Ion pair formation between basic residues at 144 of the Cyt b polypeptide and the ubiquinones at the Qo site of the Cyt bc1 complex.

Loci of spontaneous Qo site inhibitor resistant mutants in the cyt bc1 complex of the photosynthetic bacterium Rhodobacter capsulatus are M140, F144, G152, G158, and T163 of the cyt b polypeptide. In this report, we have studied the effects of arginine (R) substitution at these positions with a view to test for specific interactions with the [2Fe-2S] cluster, cyt bL with Qo site ubiquinone (Q), or hydroquinone (QH2). All the arginine mutants displayed severely or completely impeded photosynthetic growth resulting from dysfunctional cyt bc1 complexes. The source of dysfunction in G158R and T163R was identified by a > 1000-fold decrease in the Qo site affinity for QH2 and Q, sufficient to empty the site in the presence of the 30 mM ubiquinone pool of the chromatophore membrane; they appear similar to the class of mutants described in the preceding paper [Ding, H., Moser, C. C., Robertson, D. E., Tokito, M., Daldal, F., & Dutton, P. L (1995) Biochemistry 34, 15979-15996]. The source(s) of dysfunction of M140R and G152R is not so apparent since they possess Qo sites with normal QH2/Q affinity; they appear to be members to the class of mutants identified and characterized in the following paper [Saribaş, S., Ding, H., Dutton, P. L., & Daldal, F. (1995) Biochemistry 34, 16004-16012]. The present paper focuses on the unique affects of F144R. Redox potential and EPR spectral properties of the Qo site of F144R showed that arginine forms an ion pair with the head group of an anionic ubiquinone, tentatively suggested to be a ubihydroquinone anion (QH-) in the Qos domain.(ABSTRACT TRUNCATED AT 250 WORDS)