Determination of Proton Transfer Rates by Chemical Rescue: Application to Bacterial Reaction Centers
Bibliographic record
Abstract
The bacterial reaction center (RC) converts light into chemical energy through the reduction of an internal quinone molecule Q(B) to Q(B)H(2). In the native RC, proton transfer is coupled to electron transfer and is not rate-controlling. Consequently, proton transfer is not directly observable, and its rate was unknown. In this work, we present a method for making proton transfer rate-controlling, which enabled us to determine its rate. The imidazole groups of the His-H126 and His-H128 proton donors, located at the entrance of the transfer pathways, were removed by site-directed mutagenesis (His --> Ala). This resulted in a reduction in the observed proton-coupled electron transfer rate [(Q(A)(-)(*)Q(B))Glu(-) + H(+) --> (Q(A)Q(B)(-)(*))GluH], which became rate-controlled by proton uptake to Glu-L212 [Adelroth, P., et al. (2001) Biochemistry 40, 14538-14546]. The proton uptake rate was enhanced (rescued) in a controlled fashion by the addition of imidazole or other amine-containing acids. From the dependence of the observed rate on acid concentration, an apparent second-order rate constant k((2)) for the "rescue" of the rate was determined. k((2)) is a function of the proton transfer rate and the binding of the acid. The dependence of k((2)) on the acid pK(a) (i.e., the proton driving force) was measured over 9 pK(a) units, resulting in a Brönsted plot that was characteristic of general acid catalysis. The results were fitted to a model that includes the binding (facilitated by electrostatic attraction) of the cationic acid to the RC surface, proton transfer to an intermediate proton acceptor group, and subsequent proton transfer to Glu-L212. A proton transfer rate constant of approximately 10(5) s(-)(1) was determined for transfer from the bound imidazole group to Glu-L212 (over a distance of approximately 20 A). The same method was used to determine a proton transfer rate constant of 2 x 10(4) s(-)(1) for transfer to Q(B)(-)(*). The relatively fast proton transfer rates are explained by the presence of an intermediate acceptor group that breaks the process into sequential proton transfer steps over shorter distances. This study illustrates an approach that could be generally applied to obtain information about the individual rates and energies for proton transfer processes, as well as the pK(a)s of transfer components, in a variety of proton translocating systems.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".