Pepsinogen is a proenzyme secreted by chief cells. It is converted into active form i.e pepsin by HCl. HCl provides the optimum pH for the enzyme pepsin. Once pepsin is formed it itself converts pepsinogen into pepsin. This proces is called autocatalysis.
Diphyodont is the condition of having two set of teeth during the life of an organism. For example : humans are diphyodont as they have two set of teeth namely temporary milk or deciduous teeth which are replaced by permanent or adult teeth.