malic acid in grapes. This is the fourth installment. The previous blog were these:

• Malic Acid in Grapes: Part 1 Our Titratable Acid Conundrum
• Malic Acid in Grapes: Part 2 Chemical Structure and pKa
• Malic Acid in Grapes: Part 3 The Importance of Acidity

The question is, what is the role of malic acid in the grape.
The accumulation of malic acid during the first stage of berry development occurs in both leaves as well as the immature grapes. In pre-veraison grapes, sucrose transported from the leaves act as the major source of fuel for ATP synthesis and enables the synthesis of malate in the grapes reaching it's maximum level of accumulation just prior to veraison.
The switch from net accumulation of malate to degradation occurs just before veraison. Post veraison, malate is liberated from the vacuole and becomes available for catabolism through various avenues including:

• the TCA cycle and respiration
• gluconeogenesis
• amino acid interconversions
• ethanol fermentation
• production of complex secondary compounds such as anthocyanins and flavonols

The decrease in malic acid content begins at the onset of ripening where it plays a significant role in anabolic reactions.
Malic acid is an important participant in numerous cellular functions of the grape vine such as:

• dark fixation of carbon dioxide
• acid catabolizing processes of fruit ripening
• controlling stomatal aperture
• improving plant nutrition
• increasing resistance to heavy metal toxicity

Malate is the only high proportion organic acid that is actively metabolized throughout the ripening of grapes. With the accumulation of sugars and inhibition of glycolysis in ripening grape, malate is likely a vital source of carbon for these pathways.
References:
1. Crystal Sweetman, Laurent G. Deluc, Grant R. Cramer, Christopher M.Ford, and Kathleen L.Soole, "Regulation of malate metabolism in grape berry and other developing fruits", Phytochemistry, Volume 70, Issues 11–12, July–August 2009, Pages 1329-1344.