Combine yeast and warm water in a bowl.

Let stand for 5 minutes, or until bubbly to activate the yeast.

Add buttermilk to the yeast mixture.

In a large bowl, combine flour, sugar, baking powder, baking soda, and salt.

Cut in shortening using a pastry blender or your fingers until the mixture resembles coarse meal.

Add the buttermilk mixture to the dry ingredients, mixing with a fork until just moistened; avoid overmixing.

Turn the dough out onto a lightly floured surface.

Knead lightly 3 or 4 times to bring the dough together.

Roll the dough to 1/2 inch thickness.

Cut out biscuits using a 2.75-inch biscuit cutter.

Place the biscuits on a lightly greased baking sheet, touching each other.

Cover the biscuits and let them rise for 1 hour.

Preheat oven to 450°F (232°C).

Bake in the preheated oven for 10-12 minutes, or until golden brown.