ABSTRACT
The relationship between membrane protein structure and thermal stability has been examined in the reaction centre from the bacterium Rhodobacter sphaeroides, a complex membrane protein comprising three polypeptide chains and 10 cofactors. The core of this protein exhibits an approximate twofold symmetry, the cofactors being held in two membrane-spanning branches by two polypeptides, termed L and M, that have very similar folds. In assays of the thermal stability of wild-type and mutant reaction centres embedded in the native bilayer membrane, replacement of a Phe at position 197 of the M polypeptide by His produced an increase in stability, whereas an opposing replacement of His by Phe at the symmetrical position 168 of the L-polypeptide produced a decrease in stability. In light of the known X-ray crystal structures of wild-type and mutant variants of this protein, and further mutagenesis, it is concluded that these stability changes result from the introduction or removal, respectively, of a hydrogen bond between the side-chain of the His and that of an Asn located two positions along the M or L polypeptide chain, in addition to a hydrogen bond between the His side-chain and an adjacent bacteriochlorophyll cofactor.
Subject(s)
Membrane Proteins/chemistry , Membrane Proteins/metabolism , Peptides/chemistry , Photosynthetic Reaction Center Complex Proteins/chemistry , Photosynthetic Reaction Center Complex Proteins/metabolism , Temperature , Coenzymes/chemistry , Coenzymes/metabolism , Enzyme Stability , Hydrogen Bonding , Kinetics , Membrane Proteins/genetics , Models, Molecular , Mutagenesis, Site-Directed , Mutation , Photosynthetic Reaction Center Complex Proteins/genetics , Protein Conformation , Protein Engineering , Protein Unfolding , Rhodobacter sphaeroidesABSTRACT
The analysis of FDMR spectra, recorded at multiple emission wavelengths, by a global decomposition technique, has allowed us to characterise the triplet populations associated with Photosystem I and Photosystem II of thylakoids in the green alga Chlamydomonas reinhardtii. Three triplet populations are observed at fluorescence emissions characteristic of Photosystem II, and their zero field splitting parameters have been determined. These are similar to the zero field parameters for the three Photosystem II triplets previously reported for spinach thylakoids, suggesting that they have a widespread occurrence in nature. None of these triplets have the zero field splitting parameters characteristic of the Photosystem II recombination triplet observed only under reducing conditions. Because these triplets are generated under non-reducing redox conditions, when the recombination triplet is undetectable, it is suggested that they may be involved in the photoinhibition of Photosystem II. At emission wavelengths characteristic of Photosystem I, three triplet populations are observed, two of which are attributed to the P(700) recombination triplet frozen in two different conformations, based on the microwave-induced fluorescence emission spectra and the triplet minus singlet difference spectra. The third triplet population detected at Photosystem I emission wavelengths, which was previously unresolved, is proposed to originate from the antenna chlorophyll of the core or the unusually blue-shifted outer antenna complexes of this organism.
