Summary
Amino acid propensities in the proteome correlate with differences in metabolic flux (1). The most abundant amino acid (leucine) is 10x more common in proteins than least abundant (cysteine), and differences like this could be explained by metabolic cost alone.
Figures
Ref (1)
1.
Krick T, Verstraete N, Alonso LG, Shub DA, Ferreiro DU, Shub M, et al. Amino Acid Metabolism Conflicts with Protein Diversity. Molecular Biology and Evolution. 2014;31(11):2905–12. Available from: https://doi.org/10.1093/molbev/msu228