Bacterial Reaction Centers with Modified Tetrapyrrole Chromophores

Abstract

II. Norris, James R., Date. QP517.P45P46 1993 '3342-dc20 92-43633This book is printed on acid-free paper.© Academic Press, Inc. References 78 Genetic Analysis of Photosynthetic Membrane Biogenesis in Rhodobacter sphaeroides I. IntroductionSeveral different methods are available for preparing reaction centers with nonnative chromophores: (1) (re)assembly from the Polypeptides, pigments, and other components such as quinones, iron, and possibly others; (2) exchange of chromophores into native or partly denatured reaction centers; (3) modification of chromophores in native or partly denatured reaction centers by chemical or enzymatic means; (4) modification of binding sites to incorporate different chromophores, for example, by mutagenesis, and (5) mutagenic modifications of the pigments biosynthetic pathways. With the exception of the last one, all these approaches have been used on different Complete reconstitution of the complexes (Method 1) from the isolated constituent Polypeptides and suitably modified pigments is likely to have the greatest potential, but to date has been impossible with reaction centers. This technique has been applied successfully to the core light-harvesting complexes (LHCI) of purple bacteria (Miller et al, 1987;Ghosh et al, 1988;Parkes-Loach et al, 1988 Chang et al, 1990a,b;Heller and Loach, 1990) and to the peripheral light-harvesting chromoprotein LHCII-ß of higher plants (Plumley and Schmidt, 1987). The method has been extended by Paulsen etat. (1990Paulsen etat. ( ,1991 to the full-length and modified LHCII-ß precursor Polypeptides expressed in and isolated from Escherichia coli and by Schmidt et al. (1991) to other green plant peripheral light-harvesting complexes.Site-directed mutagenesis of the binding pockets (Method 4) has been used extensively to exchange bacteriochlorophylls (Behls) with their metalfree derivatives, for example, bacteriopheophytins (Bphes) and vice versa in reaction centers (RCs) from Rhodobacter (Rb.) capsulatus and Rb. sphaeroides (Table I)-With only a Single exception, whenever the histidine(s) ligating the central magnesium atom(s) of the Bchl(s) were replaced by hydrophobic amino acids, stably assembled RCs were obtained in which the Behls at the respective site(s) were replaced by Bphe to yield stably assembled and isolatable RCs (Bylina and Youvan, 1988; Coleman and Youvan, 1990;Robles et al, 1990;Schenck et al, 1990;Woodbury et al, 1990). In reversing this reaction, replacement of leucine or isoleucine next to the central N-4 cavity of Bphe with histidine resulted in binding of Behl at the respective site. Some results are available with amino acid residues of intermediate ligation strength. Serine or threonine selected binding of Behl at BA and BB (Bylina et al, 1990); glutamine (mutants H-L173Q and H-M200Q 2 ) at the sites of the special pair resulted in binding of Behl also (Bylina and Youvan, 1988). Omission of Bphe HL was found in a mutant of Rb. capsulatus in which part of the D helix of subunit L was replaced by the symmetrically equiv...

Relative binding affinities of chlorophylls in peridinin–chlorophyll–protein reconstituted with heterochlorophyllous mixtures

“…It would be interesting to see if crystal structures of the reconstituted complexes containing Chl b or acChl a will provide direct proof of these conjectures. A considerable tolerance to variations of the substituents of rings A and B has been found previously for a variety of Chl binding sites in photosynthetic pigment protein complexes, for enzymes involved in Chl biosynthesis (Scheer and Hartwich 1995;Scheer 2003), and for heme binding sites in myoglobin and hemoglobin (Pröll et al 2006). In some of these cases, replacements are even possible when no suitable H-bond donors are obvious in the crystal structures.…”

Probing native‐like orientation of pigments in modified reaction centers from Rhodobacter sphaeroides R26 by linear dichroism

“…In the BChls investigated, changes have been introduced again at C-3 and also at C-132 located at the isocyclic ring. No chlorins can be introduced into the BA,B binding sites [4]. The spectral changes upon modifying C-3 are similar to those in the BPhe a; those related to the 132-OH substituents are negligible for the QY and < 5 nm for the Qx band [11].…”

“…These numbers correspond to an exchange of only one or both BPhes, respectively, and have been interpreted to reflect either selective exchange at one site only, probably HB, or exchange at both sites, HA and HB [2,3]. Bacteriochlorophyll a (BChl) at the 'monomeric' sites BA and BB can likewise be replaced by modified bacteriochlorophylls [4]. In this case, site selectivity is less pronounced, but a selective exchange at site BA is possible in RC containing a carotenoid, e.g.…”

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Key words: Pigment modification; Low-temperature spectroscopy; Reaction center; Asymmetry; Gel squeezing; Linear dichroism questions concerning structure-function relationships in reaction centers [4]. An important conclusion from these studies was that the monomeric BChl a at site BA is a real intermediate in the ultrafast electron transfer from P to HA [7].

In all these studies, it has tacitly been assumed that the modified pigments are introduced into the binding site with the same orientation as the pigment original present and that the replacement or the treatment during exchange does not change the structure of the complex.

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“…Key words: Pigment modification; Low-temperature spectroscopy; Reaction center; Asymmetry; Gel squeezing; Linear dichroism questions concerning structure-function relationships in reaction centers [4]. An important conclusion from these studies was that the monomeric BChl a at site BA is a real intermediate in the ultrafast electron transfer from P to HA [7].…”