P.S It's not 7!
Glycine is the only amino acid which is _________.
I think it's
It's the count of horizontally symmetric letters (A, H, I, M, O, T, U, V, W, X, Y) in the name.
And the hint
Glycine is achiral (symmetric).
This is a long shot, but I think it's
Israel is in an odd (5) position.
Glycine has been found in outer space, but Alanine has not. If we replace Croatia with Israel, we get a pattern of two countries that have had citizens in space, then one who has not.
Keeping with the Alanine/Glycine theme from the hint, you can synthesize with the names. enGland mAlaysia sinGapore = GAG; hunGary isrAel venezuelA = GAA, both being the codons for Glutamic Acid.