Table 1. Kinetic parameters for threonine and serine activation by various ThrRSs.

ThrRS species	Amino acid	kcat, s-1	KM, mM	kcat/KM, s-1/mM	kcat/KM, relative*
S. solfataricus†	Threonine	13.5 ± 0.2	0.11 ± 0.01	124 ± 11	1
	Serine	12.3 ± 0.2	55 ± 5	0.22 ± 0.02	1.8 × 10-3
M. jannaschii†	Threonine	1.9 ± 0.02	0.10 ± 0.01	19 ± 2	1
	Serine	1.3 ± 0.01	25 ± 2	0.05 ± 0.01	2.6 × 10-3
E. coli‡	Threonine	36	0.11	320	1
	Serine	26	81.5	0.32	1 × 10-3

^*k_cat/K_M is relative to threonine in each set, which was set at 1. Individual kinetic parameters are based on the average of three determinations.

^†Reactions performed at 60°C.

^‡As reported in ref. 12, reactions performed at 37°C.