Preheat the oven to 350°F. Grease and flour a 9-x-5-inch loaf pan and line the bottom with a rectangle of parchment.

In a large bowl, whisk the sugar, eggs, melted butter, orange juice, and buttermilk until smooth. Whisk in the pumpkin. Sift the all-purpose flour, whole wheat flour, baking powder, baking soda, salt, and pumpkin pie spice over the surface and stir with a wooden spoon just until combined.

Pour into the prepared pan and sprinkle the top with the turbinado sugar (if using). Bake until a toothpick inserted in the center comes out clean and the edges are just starting to pull away from the sides of the pan, 45 to 50 minutes. Cool the bread in the pan for 20 minutes, then turn out and cool completely on a wire rack.