ELECTROSTATIC DOMINOES - LONG-DISTANCE PROPAGATION OF MUTATIONAL EFFECTS IN PHOTOSYNTHETIC REACTION CENTERS OF RHODOBACTER-CAPSULATUS

Citation
P. Sebban et al., ELECTROSTATIC DOMINOES - LONG-DISTANCE PROPAGATION OF MUTATIONAL EFFECTS IN PHOTOSYNTHETIC REACTION CENTERS OF RHODOBACTER-CAPSULATUS, Biochemistry, 34(26), 1995, pp. 8390-8397
Citations number
35
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Biology
Journal title
ISSN journal
0006-2960
Volume
34
Issue
26
Year of publication
1995
Pages
8390 - 8397
Database
ISI
SICI code
0006-2960(1995)34:26<8390:ED-LPO>2.0.ZU;2-V
Abstract
Two point mutants from the purple bacterium Rhodobacter capsulatus, bo th modified in the M protein of the photosynthetic reaction center, ha ve been studied by flash-induced absorbance spectroscopy. These strain s carry either the M231Arg --> Leu or M43Asn --> Asp mutations, which are located 9 and 15 Angstrom, respectively, from the terminal electro n acceptor Q(B). In the wild-type Rb. sphaeroides structure, M231Arg i s involved in a conserved salt bridge with H125Glu and H232Glu and M43 Asn is located among several polar residues that form or surround the Q(B) binding site. These substitutions were originally uncovered in ph enotypic revertants isolated from the photosynthetically incompetent L 212Glu-L213Asp --> Ala-Ala site-specific double mutant. As second-site suppressor mutations, they have been shown to restore the proton tran sfer function that is interrupted in the L212Ala-L213Ala double mutant . The electrostatic effects that are induced in reaction centers by th e M231Arg --> Leu and M43Asn --> Asp substitutions are roughly the sam e in either the double-mutant or wild-type backgrounds. In a reaction center that is otherwise wild type in sequence, they decrease the free energy gap between the Q(A)(-) and Q(B)(-) states by 24 +/- 5 and 45 +/- 5 meV, respectively. The pH dependences of K-2, the Q(A)(-)Q(B) <- > Q(A)Q(B)(-) equilibrium constant, are altered in reaction centers th at carry either of these substitutions, revealing differences in the p K(a)s of titratable groups compared to the wild type. These results co nfirm that interactions among distant residues influence the electrost atic potential in the immediate vicinity of Q(B) to ensure the efficie nt conduction of protons through the protein matrix and their delivery to the reduced quinone. It is possible that these influences are prop agated over such large distances by mutation-induced realignments of s alt bridges within a network of acidic and basic residues that is loca ted in this region of the reaction center, which could serve as a rela y mechanism to partially relocate the new negative charge much closer to the quinone.