In a small bowl, dissolve yeast in warm water. Let stand until creamy, about 5 minutes. Add buttermilk to yeast mixture, and set aside.
In a large bowl, combine flour, sugar, baking powder, baking soda, and salt. Cut in shortening with a pastry blender until mixture resembles coarse meal. Stir in yeast mixture until dry ingredients are moistened. Turn dough out onto a floured surface, and knead 4 or 5 times.
On a lightly floured surface, roll dough to 1/2 inch thickness. Cut out biscuits with a 2 1/2 inch round cutter. Place on lightly greased baking sheets, barely touching each other. Cover, and let rise in a warm place free from drafts for 1 hour, or until almost doubled in size. Preheat oven to 425 degrees F (220 degrees C).
Bake in preheated oven for 10 to 12 minutes, or until browned.