Perturbation of the primary acceptor chlorophyll site in the heliobacterial reaction center by coordinating amino acid substitution

Gregory S. Orf, Kevin E. Redding

Research output: Contribution to journalArticlepeer-review

Abstract

All known Type I photochemical reaction center protein complexes contain a form of the pigment chlorophyll a in their primary electron acceptor site (termed ec3). In the reaction center from the primitive heliobacteria (HbRC), all of the pigment cofactors are bacteriochlorophyll g except in the ec3 sites, which contain 81-hydroxychlorophyll a. To explore the energetic flexibility of this site, we performed site-directed mutagenesis on two of the amino acids of the PshA core polypeptide responsible for coordinating the 81-hydroxychlorophyll a. These two amino acids are serine-545, which coordinates the central Mg(II) through an intermediary water molecule, and serine-553, which participates in a hydrogen bond with the 131-keto O atom. Mutagenesis of serine-545 to histidine (S545H) changes how the chlorophyll's central Mg(II) is coordinated, with the result of decreasing the chlorophyll's site energy. Mutagenesis of serine-545 to methionine (S545M), which was made to mimic the ec3 site of Photosystem I, abolishes chlorophyll binding and charge separation altogether. Mutagenesis of serine-553 to alanine (S553A) removes the aforementioned hydrogen bond, increasing the site energy of the chlorophyll. In the S545H and S553A mutants, the forward and reverse electron transfer rates from ec3 are both faster. This coincides with a decrease in both the quantum yield of initial charge separation and the overall photochemical quantum yield. Taken together, these data indicate that wild-type HbRC is optimized for overall photochemical efficiency, rather than just for maximizing the forward electron transfer rate. The necessity for a chlorophyll a derivative at the ec3 site is also discussed.

Original languageEnglish (US)
Article number148324
JournalBiochimica et Biophysica Acta - Bioenergetics
Volume1862
Issue number1
DOIs
StatePublished - Jan 1 2021
Externally publishedYes

Keywords

  • Charge separation
  • Chlorophyll
  • Heliobacteria
  • Mutagenesis
  • Reaction center
  • Transient absorption

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Cell Biology

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