TABLE 5.

Observed bimolecular rate constants for NO association and calculated rate parameters for ligand entry into 86(G12) and 55(E18) mutants of CerHb at pH 7.0, 20 °C

CerHb	k′NO	kgem,CO	Fgem,CO	k′entry,CO
	μm−1s−1	μs−1		μm−1s−1
Ala-55/Leu-86(wt)a	230 ± 30	80 ± 40	0.05 ± 0.03	640 ± 400
Ala-86	200	≥150, 15	0.13	190
Phe-86	110	≥150, 53	0.17	140
Trp-86	67	∼80	0.52	31
Trp-55	45	9.4	0.44	33

^a The numbers for wt CerHb are based on the average of all determinations for the past 6 years (≥10 separate preparations, see Table 3).