Table 2.

Experimentally fit rate constants (min⁻¹) for loss of the Cys(thiolate)-ligated heme Soret of Fe(II) R266K hCBS. The loss of the Cys(thiolate)-ligated heme Soret was monitored at at 447 nm for Fe(II) R266K hCBS and was fit to a biexponential decay.

Rate of Cys52 Ligand Loss (min−1)
	37°Ca	30°C	20°C	10°C
k1	(9 ± 1.1) × 10−2	(5 ± 1.4) × 10−2	(1.4 ± 0.1) × 10−2	(9.1 ± 0.5) × 10−3
k2	(1.2 ± 0.5) × 100	(6.6 ± 0.8) × 10−1	(2.5 ± 0.3) × 10−1	(1.0 ± 0.3) × 10−1

^aUpon equilibration at physiological temperature (37°C), Fe(II) R266K hCBS exists almost exclusively as the ligand-switched form (424 nm Soret), which prevented the measurement of k₁ and k₂ at this temperature; reported values were extrapolated from Arrhenius plots of k₁ and k₂ (Figure S3).