Yeast enzyme. Molecular model of an enzyme from baker's yeast (Saccharomyces cerevisiae). This is the 20S proteasome. A proteasome is a complex type of proteinase (protein-digesting enzyme). The barrel-shaped structure consists of four ring layers (seen here from bottom left to top right), forming a central space down the middle where the catalytic protein breakdown takes place. Each ring layer consists of 7 proteasome sub-units.