TABLE 1.

Kinetic characteristics of strains with Asn-221 of cytochrome b modified

Strain	Photosynthetic growth	Reduction of heme bHa	Fraction of bH reducedb	Oxidation of heme bHc	Reoxidation (t1/2/ms)d
Wild type	+++	455	<0.05	>353	<1.6
N221D	+++	Similar to wild typee	0.8		15
N221H	+++	420	0.68	75	12
N221P	+++	442	0.167	342	2.0
N221S	+++	Similar to wild type	0.5		10.0
N221T	+++	Similar to wild type	0.37		8.0
N221I	+++	Similar to wild type	0.75		14

^aRate of reduction of heme b_H measured following a 5-μs flash at E_h ~ 120 mV in the presence of antimycin (units, mol of cytochrome b_H/mol of bc₁ complex/s) (see Fig. 4 for experimental details).

^bFraction of heme b_H reduced at the maximal amplitude of the kinetic trace in the absence of antimycin, compared with 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 of cytochrome b_H/mol of bc₁ complex/s). 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.

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

^eRates of Q_o-site turnover were similar to wild-type, as judged by kinetics measured over a longer time scale. In some strains the rates measured in the presence of antimycin were inhibited compared with the rates without inhibitor because of a dramatic lowering of the E_m of heme b_H in the presence of antimycin. In these strains, the turnover of the Q_o-site was estimated from the rate in the absence of antimycin, since the degree of inhibition allowed a significant reduction. The kinetic and thermodynamic properties of the mutant strains are described in detail elsewhere (29) (J. T. Holland, V. P. Shinkarev, and A. R. Crofts, manuscript in preparation).