Table 2.

Reactivity of cysteines of native Bcp1 and its mutants toward thiol reagents, pH 7.4.

Reagents	Free Cys	Free GSH	Reduced Bcp1		Reduced C45S		Reduced C50S
	k (M−1 s−1)	k (M−1 s−1)	k (M−1 s−1) (-SH/mole Bcp1)	E.R.	k (M−1 s−1) (-SH/mole Bcp1)	E.R.	k (M−1 s−1) (-SH/mole Bcp1)	E.R.
GSSG	0.7a (0.2)b	—	0.35 ± 0.02 (1)	0.5 1.7	—	—	—	—
Cystine	—	12	115 ± 10 (1)	9.6	4.2 ± 0.4 (0.6)	0.35	3.5 ± 0.2 (0.65)	0.29
Cystamine	—	55	33 ± 3 (1)	0.6	2.1 ± 0.2 (0.6)	0.04	3.5 ± 0.1 (0.65)	0.06
Homocystine	—	0.6	15 ± 1 (1)	25	3.8 ± 0.2 (0.6)	6.3	0.5 ± 0.1 (0.65)	0.8
DTNB
(pH 5.0)	—	20	360 ± 20 (1)	18	—	—	—	—
DTNB (pH 7.4)	—	5000	—	—	1400 ± 200 (0.6)	0.28	—	—
DTNB (pH 6.0)	—	150	—	—	—	—	925 ± 50 (0.65)	6.2
CDNB	—	0.07	0.55 ± 0.03 (1)	7.8	0.18 ± 0.02 (0.6)	2.6	0.39 ± 0.02 (0.65)	5.6
NBD-Cl	—	8	27 ± 3 (1)	3.4	3.4 ± 0.2 (0.6)	0.4	4.9 ± 0.2 (0.65)	0.6

^aExperimental value for free cys (pK_a = 8.55);

^bTheoretical value for unperturbed protein cysteine (pK_a = 9.1);

E.R., Enhanced reactivity;

Data are Mean ± S.D.