Yeast enzyme. Molecular model of an enzyme from Saccharomyces cerevisiae (Baker's yeast). This enzyme is 20S proteasome. A proteasome is a complex type of proteinase (protein-digesting enzyme). The helical ribbons, arrows and sheets show the secondary structure (the shape of the enzyme). The barrel-shaped structure consists of four ring layers (here seen side on), forming a central space down the middle (running left to right) where the catalytic protein breakdown takes place. Each ring layer consists of 7 proteasome sub-units.