Pathway of proton transfer in bacterial reaction centers: Replacement of serine-L223 by alanine inhibits electron and proton transfers associated with reduction of quinone to dihydroquinone

Journal Article · Sat Sep 01 00:00:00 EDT 1990 · Proceedings of the National Academy of Sciences of the United States of America; (United States)

DOI:https://doi.org/10.1073/pnas.87.17.6803· OSTI ID:5489388

Paddock, M L; McPherson, P H; Feher, G; Okamura, M Y ^[1]

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1. Univ. of California, San Diego, La Jolla (USA)

The pathway of proton transfer in the reaction center (RC) from Rhodobacter sphaeroides was investigated by site-directed mutagenesis. Ser-L223, a putative proton donor that forms a hydrogen bond with the secondary quinone acceptor Q{sub B}, was replaced with Ala and Thr. RCs with Ala-L223 displayed reduced electron transfer and proton uptake rates in the reaction Q{sub A}{sup {minus}}Q{sub B}{sup {minus}} + 2H{sup +} {yields} Q{sub A}Q{sub B}H{sub 2}. The rate constant for this reaction, k{sub AB}{sup (2)}, was found to be reduced {approx}350-fold to 4.0 {plus minus} 0.2 s{sup {minus}1}. Prton uptake measurements using a pH indicator dye showed a rapid uptake of 1 H{sup +} per RC followed by a slower uptake of 1 H{sup +} per RC at a rate of 4.1 {plus minus} 0.1 s{sup {minus}1}; native RCs showed a rapid uptake of 2H{sup +} per RC. Evidence is provided that these changes were not due to gross structural changes in the binding site of Q{sub B}. RCs with Thr-L223 showed little reduction in the rats of electron and proton transfer. These results indicate that proton transfer from the hydroxyl group of Ser-L223 or Thr-L223 is required for fast electron and proton transfer associated with the formation of the dihydroquinone QH{sub 2}. In contrast, previous work showed that replacing Glu-L212, another putative proton donor to Q{sub B}, with Gln slowed proton uptake from solution without significantly altering electron transfer. The authors propose a model that involves two distinct proton transfer steps. The first step occurs prior to transfer of the second electron to Q{sub B} and involves proton transfer from Ser-L223. The second step occurs after this electron transfer through a pathway involving Glu-L212.

• OSTI ID: 5489388
• Journal Information: Proceedings of the National Academy of Sciences of the United States of America; (United States), Vol. 87:17; ISSN 0027-8424
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
PHOTOSYNTHETIC REACTION CENTERS
BIOLOGICAL PATHWAYS
QUINONES
REDUCTION
ALANINES
CYTOCHROMES
ELECTRON SPIN RESONANCE
ELECTRON TRANSFER
HYDROGEN TRANSFER
PHOTOSYNTHETIC BACTERIA
PROTONS
SERINE
STOICHIOMETRY
AMINO ACIDS
AROMATICS
BARYONS
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
ELEMENTARY PARTICLES
FERMIONS
HADRONS
HYDROXY ACIDS
MAGNETIC RESONANCE
NUCLEONS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
PIGMENTS
PROTEINS
RESONANCE
140505* - Solar Energy Conversion- Photochemical
Photobiological
& Thermochemical Conversion- (1980-)