Table 1.

Summary of All-trans Retinol Binding to X21RBP and Its Tryptophan to Phenylalanine Substitution Mutants

Protein	Sites (N) Kd (μM)	Enhancement	Quenching	Energy Transfer
X2IRBP	N	1.27 ± 0.03	1.26 ± 0.02	1.26 ± 0.02
	Kd	0.07 ± 0.02	0.008 ± 0.003	0.033 ± 0.008
W450F	N	0.62 ± 0.09	†	0.83 ± 0.08
		0.72 ± 0.06*		ND
	Kd	0.45 ± 0.11	†	0.29 ± 0.08
		0.48 ± 0.05*		ND
W587F	N	1.07 ± 0.01	0.97 ± 0.02	1.20 ± 0.01
		1.40 ± 0.04*	1.31 ± 0.03*	ND
	Kd	0.07 ± 0.01	0.05 ± 0.01	0.10 ± 0.01
		0.05 ± 0.04*	0.06 ± 0.01*	ND

Calculated parameters were derived from titrations illustrated in Figure 4, except for those indicated by *, which correspond to titrations in Figure 5 where the protein used came from a separate expression and purification. ND, not done.

^†Not calculated, insufficient quenching.