Glutamine
In human blood, glutamine is the most abundant free amino acid.
Amino acid replacement
where "c" = composition, "p" = polarity, and "v" = molecular volume; and are constants of squares of the inverses of the mean distance for each property, respectively equal to 1.833, 0.1018, 0.000399. According to Grantham's distance, most similar amino acids are leucine and isoleucine and the most distant are cysteine and tryptophan.