Characterization of rhodosaminyl transfer by the AknS/AknT glycosylation complex and its use in reconstituting the biosynthetic pathway of aclacinomycin A

The tetracyclic core of anthracycline natural products with antitumor activity such as aclacinomycin A are tailored during biosynthesis by regioselective glycosylation. We report the first synthesis of TDP-L-rhodosamine and demonstrate that the glycosyltransferase AknS transfers L-rhodosamine to the aglycone to initiate construction of the side-chain trisaccharide. The partner protein AknT accelerates AknS turnover rate for L-rhodosamine transfer by 200-fold. AknT does not affect the Km but rather affects the kcat. Using these data, we propose that AknT causes a conformational change in AknS that stabilizes the transition state and ultimately enhances transfer. When the subsequent glycosyltransferase AknK and its substrate TDP-L-fucose are also added to the aglycone, the disaccharide and low levels of a fully reconstituted trisaccharide form of aclacinomycin are observed.     

Kinetic selectivity in the N-alkylation of 2-pyridyl porphyrins: a facile approach to the alphaalphabetabeta scaffold

meso-Tetrakis(2-pyridyl)-porphyrin (2-PyP) was tetra-N-alkylated with three different alpha-bromoacetamides to generate a series of water-soluble N-alkylpyridinium porphyrins (1-3). The product mixtures showed a marked preference for the formation of the alphaalphabetabeta atropisomer. With alpha-bromo-N-n-butylacetamide, the corresponding alphaalphabetabeta 2-tetrakis (N-n-butylacetamido)-pyridyl porphyrin (2-TnBuPyP, 3) was obtained in 69% isolated yield in a single step. Prolonged heating lead to equilibration of the rotational isomers for the less bulky alkyl groups, indicating that the observed preference is a kinetic effect. The intermediate products for the N-bornyl case were identified by LC/ESI-MS to deduce an explanation for the observed nonstatistical selectivity.     

Solid-state NMR evidence for a protonation switch in the binding pocket of the H1 receptor upon binding of the agonist histamine

G protein coupled receptors (GPCRs) represent a major superfamily of transmembrane receptor proteins that are crucial in cellular signaling and are major pharmacological targets. While the activity of GPCRs can be modulated by agonist binding, the mechanisms that link agonist binding to G protein coupling are poorly understood. Here we present a method to accurately examine the activity of ligands in their bound state, even at low affinity, by solid-state NMR dipolar correlation spectroscopy and confront this method with the human H1 receptor. The analysis reveals two different charge states of the bound agonist, dicationic with a charged imidazole ring and monocationic with a neutral imidazole ring, with the same overall conformation. The combination of charge difference and pronounced heterogeneity agrees with converging evidence that the active and inactive states of the GPCR represent a dynamic equilibrium of substates and that proton transfer between agonist and protein side chains can shift this equilibrium by stabilizing the active receptor population relative to the inactive one. In fact, the data suggest a global functional analogy between H1 receptor activation and the meta I/meta II charge/discharge equilibrium in rhodopsin (GPCR). This corroborates current ideas on unifying principles in GPCR structure and function.     

The traveling salesman problem on cubic and subcubic graphs

We study the traveling salesman problem (TSP) on the metric completion of cubic and subcubic graphs, which is known to be NP-hard. The problem is of interest because of its relation to the famous 4/3-conjecture for metric TSP, which says that the integrality gap, i.e., the worst case ratio between the optimal value of a TSP instance and that of its linear programming relaxation (the subtour elimination relaxation), is 4/3. We present the first algorithm for cubic graphs with approximation ratio 4/3. The proof uses polyhedral techniques in a surprising way, which is of independent interest. In fact we prove constructively that for any cubic graph on $n$ vertices a tour of length $4n/3-2$ exists, which also implies the 4/3-conjecture, as an upper bound, for this class of graph-TSP. Recently, Mömke and Svensson presented an algorithm that gives a 1.461-approximation for graph-TSP on general graphs and as a side result a 4/3-approximation algorithm for this problem on subcubic graphs, also settling the 4/3-conjecture for this class of graph-TSP. The algorithm by Mömke and Svensson is initially randomized but the authors remark that derandomization is trivial. We will present a different way to derandomize their algorithm which leads to a faster running time. All of the latter also works for multigraphs.     

The Effect of a Horseshoe Crab Citizen Science Program on Middle School Student Science Performance and STEM Career Motivation

The purpose of the present quasi‐experimental study was to examine the impact of a horseshoe crab citizen science program on student achievement and science, technology, engineering, and mathematics (STEM) career motivation with 86 (n = 86) eighth‐grade students. The treatment group conducted fieldwork with naturalists and collected data for a professional biologist studying horseshoe crab speciation and a mock survey. The comparison group studied curriculum related to horseshoe crabs in the science classroom. A series of measures related to self‐efficacy, interest, outcome expectations, choice goals, and content knowledge were given to participants before and after the intervention. It was hypothesized that students would report higher motivational beliefs regarding science and show higher levels of achievement following the intervention than the comparison group. Support was shown for most of the hypotheses. In addition, path analyses indicated that students' motivational beliefs influence content knowledge and outcome expectations, which in turn affect their career goals. These results have implications for incorporating authentic fieldwork within a formal school structure as an effective method for promoting student achievement and STEM career motivation.     

Optimal control of swinging alliances in a parabolic competition model

A system of parabolic partial differential equations describes the interaction of three populations, modeling a dynamic competition/cooperation scenario. More precisely, two populations are always competing with each other, but the third population can switch the mode of alliance with the other two populations between cooperation and competition. The control is a function measuring the strength and nature of the alliance and the goal is to maximize the population with the swinging alliance while keeping the other two populations close to each other and minimizing the cost of the alliance action. Various scenarios are illustrated with numerical results.     

An age-structured model for the spread of epidemic cholera: Analysis and simulation

Occasional outbreaks of cholera epidemics across the world demonstrate that the disease continues to pose a public health threat. Traditional models for the spread of infectious diseases are based on systems of ordinary differential equations. Since disease dynamics such as vaccine efficacy and the risk for contracting cholera depend on the age of the humans, an age-structured model offers additional insights and the possibility of studying the effects of treatment options. The model investigated is given as a system of hyperbolic (first-order) partial differential equations in combination with ordinary differential equations. First, using a representation from the method of characteristics and a fixed point argument, we prove the existence and uniqueness of a solution to our nonlinear system. Then we present a finite difference approximation to the model and study the effect of high and low rates of shedding of cholera vibrios on the dynamics of the spread of the disease. The simulations demonstrate the explosive nature of cholera outbreaks that is observed in reality. The contrast of results for high and low rates of shedding of vibrios suggest a possible underlying cause for this effect.