Distance metrics for heme protein electron tunneling

AbstractThere is no doubt that distance is the principal parameter that sets the order of magnitude for electron-tunneling rates in proteins. However, there continue to be varying ways to measure electron-tunneling distances in proteins. This distance uncertainty blurs the issue of whether the intervening protein medium has been naturally selected to speed or slow any particular electron-tunneling reaction. For redox cofactors lacking metals, an edge of the cofactor can be defined that approximates the extent in space that includes most of the wavefunction associated with its tunneling electron. Beyond this edge, the wavefunction tails off much more dramatically in space. The conjugated porphyrin ring seems a reasonable edge for the metal-free pheophytins and bacteriopheophytins of photosynthesis. For a metal containing redox cofactor such as heme, an appropriate cofactor edge is more ambiguous. Electron-tunneling distance may be measured from the conjugated heme macrocycle edge or from the metal, which can be up to 4.8 Å longer. In a typical protein medium, such a distance difference normally corresponds to a ~1000 fold decrease in tunneling rate. To address this ambiguity, we consider both natural heme protein electron transfer and light-activated electron transfer in ruthenated heme proteins. We find that the edge of the conjugated heme macrocycle provides a reliable and useful tunneling distance definition consistent with other biological electron-tunneling reactions. Furthermore, with this distance metric, heme axially- and edge-oriented electron transfers appear similar and equally well described by a simple square barrier tunneling model. This is in contrast to recent reports for metal-to-metal metrics that require exceptionally poor donor/acceptor couplings to explain heme axially-oriented electron transfers

Electron tunneling chains of mitochondria

AbstractThe single, simple concept that natural selection adjusts distances between redox cofactors goes a long way towards encompassing natural electron transfer protein design. Distances are short or long as required to direct or insulate promiscuously tunneling single electrons. Along a chain, distances are usually 14 Å or less. Shorter distances are needed to allow climbing of added energetic barriers at paired-electron catalytic centers in which substrate and the required number of cofactors form a compact cluster. When there is a short-circuit danger, distances between shorting centers are relatively long. Distances much longer than 14Å will support only very slow electron tunneling, but could act as high impedance signals useful in regulation. Tunneling simulations of the respiratory complexes provide clear illustrations of this simple engineering

University–Museum Partnerships for K-12 Engineering Learning: Understanding the Utility of a Community Co-Created Informal Education Program in a Time of Social Disruption

This study explores the impact of COVID-19 on informal learning institutions, primarily science museums, through the lens of an activity kit co-created by CELL-MET—a cross-university, engineering research center—and museum partners. While formal learning organizations, like K-12 schools, play a critical role in the education process through standardized teaching, informal learning organizations also make important contributions to the engineering education ecosystem, such as by fostering engineering identity development, especially for learners and their families. This is particularly valuable for young learners from underrepresented and under-resourced communities. In this study, two questions are addressed: (1) How were museums impacted by COVID-19 and the resulting disruptions to their operations, and how did they respond? (2) To what extent were museums able to implement and adapt EEK! to reach under-served youth in the face of social disruption? When the world was experiencing social disruption from the spread of COVID-19, the authors realized they had an opportunity to test the utility and adaptability of their model of engineering activity co-creation. Approximately six months into the launch of both EEK! and the global pandemic, a 29-item survey was distributed to EEK! recipient institutions. Of the museum respondents, 97% reported experiencing full closures and 73% reported layoffs and furloughs. Despite these challenges, 78% implemented EEK!, with 70% of the institutions creating new virtual programming, and 38% adapting EEK! for remote facilitation, including real-time virtual events, self-guided activities, and at-home activity kits. Museums were equally impacted by COVID-19 policies and closures, but have not received the public attention and support that K-12 schools have received. Nonetheless, they have responded with ingenuity in using and adapting EEK!. Given their K-12 partnerships, flexibility, and ability to engage learners, museums are undervalued collaborators for universities trying to impact the K-12 engineering education ecosystem

Quinone and non-quinone redox couples in Complex III

The Q cycle mechanism proposed by Peter Mitchell in the 1970’s explicitly considered the modification of ubiquinone two-electron redox properties upon binding to Complex III to match the thermodynamics of the other single-electron redox cofactors in the complex, and guide electron transfer to support the generation of a proton electro-chemical gradient across native membranes. A better understanding of the engineering of Complex III is coming from a now moderately well defined thermodynamic description of the redox components as a function of pH, including the Qi/heme b(H) cluster. The redox properties of the most obscure component, Qo, is finally beginning to be resolved

Rheostat Re-Wired: Alternative Hypotheses for the Control of Thioredoxin Reduction Potentials

<div><p>Thioredoxins are small soluble proteins that contain a redox-active disulfide (CXXC). These disulfides are tuned to oxidizing or reducing potentials depending on the function of the thioredoxin within the cell. The mechanism by which the potential is tuned has been controversial, with two main hypotheses: first, that redox potential (<i>E_m</i>) is specifically governed by a molecular ‘rheostat’—the XX amino acids, which influence the Cys pK_a values, and thereby, <i>E_m</i>; and second, the overall thermodynamics of protein folding stability regulates the potential. Here, we use protein film voltammetry (PFV) to measure the pH dependence of the redox potentials of a series of wild-type and mutant archaeal Trxs, PFV and glutathionine-equilibrium to corroborate the measured potentials, the fluorescence probe BADAN to measure pK_a values, guanidinium-based denaturation to measure protein unfolding, and X-ray crystallography to provide a structural basis for our functional analyses. We find that when these archaeal thioredoxins are probed directly using PFV, both the high and low potential thioredoxins display consistent 2H⁺:2e^- coupling over a physiological pH range, in conflict with the conventional ‘rheostat’ model. Instead, folding measurements reveals an excellent correlation to reduction potentials, supporting the second hypothesis and revealing the molecular mechanism of reduction potential control in the ubiquitous Trx family.</p></div

Electrochemical potentials and Cys pKa values of thioredoxins.

<p>^adetermined by PFV at pH 7.0</p><p>^bfrom Ref [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122466#pone.0122466.ref003" target="_blank">3</a>]</p><p>^cfrom reference [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122466#pone.0122466.ref005" target="_blank">5</a>]</p><p>^d from Ref [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122466#pone.0122466.ref006" target="_blank">6</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122466#pone.0122466.ref008" target="_blank">8</a>].</p><p>Electrochemical potentials and Cys pKa values of thioredoxins.</p