Effects of Amine Donors on Reaction Equilibria. To support our hypothesis that transamination with L-Lys is driven by byproduct cyclization, we compared the thermodynamics of amine transfer to 3 from other commonly deployed amine donors by means of the equilibrium constant (Keq), which directly reflects the ratio of products to substrates of a closed system that has reached a steady state. We calculated the apparent Keq (K eq app ) of individual PRO-TRANS(248) reaction mixtures with 2 and a collection of amine donors by measuring the [3] at equilibrium (see Supporting Information for details), which we determined to be established through regular sampling (Figure ). In the case of lysine, spontaneous byproduct cyclization should favor the formation of 3 compared to those donors that lack such a capacity. Indeed, the K eq app calculated from reactions with L-Lys (5.9) is more than fourteenfold greater than those with other commonly deployed proteinogenic amino acids, such as L-Ala (0.046), L-glutamic acid (L-Glu, 0.060), L-glutamine (L-Gln, 0.42), L-aspartic acid (L-Asp, 0.0080), and L-asparagine (L-Asn, 0.014). Notably, L-Gln, which has likewise been leveraged as a smart donor because of its resulting ketone's propensity to cyclize, 30 also elicits an elevated K eq app , lending further credence to the proposed mechanism.