Biology Question #2574

Sarah, a 17 year old female from the Internet asks on February 14, 2005,

Why doesn't pepsin break down gelatin on developed photographic film or does it just take a very long time? I did an investigation to see how pH affected the time taken to break down the layer of gelatin on film and pepsin didn't work at all, not even in acidic conditions, whereas trypsin worked in minutes.

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The answer

Wendy Hutchins answered on April 1, 2005

Trypsin and pepsin are digestive enzymes. They are large tangled chains of amino acids folded into shapes that promote the cleavage of protein molecules. But trypsin and pepsin themselves are proteins. Let me ask you this: why don't they digest themselves? The reason also explains why pepsin won't digest gelatin. Enzymes are specific. Each kind can cleave (or digest) only a certain type of molecule. Trypsin and pepsin do not have the right molecular shape to attack themselves. A gelatin molecule has a particular shape that "fits" trypsin, making it digestible, but it won't fit pepsin, so it doesn't get broken down.

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