system
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human
Why exactly are glycine and proline considered "special" amino acids?

I heard this many times throughout my biochemistry class and now I'm hearing it again in biopharmaceutical science, but I've never understood what really separates them from the other amino acids.
gpt
The answer here boils down to degrees of freedom. Because it lacks a beta substituent, glycine can adopt a number of psi- and phi- angle combinations that are energetically disfavored for other amino acids. As a consequence, the Ramachandran plot for glycine is radically different; additionally glycines tend to favor flexibility in the local backbone (because the entropic cost of folding becomes higher).

In the opposite direction, the fact that proline is cyclic significantly reduces its range of favorable angular configurations. Additionally, its cyclic character prevents it from forming hydrogen bonds at the amide (making it a "helix breaker"). However, for proline the cis- configuration of the peptide bond is not as unfavorable as it is for other amino acids, which opens up some additional conformational space.

Both amino acids have significant effects on the local flexibility of the chain and have angular restrictions that differ substantially from other amino acids. This is why they are "special".