Table 1. Kinetic parameters of mThrRS or mThrRS-L-N^T for Thr or Ser in amino acid activation^a.

Enzyme	Amino acid	k cat (s−1)	K M (mM)	k cat/KM (mM−1 s−1)	Discriminator factor (D)b
mThrRS	Thr	11.16 ± 0.85	0.30 ± 0.05	37.20	1
	Ser	2.73 ± 0.32	76.64 ± 6.38	0.036	1033
mThrRS-L-NT	Thr	25.61 ± 2.01	3.26 ± 0.29	7.86	1
	Ser	1.10 ± 0.08	152.12 ± 10.21	0.0072	1092

^aResults are the average of three independent repeats, with standard deviations indicated.

^b D was calculated using the formula D = (k_cat/K_M)_Thr/(k_cat/K_M)_Ser.