Yeast enzyme. Molecular model of an enzyme from Saccharomyces cerevisiae (Bakers yeast). This enzyme is 20S proteasome. A proteasome is a complex type of proteinase (proteindigesting enzyme). The helical ribbons, arrows and sheets show the secondary structure (the shape of the enzyme). The barrelshaped 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 subunits.