Different types of proteases help cleave the proteins in different formats. There are serine, aspartate, metalloproteases, and many other classes. All use different mechanisms to cleave the peptide bonds to begin protein degradation. For example, the serine proteases, such as trypsin, engage in a nucleophilic attack on the hydroxyl oxygen of the serine on the peptide bond's carbonyl carbon in order to cleave this bond. An acyl-enzyme intermediate is created and the mechanism continues to hydrolize the other remaining linkages. On the other hand, metalloproteases, such as zinc proteases, incorporate metals to break the bonds. With zinc, its active site incorporates the zinc ion, water, and histidines (which are ligands to the zinc ion). The zinc protease also engages in a nucleophilic attack but on the carbonyl carbon, using the water's oxygen atom. The active site's base helps this process along by taking a proton from that water.