in different parts of the of cell including the outermost part of the cell membrane, chemicals called enzymes snip the app into small pieces. these enzymes that do the snipping are alpha-secretase, beta-secretase, and gamma-secretase. depending on which enzyme is doing the snipping and what parts of the app are snipped, two different things can happen. one that is helpful and one that causes the formation of beta-amyloid plaques.