I was stuck here between C and D. I know that proteases function to digest proteins into smaller fragments, but this didn't make sense to me in the context of the passage. Why would breaking something down cause it to function better? I would think proteases would cause a breakdown/destructive process (not enhancing function)

In the passage, it says that proteases "trigger the conformational changes in EGP that are necessary for fusion." That led me to choose D


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MCAT Tutor
2+ Year Member
Nov 6, 2014
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Hi there, good question!

There are plenty of biological situations where breaking something down is necessary to its function. For example, consider zymogens (the inactive precursors of enzymes). Zymogens typically need to be cleaved into their active forms; for example, angiotensinogen must be cleaved by renin to form angiotensin I. And actually, angiotensin I is then cleaved by another peptidase (angiotensin-converting enzyme, or ACE), which forms the active angiotensin II. The key is that proteases can function in very different and highly specific ways. They don't necessarily trigger the complete breakdown of the protein. For example, ACE just cleaves two residues from the C-terminus of angiotensin I.

If you're curious about an example that is more similar to the passage discussion of viruses, you could check out HIV-1 protease (HIV-1 protease - Wikipedia). Of course, any information about that specific protease would fall outside the scope of the MCAT.

Good luck with your prep!
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