When you eat a miracle berry, the glycoprotein called Miraculin lays a so-called blanket over your sweet taste buds. What is the difference between a protein and a glycoprotein? A protein is an organic compound deemed to be necessary for life and contains nitrogen, carbon, oxygen, and hydrogen. A glycoprotein is a protein that contains carbohydrates. Unlike protein, it only contains three out of the four similar types of atoms: carbon, hydrogen, and oxygen.
Now onto the science. When the pH of your tongue is neutral, 6.2 to 7.6, the Miraculin bonds to your sweet taste bud receptors (hT1R2-hT1R3), and is the antagonist for your sweet taste buds. An antagonist is when a molecule that binds to these taste buds and does not activate them. Miraculin is also an agonist in that when flavors hit your tongue, the Miraculin tells your brain it's so sweet, in a way, rewires your firing from your tongue to your brain.
When a low pH food is eaten, like lemons, the Miraculin blanket changes shape around your receptor (hT1R2) to the change in pH on your tongue which creates the sweetness from the agonist role Miraculin plays. When not consuming foods, Miraculin plays the antagonist role, no firing sweet signals and is relatively tasteless. Sour foods especially change the shape of the Miraculin molecule and the taste buds around it which intensifies the sweetness.