Table 1.

Dissociation constants and rate data

Protein	Binding site	Kd, pM	kon, M−1·s−1	koff, s−1	t1/2, s
268//NRE	N	190 ± 50	2.5 ± 0.4 × 107	4.7 ± 2.9 × 10−3	150
268//NRE	Z	10*
268//NRE	NZ	<1.0†	2.5 ± 0.2 × 108	5.2 ± 0.9 × 10−7	1.3 × 106
268//NRE	N/Z	<1.0†	2.5 ± 0.2 × 108	9.2 ± 0.7 × 10−7	7.5 × 105
268//NRE	N//Z	<1.0†	2.6 ± 0.6 × 108	7.7 ± 1.3 × 10−7	9.0 × 105
NRE	N/Z	180 ± 43	>7.3 × 107	>5.9 × 10−2	<12
Zif268	NZ	12 ± 3
Zif268	N/Z	14 ± 4	>7.0 × 108	1.4 ± 0.4 × 10−2	39
Zif268	N//Z	14 ± 1

All the constants were determined in at least two separate experiments, and the SEM is indicated. 

^*An exact K_d value could not be determined because this complex gave a smeared band on the gels. 

^†As explained in the text, these K_dvalues could not be measured directly. Estimating K_dfrom the ratio k_off/k_on gives values of 2.1 fM at the NZ site, 3.7 fM at the N/Z site, and 3.0 fM at the N//Z site.