Table 1.

Specific activity and K_d for E52X (X = Ala, Asp, or Gln)-β2

β2	Y122•/β2	Specific activitya	Wild-type activityb	Kda
		nmol/min/mg	%	μm
WT	1.2	7000	100.0	0.18b
E52A	1.0	13.4c	0.2	0.96
E52D	1.2	10.2c	0.2	2.33
E52Q	1.1	8.6c	0.1	0.12
F3Y122•	0.7	686	9.8
E52Q/F3Y122•	0.9	5.8c	0.1	<4 nmd

^a Specific activity was determined by the radioactive assay (49) and K^d was determined by the competitive binding method (12). All data are representative of at least two independent experiments.

^b Previously reported (12).

^c The wild-type NrdB that co-purifies with mutants may cause the low activity.

^d Upper limit for K_d of E52Q/F₃Y_122^•.