Modulating radical propagation in proteins by proton-coupled electron transfer and hydrogen bonding

This study demonstrates that modulating radical propagation in Cytochrome c peroxidase variants is achieved through proton-coupled electron transfer and hydrogen bonding, which elevate the formal potential of tyrosine radicals to facilitate efficient long-range electron transfer and charge migration.

The Gist

The Big Picture: The Protein "Relay Race"

Imagine a protein (a tiny machine inside your body) as a long, winding hallway. Sometimes, this machine needs to move a tiny spark of energy—an electron—from one end of the hallway to the other. This is called Electron Transfer (ET).

The problem? The hallway is too long for the spark to jump all the way across in one go. It needs "stepping stones" to hop from one spot to the next. In the protein studied here (Cytochrome c Peroxidase, or CcP), the stepping stones are special amino acids called Tryptophan and Tyrosine.

The scientists wanted to figure out how to control these stepping stones. They wanted to know: How do we make the spark hop faster? And how do we stop it from hopping to the wrong place?

---

The Main Characters

1. The Spark (The Electron): The energy that needs to move.
2. The Relay Station (Residue 191): A specific spot in the hallway where the spark usually stops.
   • The Original: A Tryptophan (Trp) amino acid. It's a great stepping stone.
   • The Replacement: The scientists swapped this for a Tyrosine (Tyr). Unfortunately, Tyrosine is a "clumsy" stepping stone. It's too heavy and sticky; the spark gets stuck, and the machine stops working.
3. The Helper (Residue 232): A nearby amino acid (usually Glutamate or Histidine) that acts like a proton manager. It can grab or release a tiny hydrogen particle (a proton) to help the spark move.
4. The Power Source:
   • Scenario A (The Peroxide Engine): The natural way the machine works, using hydrogen peroxide to create a high-energy spark.
   • Scenario B (The Flashlight Engine): The scientists replaced the machine's core with a Zinc-Porphyrin (ZnP). When they shine a light on it, it creates a spark. This lets them study the process in slow motion.

---

The Discovery: The "Proton Handshake"

The scientists discovered that simply swapping the stepping stone (Trp for Tyr) broke the machine. But, if they added a Helper (Residue 232) right next to the new stone, the machine started working again!

Here is the magic trick they found: Proton-Coupled Electron Transfer (PCET).

Think of it like a dance:

• To move the spark (electron), the Tyrosine stepping stone must first let go of a proton (a hydrogen particle).
• The Helper (Residue 232) acts as a partner who catches that proton.
• Once the proton is caught, the Tyrosine becomes "lighter" and more energetic, allowing the spark to hop onto it easily.

The Analogy: Imagine trying to push a heavy boulder (the electron) up a hill.

• Without the Helper: The boulder is too heavy; you can't push it.
• With the Helper: The Helper grabs a heavy backpack (the proton) off your back. Now you are lighter, and you can push the boulder up the hill with ease.

The Twist: Two Different Rules for Two Different Engines

The most surprising part of the paper is that the "Helper" behaves differently depending on which engine is running.

1. The Peroxide Engine (Natural System)

• How it works: The machine is already super-charged. The spark is very strong.
• The Problem: The Tyrosine stepping stone is too "high-potential" (too energetic). It needs to be calmed down slightly to accept the spark from the next station.
• The Helper's Job: The Helper holds onto a proton and forms a hydrogen bond (a sticky connection) with the Tyrosine. This connection lowers the energy of the Tyrosine just enough so it can catch the spark and pass it along.
• The pH Effect: If you make the environment more basic (higher pH), the Helper lets go of the proton. The connection breaks, the Tyrosine gets too energetic again, and the machine slows down.

2. The Flashlight Engine (ZnCcP System)

• How it works: The spark from the flashlight is weaker. It needs a boost to get onto the Tyrosine.
• The Problem: The Tyrosine is too "stubborn" to let the weak spark hop on.
• The Helper's Job: The Helper steals a proton from the Tyrosine. This makes the Tyrosine "hungry" for an electron. It lowers the barrier, allowing the weak flashlight spark to hop on.
• The pH Effect: If you make the environment more basic (higher pH), the Helper is ready to steal the proton. The machine speeds up! (This is the opposite of the Peroxide engine).

The "Radical Migration" Mystery

The scientists also used a special camera (EPR spectroscopy) to take pictures of where the spark was hiding.

• In the original machine (Tryptophan): The spark stays right at the main stepping stone. It's a good, stable spot.
• In the modified machine (Tyrosine + Helper): The spark doesn't just stay put. Once it hops onto the Tyrosine, it sometimes gets "spooked" and jumps further down the hallway to other random spots in the protein.
• Why? The Helper's connection changes the energy landscape. If the connection is just right, the spark flows smoothly. If the connection is wrong (or missing), the spark gets lost and wanders off to the wrong places.

The Takeaway

This paper teaches us that proteins are like sophisticated electrical circuits where the "wiring" is controlled by tiny hydrogen bonds.

By adding or removing a single proton (a hydrogen particle) and changing the pH (acidity), nature (and scientists) can:

1. Turn the machine on or off.
2. Speed it up or slow it down.
3. Decide exactly where the energy goes.

It's like having a dimmer switch and a traffic controller built into the same tiny molecule. This helps us understand how our bodies manage energy and how we might design better artificial machines (like solar cells or batteries) that mimic these biological tricks.

Technical Summary

1. Problem Statement

Long-range electron transfer (ET) in proteins is essential for metabolic pathways like respiration and photosynthesis. This process often relies on "hole-hopping" mechanisms where redox-active residues, primarily tryptophan (Trp) and tyrosine (Tyr), act as relay sites to transport charge over distances exceeding 10–30 Å.

• The Challenge: While Trp and Tyr are both oxidizable, they are not functionally interchangeable due to differences in their radical cation pKa values and sensitivities to the local environment.
• Specific Context: In yeast cytochrome c peroxidase (CcP), the native Trp191 (W191) acts as a critical relay site to transfer electrons from cytochrome c (Cc) to the heme active center. Substituting W191 with Tyr (Y191) renders the enzyme inactive because the Y191 radical potential is insufficient to oxidize Cc. Previous work showed that introducing a general base (Glu232 or His232) adjacent to Y191 could "rescue" activity, but the precise mechanistic role of proton-coupled electron transfer (PCET) and hydrogen bonding in modulating the radical's formal potential and propagation pathways remained unclear.
• Knowledge Gap: There is a lack of direct experimental measurement of the equilibrium between the photo-generated porphyrin radical (ZnP•+) and the relay residue radical (W/Y191•+), and how proton management dictates whether the radical stays at the relay site or propagates to peripheral sites.

2. Methodology

The authors employed a multi-faceted approach combining protein engineering, spectroscopy, and computational modeling:

• Protein Engineering & Mutagenesis:

  • Fluorination: Used global incorporation of the non-canonical amino acid 4-fluoro-glutamic acid (F-Glu) into E. coli to replace Glu232. F-Glu has a lower pKa (~2 units lower) than Glu, allowing the authors to test the hypothesis that a hydrogen bond from a protonated base is required to elevate the Y191• potential.
  • Variants: Generated CcP variants including W191 (wild-type), Y191, Y191:E232, Y191:H232, and Y191:F-E232 (with F-Glu).
  • Heme Replacement: Replaced the native heme with Zinc Porphyrin (ZnP) to create ZnCcP, enabling photo-initiated ET studies distinct from the peroxide-driven native system.

• Kinetic Assays:

  • Peroxide-driven (FeCcP): Measured multiple-turnover rates of Cc oxidation by Compound I (Cpd I) across pH 5–7 using UV-Vis spectroscopy.
  • Photo-driven (ZnCcP): Used time-resolved transient absorption (TA) spectroscopy to measure back-electron transfer rates ($k_{eb}$) between ZnP•+ and Cc in the presence of various pH conditions and solvent isotope effects (H₂O vs. D₂O).

• Radical Characterization:

  • Sacrificial Acceptor: Replaced Cc with the irreversible oxidant [Co(NH₃)₅Cl]²⁺ (CoN₅). This prevents recombination, allowing stable accumulation of protein radicals for characterization.
  • EPR Spectroscopy: Utilized continuous-wave (cw-EPR) at various temperatures (10 K, 110 K, 293 K) and pulsed EPR techniques (ENDOR and ESEEM) to identify radical species (ZnP•+, Trp•+, Tyr•) and their spin distributions.
  • Solvent Isotope Effects: Measured Kinetic Solvent Isotope Effects (KSIE) to confirm PCET involvement.

• Computational Modeling:

  • Performed QM/MM (Quantum Mechanics/Molecular Mechanics) molecular dynamics simulations using the wB97x functional (chosen for superior spin density accuracy over B3LYP) to track hole localization and spin density distribution along trajectories.

3. Key Contributions

• Mechanistic Distinction: Demonstrated that the mechanism for activating the Y191 relay site differs fundamentally between the peroxide-driven (FeCcP) and photo-driven (ZnCcP) systems.
• Direct Measurement of Radical Equilibrium: Provided experimental evidence and computational support for the equilibrium constant ($K_{eq}$) between ZnP•+ and W191•+, placing the Trp radical ~15 mV lower in potential than ZnP•+.
• Role of Hydrogen Bonding: Established that hydrogen bonding to the radical cation is a dual-function switch: it can either elevate the potential of a formed radical (to oxidize a donor) or facilitate the formation of the radical via PCET (by lowering the oxidation barrier).
• Radical Propagation Control: Showed that the nature of the adjacent base (Glu vs. His) and the pH dictate whether the radical remains at the 191 site or propagates to peripheral Trp residues.

4. Key Results

A. Peroxide-Driven System (FeCcP)

• Hydrogen Bond Requirement: In the Y191 variant, activity is rescued by E232 or H232. The hydrogen bond from the protonated base (E/H232-H⁺-Y191•) elevates the formal potential of the Y191 radical, enabling it to oxidize Cc.
• Fluorination Validation: The Y191:F-E232 variant (with lower pKa F-Glu) showed activity only at lower pH (<4) compared to Y191:E232. This confirms that the protonated state of the base is essential to maintain the high potential required for ET. As pH increases, deprotonation lowers the Y191• potential, shutting down ET.

B. Photo-Driven System (ZnCcP)

• Opposite pH Trend: Unlike FeCcP, ET rates in ZnCcP increase with pH. This indicates that the rate-limiting step is the formation of the Y191 radical via PCET.
• PCET Mechanism: At higher pH, the conjugate base (E232/H232) deprotonates, accepting a proton from Y191 during oxidation by ZnP•+. This concerted PCET lowers the barrier for Y191 oxidation.
• Solvent Isotope Effects: Significant KSIEs were observed only in the pH ranges where rates increased, confirming that proton transfer is coupled to electron transfer.

C. Radical Distribution and Propagation

• Equilibrium: Using CoN₅ to trap radicals, EPR and QM/MM calculations revealed that the radical equilibrium ($K_{eq}$) favors the Trp•+ (or Tyr•+) site over ZnP•+ by a ratio of ~1.8:1. The W191•+ potential is ~15 mV below ZnP•+.
• Exchange Rate: Despite close proximity (7–8 Å), the exchange rate between ZnP•+ and W191•+ is slow (<10⁷ s⁻¹), likely due to orbital orientation constraints.
• Peripheral Propagation:
  • In Y191:H232 and Y191:E232 variants at specific pH ranges, the radical does not stay at 191. Instead, it propagates to peripheral Trp residues (detected via ¹⁴N ESEEM and broad low-field EPR signals).
  • This propagation requires two conditions: (1) PCET to form Y191•, and (2) a subsequent hydrogen bond from the base to Y191• to raise its potential sufficiently to oxidize peripheral sites.
  • Y191:H232 is unique in propagating radicals effectively at lower pH (6–8) compared to E232 variants, likely due to the neutral imidazole of His232 maintaining a hydrogen bond even when deprotonated.

5. Significance

• Engineering Protein ET: The study provides a blueprint for engineering long-range ET systems. It demonstrates that "proton management" (tuning pKa and hydrogen bonding networks) can be used to separate charge migration from recombination and direct radicals to specific sites.
• Biological Relevance: The findings offer mechanistic insights into natural systems like Photosystem II (PSII), where TyrZ and His residues utilize similar PCET strategies for water splitting. It explains how proteins can switch between different radical propagation pathways based on environmental pH.
• Fundamental Biophysics: The work resolves the debate on the relative potentials of ZnP•+ and W191•+ in CcP, correcting previous computational models and highlighting the critical, tunable role of the local electrostatic environment in redox catalysis.
• Allosteric Regulation: The results suggest that radical propagation in proteins could be subject to allosteric regulation via protonation states, offering new avenues for designing bio-inspired molecular electronics and sensors.