Amino acid specificity spectra of Class I LeuRS and Class II HisRS2 Urzymes.
The mean net free energy for specificity of Class I and II Urzymes for homologous vs. heterologous substrates, i.e., ΔG(kcat/KM)ref – ΔG(kcat/KM)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. (From Carter, CW Jr., (2015) MDPI Life 5: 294–320).