What is the reason a grain of salt alters the perceived bitterness of coffee by such a high degree?
Is simply a dissolution no reaction occur and as said Niculau the pH change, if occur, is absolutely negligible.



Salt in general affect taste lowering the bitterness of the food enhancing the positive sensory attributes of foods. In fact the reason of this behavior is the sodium ion $Na^+$ (even litium ion works) and his interaction with the receptors in taste buds. So sodium acetate therethically works as well, Gery Beuchamp has studied many of this salt, quoting from NCBI:


  various sodium-containing ingredients have been known to reduce the
  bitterness of certain compounds found in foods, including quinine
  hydrochloride, caffeine, magnesium sulfate, and potassium chloride


This is an article about the suppression of bitterness by sodium. From here I'm afraid is more a question of physiology.

I read this the first time here:

This, Herve. Molecular gastronomy: exploring the science of flavor. Columbia University Press, 2006 p. 94