Reverse Ordered Sequential Mechanism for Lactoperoxidase with Inhibition by Hydrogen Peroxide

Lactoperoxidase (LPO, FeIII in its resting state in the absence of substrates)—an enzyme secreted from human mammary, salivary, and other mucosal glands—catalyzes the oxidation of thiocyanate (SCN−) by hydrogen peroxide (H2O2) to produce hypothiocyanite (OSCN−), which functions as an antimicrobial agent. The accepted catalytic mechanism, called the halogen cycle, comprises a two-electron oxidation of LPO by H2O2 to produce oxoiron(IV) radicals, followed by O-atom transfer to SCN−. However, the mechanism does not explain biphasic kinetics and inhibition by H2O2 at low concentration of reducing substrate, conditions that may be biologically relevant. We propose an ordered sequential mechanism in which the order of substrate binding is reversed, first SCN− and then H2O2. The sequence of substrate binding that is described by the halogen cycle mechanism is actually inhibitory.

NIGERIA: POWER-SHARING AND THE RESURGENCE OF SEPARATIST AGITATION. THE PROSPECT OF A CONSOCIATIONAL MODEL

The theory of consociationalism has been extensively discussed in literature; however, its feasibility in managing conflicts in deeply divided societies is heavily contested. The few studies that have examined how the theory applies in real-world situations remain inconclusive. The present work, therefore, explored the prospect of consociational power-sharing model in addressing the problem of under-representation, political exclusion, and marginalization in Nigeria. This is against the backdrop of the incessant separatist agitations in Nigeria, which has undermined the peace, stability, and unity of the country. Using qualitatively analyzed data from secondary sources, this study argues that even though Nigeria does not meet any of the favorable conditions of consociationalism set out by Lijphart (1985), it would still benefit from consociational power-sharing. The study proposes the adoption of semi-presidentialism based on the principle of grand coalition and proportionality and gives an assessment of how it could work for Nigeria. It contends that the rotation of power among the six geo-political zones in the country and the adoption of proportional sequential mechanism would facilitate elite cooperation and inclusion of all segments of the society in the political process, thereby easing the fear of sectional domination in Nigeria. Keywords: power-sharing, consociationalism, separatist agitation, under-representation, political exclusion, Nigeria.

Invitation Games: An Experimental Approach to Coalition Formation

This paper studies how to form an efficient coalition—a group of people. More specifically, we compare two mechanisms for forming a coalition by running a laboratory experiment and reveal which mechanism leads to higher social surplus. In one setting, we invite the subjects to join a meeting simultaneously, so they cannot know the other subjects’ decisions. In the other setting, we ask them sequentially, which allows each subject to know his or her predecessor’s choice. Those who decide to join the meeting form a coalition and earn payoffs according to their actions and individual preferences. As a result, we obtain the following findings. First, the sequential mechanism induces higher social surplus than the simultaneous mechanism. Second, most subjects make choices consistent with the subgame-perfect Nash equilibrium in the sequential setting and choose the dominant strategy in the simultaneous setting, when a dominant strategy exists. Finally, when the subjects need to look further ahead to make a theoretically rational choice, they are more likely to fail to choose rationally.

Structures and mechanism of human glycosyltransferase β1,3-N-acetylglucosaminyltransferase 2 (B3GNT2), an important player in immune homeostasis

β1,3-N-acetylglucosaminyltransferases (B3GNTs) are Golgi-resident glycosyltransferases involved in the biosynthesis of poly-N-acetyl-lactosamine chains. They catalyze the addition of the N-acetylglucosamine to the N-acetyl-lactosamine repeat as a key step of the chain elongation process. Poly-N-acetyl-lactosamine is involved in immune system in many ways. Particularly, its long chain has been demonstrated to suppress excessive immune responses. Among the characterized B3GNTs, B3GNT2 is the major poly-N-acetyl-lactosamine synthase and deletion of its coding gene dramatically reduced the cell surface poly-N-acetyl-lactosamine and led to hypersensitive and hyperresponsive immunocytes. Despite the extensive functional studies, no structural information is available to understand the molecular mechanism of B3GNT2, as well as other B3GNTs. Here we present the structural and kinetic studies of the human B3GNT2. Five crystal structures of B3GNT2 have been determined in the unliganded, donor substrate-bound, acceptor substrate-bound and product(s)-bound states at resolutions ranging from 1.85 Å to 2.35 Å. Kinetic study shows that the transglycosylation reaction follows a sequential mechanism. Critical residues involved in recognition of both donor and acceptor substrates as well as catalysis are identified. Mutations of these invariant residues impair B3GNT2 activity in cell assays. Structural comparison with other glycosyltransferases such as mouse Fringe reveals a novel N-terminal helical domain of B3GNTs that may stabilize the catalytic domain and distinguish among different acceptor substrates.

Structural basis of mitochondrial translation

Translation of mitochondrial messenger RNA (mt-mRNA) is performed by distinct mitoribosomes comprising at least 36 mitochondria-specific proteins. How these mitoribosomal proteins assist in the binding of mt-mRNA and to what extent they are involved in the translocation of transfer RNA (mt-tRNA) is unclear. To visualize the process of translation in human mitochondria, we report ~3.0 Å resolution structure of the human mitoribosome, including the L7/L12 stalk, and eight structures of its functional complexes with mt-mRNA, mt-tRNAs, recycling factor and additional trans factors. The study reveals a transacting protein module LRPPRC-SLIRP that delivers mt-mRNA to the mitoribosomal small subunit through a dedicated platform formed by the mitochondria-specific protein mS39. Mitoribosomal proteins of the large subunit mL40, mL48, and mL64 coordinate translocation of mt-tRNA. The comparison between those structures shows dynamic interactions between the mitoribosome and its ligands, suggesting a sequential mechanism of conformational changes.

An Asymmetric Mechanism in a Symmetric Molecular Machine

The design of molecular architectures exhibiting functional motions is a promising area for disruptive technological development. Towards this goal, rotaxanes and catenanes, which undergo relative motions of their sub-units in response to external stimuli, are prime candidates. Here, we report on the computational analysis of the contraction/extension of a bistable [c2]-daisy chain rotaxane. Using free energy calculations and transition path optimizations, we explore the free energy landscape governing the functional motions of a prototypical molecular machine with atomic resolution.<br>The calculations reveal a sequential mechanism for contraction/extension in which the asynchronous gliding of each ring is preferred over the concerted movement suggested by chemical intuition. Analysis of the underlying free energy surface indicates that dissymmetric gliding is favored because it entails crossings of much smaller barriers.<br>Our findings illustrate an important design principle for molecular machines, namely that efficient exploitation of thermal fluctuations may be realized by breaking down the large-scale functional motions into smaller steps.

An Asymmetric Mechanism in a Symmetric Molecular Machine

The design of molecular architectures exhibiting functional motions is a promising area for disruptive technological development. Towards this goal, rotaxanes and catenanes, which undergo relative motions of their sub-units in response to external stimuli, are prime candidates. Here, we report on the computational analysis of the contraction/extension of a bistable [c2]-daisy chain rotaxane. Using free energy calculations and transition path optimizations, we explore the free energy landscape governing the functional motions of a prototypical molecular machine with atomic resolution.<br>The calculations reveal a sequential mechanism for contraction/extension in which the asynchronous gliding of each ring is preferred over the concerted movement suggested by chemical intuition. Analysis of the underlying free energy surface indicates that dissymmetric gliding is favored because it entails crossings of much smaller barriers.<br>Our findings illustrate an important design principle for molecular machines, namely that efficient exploitation of thermal fluctuations may be realized by breaking down the large-scale functional motions into smaller steps.

In Situ Combustion Synthesis in Air of Calcium Titanate Powders Using Minerals as a Calcium Source

Calcium titanate (CaTiO3) was synthesized through combustion in air from calcium sources of raw minerals (lime-stone and calcite), anatase titanium dioxide (A-TiO2) and magnesium (Mg). The syntheses were divided into two reactant systems (lime-stone/A-TiO2/Mg and calcite/A-TiO2/Mg. Before synthesis, the raw minerals and A-TiO2 were high-energy milled for 30 min. These powders were then separately mixed with Mg by ball milling. After synthesis, the as-combusted products were leached with 2 M HCl solution to remove by-products and impurities. A sequential mechanism for the in-situ combustion was proposed by using data from simultaneous thermal analysis (STA) together with thermodynamic values calculated with HSC software. XRD results showed that the as-leached products from both reactant systems mainly contained CaTiO3. FT-IR spectroscopy indicated that the as-leached products had Ca-Ti-O and Ti-O functional groups. In addition, SEM observation of the as-leached products revealed cuboid-like crystals with a particle size of about 100 nm.

Evaluation of a concerted vs. sequential oxygen activation mechanism in α-ketoglutarate–dependent nonheme ferrous enzymes

Determining the requirements for efficient oxygen (O2) activation is key to understanding how enzymes maintain efficacy and mitigate unproductive, often detrimental reactivity. For the α-ketoglutarate (αKG)–dependent nonheme iron enzymes, both a concerted mechanism (both cofactor and substrate binding prior to reaction with O2) and a sequential mechanism (cofactor binding and reaction with O2 precede substrate binding) have been proposed. Deacetoxycephalosporin C synthase (DAOCS) is an αKG-dependent nonheme iron enzyme for which both of these mechanisms have been invoked to generate an intermediate that catalyzes oxidative ring expansion of penicillin substrates in cephalosporin biosynthesis. Spectroscopy shows that, in contrast to other αKG-dependent enzymes (which are six coordinate when only αKG is bound to the FeII), αKG binding to FeII-DAOCS results in ∼45% five-coordinate sites that selectively react with O2 relative to the remaining six-coordinate sites. However, this reaction produces an FeIII species that does not catalyze productive ring expansion. Alternatively, simultaneous αKG and substrate binding to FeII-DAOCS produces five-coordinate sites that rapidly react with O2 to form an FeIV=O intermediate that then reacts with substrate to produce cephalosporin product. These results demonstrate that the concerted mechanism is operative in DAOCS and by extension, other nonheme iron enzymes.