Figure 6.

Amino acid specificity spectra of Class I LeuRS and Class II HisRS2 Urzymes. The net free energy for specificity of Class I and II Urzymes for homologous vs. heterologous substrates, i.e., ΔG(k_cat/K_M)_ref – ΔG(k_cat/K_M)_{amino acid (i)}, where ref is the cognate amino acid, is approximately 1 kcal/mole for both Class I and II Urzymes (center). Class I amino acids are colored blue; Class II amino acids are colored green. Bold colors denote substrates from the homologous Class; pastel colors denote heterologous substrates.