Like wine and beer, sake uses yeast to turn sugar into alcohol. But where does the sugar come from?
Grapes already contain sugar in the form of glucose and fructose, but rice and the grains used for beer store their energy as long-chain carbohydrates that yeast enzymes can't process. Grains are malted, or sprouted, to make beer. Enzymes released as the grain sprouts breaks the carbohydrate stores down into sugar needed for growth.
This isn't an option for sake, as the part of the rice grain needed for germination is removed when the rice is polished. So where does the sugar come from? The answer is from a mould, Aspergillus oryzae.
A. oryzae feeds on the rice, releasing enzymes that break down the long-chain carbohydrates into sugar, supplying the yeast that produces alcohol. Beer brewing is a two-stage process where sugars are first produced through malting, followed by fermentation where sugar is converted to alcohol. Sake brewing is slightly different, as both processes happen at the same time - koji continuously breaks carbohydrates down into sugar, and the sugar is simultaneously converted into alcohol by yeast. This process is called multiple parallel fermentation.
You can read more about A. oryzae genetics here: Genomics of Aspergillus oryzae: Learning from the History of Koji Mold and Exploration of its Future (Machida et al., 2008)