Table 4.

	Kinetic characteristics of strains with Asn-221 of cytochrome b modified
Strain	Photosynthetic growth	Reduction of bHa	Fraction of bH reduced b	Em,7, pH dependence d			Reoxid. (t½/ms) e
				cyt b150	bH, no inhib.	with AA
Wild type	+++	455	0.05	148, −59	32, −59	46, 0	<1.6
N221D	+++	(as wt)	0.8	150 −59	35, −54	−47, −59	15
N221H	+++	520	0.68	150, −59	35, −59	13, −50	12
N221P	+++	442	0.167	154, −59	38, −59	−3, −59	2.0
N221T	+++	(as wt)	0.37	140, −59	29, −50	13, 59	8.0
N221I	+++	(as wt)	0.75	104, −59	36, −59	−95, −59	14

^aNotes: Rate of reduction of heme b_H measured following a 5 µs flash at E_h ~120 mV in the presence of antimycin (units – mol cyt b_H /mol bc₁ complex / sec).

^bFraction of heme b_H reduced at the maximal amplitude (at ~3–5 ms) of the kinetic trace in the absence of antimycin, compared to the fraction reduced in the presence of antimycin.

^cRate of reoxidation of heme b_H in the absence of antimycin, measured by subtraction of the traces in the presence and absence of antimycin (units – mol cyt b_H /mol bc₁ complex / sec). The rate in the wild-type reflects the rate-limiting reduction of the low potential chain through the Q_o-site reaction. Since v = k(occupancy), and the fraction of heme b_H in the reduced form (column 4) is <<1, the rate constant must be much greater than that for the limiting step.

^dE_m,7 in mV, pH dependence of E_m in mV/pH unit, assuming a single component for each heme.

^eHalf-times for reoxidation of heme b_H measured from kinetic traces.