UBC Theses and Dissertations
Structures of photosynthetic reaction centers with alternative cofactors Hardjasa, Amelia
Rhodobacter sphaeroides is a model organism for the study of bacterial photosynthesis. The R. sphaeroides photosynthetic reaction centre (RC) is the primary site of electron transfer, which is mediated by the photosynthetic pigments bacteriochlorophyll a (BChl) and bacteriopheophytin (BPhe). The substitution of key amino acid residues can change the type of cofactors present in the RC. In particular, studies have shown that when the leucine residue in position 214 of the M subunit [(M)L214] is converted into a histidine, the BPhe normally present in the neighbouring position (HA) is replaced with a BChl. This study investigated the hypothesis that steric exclusion by the coordinating residue causes dechelation of the central magnesium ion in BChl, producing BPhe. Crystal structures of RCs where (M)L214 is substituted for glycine and alanine were determined, which demonstrated that the presence of BPhe in the HA pocket is unchanged despite decreasing the size of the residue in position (M)214. A crystal structure of an RC where (M)L214 is substituted for asparagine was also determined and showed that the replacement of BPhe with BChl at HA occurs if residue (M)214 includes an amide moiety. In the R. sphaeroides Δbchd strain, which lacks the ability to make BChl, it is believed that the RC cofactor sites are populated exclusively with zinc-bacteriochlorophyll (Zn-BChl). The crystal structures of this Zn-BChl containing RC (Zn-RC) and a Zn-RC with the (M)L214H substitution (Zn-β-RC) were solved for the first time. These structures confirmed the presence of Zn-BChl in every cofactor position and the tetracoordination of the HA Zn-BChl in the Zn-β-RC, as well as revealing that the occupancy of the HB cofactor was much lower than that of all other cofactors.
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