Three hydrophobic amino acids in Escherichia coli HscB make the greatest contribution to the stability of the HscB-IscU complex

Table 1 ITC binding isotherms for the interaction of IscU with wild-type and alanine-substituted forms of HscB¹

HscB	n	K_a(10⁵M^-1)	ΔH(kcal/mol)	ΔS(e.u.)
wild-type	1.02 ± 0.08	1.16 ± 0.28	-2.22 ± 0.34	15.7 ± 0.7
R87A	1.04	0.28	-0.88	17.4
T89A	1.08	0.59	-1.00	18.4
L92A	0.70	0.05	0.41	18.3
M93A	0.83 ± 0.04	2.55 ± 0.49	-1.25 ± 0.07	20.4 ± 0.1
L96A	0.44 ± 0.04	0.05 ± 0.01	-3.60 ± 0.00	4.8 ± 0.6
E97A	0.96	1.73	-2.60	15.2
R99A	0.69	0.19	1.18	23.6
E100A	0.91	0.24	-2.22	12.0
D103A	1.04	0.54	-1.61	16.2
E104A	0.83	1.72	-2.80	14.6
R152A	1.09	0.65	-1.34	17.5
F153A	0.78	0.05	0.58	19.0
K156A	0.93	2.59	-2.64	15.9
S160A	1.05	0.92	-1.58	17.4
E97A, E100A, E104A	0.68	0.33	-1.73	14.9

¹Stoichiometries (n), association constants (K_a), heats of binding (ΔH), and entropies of binding (ΔS) for the interaction of wild-type and alanine-substituted forms of HscB with apo-IscU, as determined by ITC. The entropic contribution was determined from the relationship, ΔG_b = ΔH_b - TΔS_b, where ΔG_b = -RTlnK_a. Owing to the low n values obtained with HscB(L92A), HscB(L96A), HscB(R99A), HscB(F153A), and HscB(E97A, E100A, E104A), the ΔH and ΔS values determined for these variants are likely only approximate. Gb values cited in the text were calculated from the relationship, ΔΔG_b = ΔG_b (mutant HscB) - ΔG_b (wild-type HscB).

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