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Pepsin digests proteins, as we know, but can it digest other enzymes that are actually made up of proteins, like digesting other pepsin molecules?

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Pepsin does cleave itself (Ingram, 1951) - this is termed autolysis. Pepsin (and other proteases) act on proteins with the right amino acid sequences. Pepsin prefers to cleave at phenylalanine, tryptophan or tyrosine residues (Lehninger, A. Principles of Biochemistry).

* Pepsin can also cleave inactive 'zymogen' enzymes to activate them. One of these zymogens is pepsinogen (an inactive form of pepsin itself). So you could argue that pepsin is cleaving itself here but I personally wouldn't, since pepsinogen isn't really pepsin.

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    $\begingroup$ Confusingly, pepsin does cleave itself - or, rather pepsinogen does to make pepsin (autocatalysis). Hmm, it also does autolysis, so I'm not sure your answer is correct ... $\endgroup$ – gilleain Nov 30 '18 at 16:25
  • $\begingroup$ @gilleain I've added a note about it. I was initially going to talk about zymogens but I felt that it would be muddying the answer. Personally I'd say pepsin and pepsinogen are different things, plus (to my mind) the question is asking whether a solution of purified pepsin will eventually eat itself. $\endgroup$ – Jam Nov 30 '18 at 17:04
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    $\begingroup$ Jam, indeed - that's autolysis! Apparently, you should store pepsin at a low temperature to stop it eating itself $\endgroup$ – gilleain Nov 30 '18 at 17:05
  • $\begingroup$ @gilleain Well I'll be! Never realised it did eat itself! Thanks for letting me know $\endgroup$ – Jam Nov 30 '18 at 17:06
  • $\begingroup$ Regarding your note on terminology: (1) digestion is common parlance for proteases and nucleases. (2) not all enzymes are proteins. $\endgroup$ – canadianer Nov 30 '18 at 20:40

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