Table 1.

Steady-state kinetics for aminoacylation by TyrRSTT

Tyr	k cat (s –1)	K m (μM)	k cat/Km (μM –1 s –1)	Discrimination factor
ATP-PPi exchange assay
l-Tyra	17.36 ± 3.9	9 ± 2	1.93	1
d-Tyrb	2.2 ± 0.65	27.6 ± 7.5	0.08	24
Aminoacylation assay
l-Tyrc	46.7 ± 6.7	3.7 ± 0.35	12.76	1
d-Tyrc	3.5 ± 0.2	5.3 ± 0.5	0.66	19

The data shown represent mean values ± SEM (n = 3), measured by ATP-PPi exchange assay at 60 °C, pH 7.5.

^aTyrRSTT was assayed at 5 nM concentration.

^bTyrRSTT was assayed at 50 nM concentration.

^cData from (31).

Discrimination factor = (k_cat/K_m)_{l-Tyr (cognate amino acid)}/(k_cat/K_m)_{d-Tyr (noncognate)}.