The diheme enzyme MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor protein to generate a tryptophan tryptophylquinone cofactor. The MauG-catalyzed reaction proceeds via a bis -Fe(IV) intermediate in which one heme is present as Fe(IV)=O and the other as Fe(IV) with axial histidine and tyrosine ligation. Herein, a unique near-infrared absorption feature exhibited specifically in bis -Fe(IV) MauG is described, and evidence is presented that it results from a charge-resonance-transition phenomenon. As the two hemes are physically separated by 14.5 Å, a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes is reversibly oxidized and reduced to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis -Fe(IV) MauG. Analysis of the MauG structure reveals that electron transfer via this mechanism is rapid enough to enable a charge-resonance stabilization of the bis -Fe(IV) state without direct contact between the hemes. The finding of the charge-resonance-transition phenomenon explains why the bis -Fe(IV) intermediate is stabilized in MauG and does not permanently oxidize its own aromatic residues.

The diheme enzyme MauG catalyzes the posttranslational modification of the precursor protein of methylamine dehydrogenase (preMADH) to complete biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Catalysis proceeds through a high valent bis-Fe(IV) redox state and requires long-range electron transfer (ET), as the distance between the modified residues of pre-MADH and the nearest heme iron of MauG is 19.4 Å. Trp199 of MauG resides at the MauG-preMADH interface, positioned midway between the residues that are modified and the nearest heme. W199F and W199K mutations did not affect the spectroscopic and redox properties of MauG, or its ability to stabilize the bis-Fe(IV) state. Crystal structures of complexes of W199F/K MauG with pre-MADH showed no significant perturbation of the MauG-preMADH structure or protein interface. However, neither MauG variant was able to synthesize TTQ from preMADH. In contrast, an ET reaction from diferrous MauG to quinone MADH, which does not require the bis-Fe(IV) intermediate, was minimally affected by the W199F/K mutations. W199F/K MauGs were able to oxidize quinol MADH to form TTQ, the putative final two-electron oxidation of the biosynthetic process, but with k cat ∕K m values approximately 10% that of wild-type MauG. The differential effects of the W199F/K mutations on these three different reactions are explained by a critical role for Trp199 in mediating multistep hopping from preMADH to bis-Fe(IV) MauG during the long-range ET that is required for TTQ biosynthesis.cytochrome | electron hopping | peroxidase | protein oxidation | protein radical L ong-range electron transfer (ET) through proteins is required for biological processes including respiration, photosynthesis, and metabolism. Mechanisms by which ET occurs over large distances to specific sites within a protein have been extensively studied (1-4). For interprotein ET, kinetic mechanisms are more complex, as the overall redox reaction requires additional steps such as protein-protein association and reorientation of the protein complex to optimize the system for ET (5, 6). "Long-range catalysis" is a related process in which the redox center that provides the oxidizing or reducing power is physically distinct from the site of chemical reaction of the substrate, so that long-range ET is required for catalysis. Thus far two enzymes have been postulated to employ long-range catalysis. Ribonucleotide reductase (RNR) catalyzes the formation of deoxyribonucleotides from ribonucleotides by long-range ET via multiple tyrosyl residues (7,8). DNA photolyase is a flavoprotein that catalyzes DNA repair of pyrimidine-pyrimidine dimers via multiple tryptophan residues (9). In these enzymes it is believed that the long-range ET proceeds by hopping (10) through residues that can stabilize a radical state, rather than via a single long-range electron tunneling event.

Plants synthesize carotenoids essential for plant development and survival. These metabolites also serve as essential nutrients for human health. The biosynthetic pathway leading to all plant carotenoids occurs in chloroplasts and other plastids and requires 15-cis-ζ-carotene isomerase (Z-ISO). It was not certain whether isomerization was achieved by Z-ISO alone or in combination with other enzymes. Here we show that Z-ISO is a bona fide enzyme and integral membrane protein. Z-ISO independently catalyzes the cis-to-trans isomerization of the 15–15′ C=C bond in 9,15,9′-cis-ζ-carotene to produce the substrate required by the following biosynthetic pathway enzyme. We discovered that isomerization depends upon a ferrous heme b cofactor that undergoes redox-regulated ligand-switching between the heme iron and alternate Z-ISO amino acid residues. Heme b-dependent isomerization of a large, hydrophobic compound in a membrane is unprecedented. As an isomerase, Z-ISO represents a new prototype for heme b proteins and potentially utilizes a novel chemical mechanism.

Interfacial evaporation using porous hydrogels has demonstrated highly effective solar evaporation performance under natural sunlight to ensure an affordable clean water supply. However, it remains challenging to realize scalable and ready‐to‐use hydrogel materials with durable mechanical properties. Here, self‐assembled templating (SAT) is developed as a simple yet effective method to fabricate large‐scale elastic hydrogel evaporators with excellent desalination performance. The highly interconnected porous structure of the hydrogels with low tortuosity and tunable pore size enables high level of tunability on the water transport rate. With superior elasticity, the porous hydrogels are easy to process with a rapid shape recovery after being rolled, folded, and twisted over hundred times, and exhibit highly effective and stable evaporation with an evaporation rate of ≈2.8 kg m−2 h−1 and ≈90 % solar‐to‐vapor efficiency. It is anticipated that this SAT strategy, without the typical need for freeze‐drying, will accelerate the industrialization of hydrogel solar evaporators for practical applications.

An intriguing mystery about tryptophan 2,3-dioxygenase is its hydrogen peroxide-triggered enzyme reactivation from the resting ferric oxidation state to the catalytically active ferrous form. In this study, we found that such an odd Fe(III) reduction by an oxidant depends on the presence of L-Trp, which ultimately serves as the reductant for the enzyme. In the peroxide reaction with tryptophan 2,3-dioxygenase, a previously unknown catalase-like activity was detected. A ferryl species (␦ ‫؍‬ 0.055 mm/s and ⌬E Q ‫؍‬ 1.755 mm/s) and a protein-based free radical (g ‫؍‬ 2.0028 and 1.72 millitesla linewidth) were characterized by Mössbauer and EPR spectroscopy, respectively. This is the first compound ES-type of ferryl intermediate from a heme-based dioxygenase characterized by EPR and Mössbauer spectroscopy. Density functional theory calculations revealed the contribution of secondary ligand sphere to the spectroscopic properties of the ferryl species. In the presence of L-Trp, the reactivation was demonstrated by enzyme assays and by various spectroscopic techniques. A Trp-Trp dimer and a monooxygenated L-Trp were both observed as the enzyme reactivation byproducts by mass spectrometry. Together, these results lead to the unraveling of an over 60-year old mystery of peroxide reactivation mechanism. These results may shed light on how a metalloenzyme maintains its catalytic activity in an oxidizing environment.Hemoproteins perform a wide range of biological functions, including oxygen transport, storage, electron transfer, monooxygenation, and reduction of dioxygen. However, they rarely express dioxygenase activity as their native biological function. Tryptophan 2,3-dioxygenase (TDO) 3 is the first described exception (1-3). This enzyme employs a b-type ferrous heme prosthetic group to catalyze the oxidative cleavage of the indole ring of L-Trp, converting it to N-formylkynurenine (NFK) (Scheme 1). This is the first and rate-limiting step of the kynurenine pathway of L-Trp metabolism, which oxidizes over 99% of L-Trp in mammalian intracellular and extracellular pools (2, 4 -9). The kynurenine pathway constitutes the major steps in biosynthesis of NAD, an essential redox cofactor in all living systems (5).TDO is a hepatic enzyme first discovered in rat liver extracts in 1936 (1). An analogous enzyme, indoleamine 2,3-dioxygenase (IDO), was isolated 31 years later from tissues other than the liver (10). Although both enzymes catalyze the same reaction, TDO is highly substrate-specific with L-Trp, whereas IDO presents a more relaxed specificity. TDO is a homotetramer with a total mass of ϳ134 kDa, whereas IDO is a monomeric protein. The two enzymes share only 14% sequence identity but conserve similar active site architectures (11-13). In addition to humans, TDO has also been found in other mammals, such as rats and mice, as well as in mosquitoes and bacteria (2, 5, 14 -18). Recently, a potential heme-dependent dioxygenase enzyme superfamily has been proposed (19). Moreover, several other heme-based proteins, such as my...

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A fundamental problem in network analysis is clustering the nodes into groups which share a similar connectivity pattern. Existing algorithms for community detection assume the knowledge of the number of clusters or estimate it a priori using various selection criteria and subsequently estimate the community structure. Ignoring the uncertainty in the first stage may lead to erroneous clustering, particularly when the community structure is vague. We instead * propose a coherent probabilistic framework for simultaneous estimation of the number of communities and the community structure, adapting recently developed Bayesian nonparametric techniques to network models. An efficient Markov chain Monte Carlo (MCMC) algorithm is proposed which obviates the need to perform reversible jump MCMC on the number of clusters. The methodology is shown to outperform recently developed community detection algorithms in a variety of synthetic data examples and in benchmark real-datasets. Using an appropriate metric on the space of all configurations, we develop non-asymptotic Bayes risk bounds even when the number of clusters is unknown. Enroute, we develop concentration properties of non-linear functions of Bernoulli random variables, which may be of independent interest in analysis of related models.

For photocatalytic solar energy conversion, the critical challenge is to enhance the solar utilization efficiency. Many efforts have focused on the development of broad-band response nanomaterials. Here, we propose an alternative approach wherein, over Ni 2 P/ TiO 2 nanoparticles without noble metal, the UV−vis part of solar energy was absorbed and converted by a semiconductor and its infrared part was separately collected and converted into thermal energy to heat the photocatalytic reaction to a certain temperature. The photothermocatalytic hydrogen activity was 3.6 times that of the sum of the photocatalytic and thermocatalytic reactions. The in situ generated oxygen vacancies in Ni 2 P/TiO 2 during the photothermocatalytic reaction were found to be responsible for the enhanced activity. Moreover, the photocurrent transient response results revealed the faster transfer of electrons from TiO 2 to Ni 2 P at higher temperature which is vital for the significantly enhanced photothermocatalytic hydrogen production. The long-term test also shows the stability of the proposed reaction system.