4-Methylideneimidazole-5-one (MIO)-containing aminomutases catalyze the conversion of
l
-α-amino acids to β-amino acids with either an (
R
) or an (
S
) configuration.
l
-Phenylalanine and
l
-tyrosine are the only two natural substrates identified to date. The enediyne chromophore of the chromoprotein antitumor antibiotic kedarcidin (KED) harbors an (
R
)-2-aza-3-chloro-β-tyrosine moiety reminiscent of the (
S
)-3-chloro-5-hydroxy-β-tyrosine moiety of the C-1027 enediyne chromophore, the biosynthesis of which uncovered the first known MIO-containing aminomutase, SgcC4. Comparative analysis of the KED and C-1027 biosynthetic gene clusters inspired the proposal for (
R
)-2-aza-3-chloro-β-tyrosine biosynthesis starting from 2-aza-
l
-tyrosine, featuring KedY4 as a putative MIO-containing aminomutase. Here we report the biochemical characterization of KedY4, confirming its proposed role in KED biosynthesis. KedY4 is an MIO-containing aminomutase that stereospecifically catalyzes the conversion of 2-aza-
l
-tyrosine to (
R
)-2-aza-β-tyrosine, exhibiting no detectable activity toward 2-aza-
l
-phenylalanine or
l
-tyrosine as an alternative substrate. In contrast, SgcC4, which stereospecifically catalyzes the conversion of
l
-tyrosine to (
S
)-β-tyrosine in C-1027 biosynthesis, exhibits minimal activity with 2-aza-
l
-tyrosine as an alternative substrate but generating (
S
)-2-aza-β-tyrosine, a product with the opposite stereochemistry of KedY4. This report of KedY4 broadens the scope of known substrates for the MIO-containing aminomutase family, and comparative studies of KedY4 and SgcC4 provide an outstanding opportunity to examine how MIO-containing aminomutases control substrate specificity and product enantioselectivity.