Lysine Compatibility with Wild-Type Transaminases. To assess whether lysine is accepted as a substrate by transaminases more broadly, we subjected a 384-member panel of wild-type transaminases to reactions with L-and D-Lys separately, monitored the conversion of pyruvate to alanine, and compiled our findings into a single heat map depicting the assay yield (AY) of alanine in each enzymatic reaction as well as each enzyme's preference (or lack thereof) for a particular lysine enantiomer (Figure , Figures ). We chose pyruvate as the amine acceptor because of its generally broad transaminase compatibility, as this would then minimize the number of low-yielding reactions that would otherwise result from incompatibility with the acceptor rather than with lysine. We categorized the results as follows: >22% AY (dark teal)-the activity of PRO-TRANS(248) under these unoptimized conditions-indicates good compatibility with lysine; 5-22% AY (light teal) suggests promising activity that may require rigorous condition optimization or protein engineering to achieve high yields; and <5% AY (gray) suggests incompatibility, at least in the context of a practical kinetic regime. By these criteria, 72% of the transaminases examined showed at least promising activity, indicating that lysine may indeed be well tolerated as an amine donor by transaminases more broadly.