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In our continued investigations of microbial globins, we solved the structure of a truncated hemoglobin from Shewanella benthica, an obligate psychropiezophilic bacterium. The distal side of the heme active site is lined mostly with hydrophobic residues, with the exception of a tyrosine, Tyr34 (CD1) and a histidine, His24 (B13). We found that purified SbHbN, when crystallized in the ferric form with polyethylene glycol as precipitant, turned into a green color over weeks. The electron density obtained from the green crystals accommodated a trans heme d, a chlorin-type derivative featuring a γ-spirolactone and a vicinal hydroxyl group on a pyrroline ring. In solution, exposure of the protein to one equivalent of hydrogen peroxide resulted in a similar green color change, but caused by the formation of multiple products. These were oxidation species released on protein denaturation, likely including heme d, and a species with heme covalently attached to the polypeptide. The Tyr34Phe replacement prevented the formation of both heme d and the covalent linkage. The ready modification of heme b by SbHbN expands the range of chemistries supported by the globin fold and offers a route to a novel heme cofactor. 

Initially proposed by Lovric and Scholz to explain redox reactions in solid-phase voltammetry, the Scholz model’s applications have expanded to redox reactions in various materials. As an extension of the Cottrell equation, the Scholz model enabled the quantification of electron hopping and ion diffusion with coefficients, De and Di, respectively. Research utilizing the Scholz model indicated that, in most cases, a huge bottleneck results from the ion diffusion which is slower than electron hopping by orders of magnitude. Therefore, electron and ion motion can be tuned and optimized to increase the charge transport and conductivity through systematic investigations guided by the Scholz model. The strategy may be extended to other solid-state materials in the future, e.g., battery anodes/cathodes. In this Perspective, the applications of the Scholz model in different materials will be discussed. Moreover, the limitations of the Scholz model will also be introduced, and viable solutions to those limitations discussed. 

Pakistan is the most glaciated country on the planet but faces increasing water scarcity due to the vulnerability of its primary water source, the Indus River, to changes in climate and demand. Glacier melt constitutes over one-third of the Indus River’s discharge, but the impacts of glacier shrinkage from anthropogenic climate change are not equal across all eleven subbasins of the Upper Indus. We present an exploration of glacier melt contribution to Indus River flow at the subbasin scale using a distributed surface energy and mass balance model run 2001–2013 and calibrated with geodetic mass balance data. We find that the northern subbasins, the three in the Karakoram Range, contribute more glacier meltwater than the other basins combined. While glacier melt discharge tends to be large where there are more glaciers, our modeling study reveals that glacier melt does not scale directly with glaciated area. The largest volume of glacier melt comes from the Gilgit/Hunza subbasin, whose glaciers are at lower elevations than the other Karakoram subbasins. Regional application of the model allows an assessment of the dominant drivers of melt and their spatial distributions. Melt energy in the Nubra/Shyok and neighboring Zaskar subbasins is dominated by radiative fluxes, while turbulent fluxes dominate the melt signal in the west and south. This study provides a theoretical exploration of the spatial patterns to glacier melt in the Upper Indus Basin, a critical foundation for understanding when glaciers melt, information that can inform projections of water supply and scarcity in Pakistan.