Biocatalytic asymmetric aldol addition into unactivated ketones.

Enzymes are renowned for their catalytic efficiency and selectivity, but many classical transformations in organic synthesis have no biocatalytic counterpart. Aldolases are prodigious C-C bond-forming enzymes, but their reactivity has only been extended past activated carbonyl electrophiles in special cases. To probe the mechanistic origins of this limitation, we use a pair of aldolases whose activity is dependent on pyridoxal phosphate. Our results reveal how aldolases are limited by kinetically favourable proton transfer with solvent, which undermines aldol addition into ketones. We show how a transaldolase can circumvent this limitation, enabling efficient addition into unactivated ketones. The resulting products are highly sought non-canonical amino acids with side chains that contain chiral tertiary alcohols. Mechanistic analysis reveals that transaldolase activity is an intrinsic feature of pyridoxal phosphate chemistry and identifies principles for extending aldolase catalysis beyond its previous limits to enable convergent, enantioselective C-C bond formation from simple starting materials.

Efficient chemoenzymatic synthesis of α-aryl aldehydes as intermediates in C-C bond forming biocatalytic cascades.

Multi-enzyme biocatalytic cascades are emerging as practical routes for the synthesis of complex bioactive molecules. However, the relative sparsity of water-stable carbon electrophiles limits the synthetic complexity of molecules made from such cascades. Here, we develop a chemoenzymatic platform that leverages styrene oxide isomerase (SOI) to covert readily accessible aryl epoxides into α-aryl aldehydes through a Meinwald rearrangement. These unstable aldehyde intermediates are then intercepted with a C-C bond forming enzyme, ObiH, that catalyzes a transaldolase reaction with l-threonine to yield synthetically challenging ß-hydroxy-α-amino acids. Co-expression of both enzymes in E. coli yields a whole cell biocatalyst capable of synthesizing a variety of stereopure non-standard amino acids (nsAA) and can be produced on gram-scale. We used isotopically labelled substrates to probe the mechanism of SOI, which we show catalyzes a concerted isomerization featuring a stereospecific 1,2-hydride shift. The viability of in situ generated α-aryl aldehydes was further established by intercepting them with a recently engineered decarboxylative aldolase to yield γ-hydroxy nsAAs. Together, these data establish a versatile method of producing α-aryl aldehydes in simple, whole cell conditions and show that these intermediates are useful synthons in C‒C bond forming cascades.

Perceptions of SNAP and Stocking Standards: A Qualitative Study of California Small Food Store Owners and Managers.

The Supplemental Nutrition Assistance Program (SNAP) is critical to alleviating food insecurity, but low diet quality among program participants is a concern. Nutrition-related interventions have focused on SNAP-authorized food retailers, but the perspectives of small food store owners and managers have not been represented in national policy discussions. This study aimed to explore the opinions of store owners/managers of SNAP-authorized small food stores about their overall perceptions of the program and the stricter stocking standards previously proposed in 2016. We conducted in-depth, semi-structured interviews with 33 small food store owners and managers in San Francisco and Oakland, California in 2016. Interviews were analyzed for thematic content using the general inductive approach. Four themes emerged from owners/managers' discussion of their overall perceptions of SNAP: the beneficial impact of SNAP on their business, how SNAP enables them to connect with the broader community, the importance of SNAP in preventing hunger, and the nutrition-related struggles that SNAP participants face. Store owners/managers had a generally favorable response towards the proposed stricter stocking standards. Additional themes discussed pertained to the concern about whether stocking changes would lead SNAP participants to purchase more healthful food and some logistical challenges related to sourcing and storing perishable foods.

l-Threonine Transaldolase Activity Is Enabled by a Persistent Catalytic Intermediate.

l-Threonine transaldolases (lTTAs) are a poorly characterized class of pyridoxal-5'-phosphate (PLP) dependent enzymes responsible for the biosynthesis of diverse ß-hydroxy amino acids. Here, we study the catalytic mechanism of ObiH, an lTTA essential for biosynthesis of the ß-lactone natural product obafluorin. Heterologously expressed ObiH purifies as a mixture of chemical states including a catalytically inactive form of the PLP cofactor. Photoexcitation of ObiH promotes the conversion of the inactive state of the enzyme to the active form. UV-vis spectroscopic analysis reveals that ObiH catalyzes the retro-aldol cleavage of l-threonine to form a remarkably persistent glycyl quinonoid intermediate, with a half-life of ∼3 h. Protonation of this intermediate is kinetically disfavored, enabling on-cycle reactivity with aldehydes to form ß-hydroxy amino acids. We demonstrate the synthetic potential of ObiH via the single step synthesis of (2S,3R)-ß-hydroxyleucine. To further understand the structural features underpinning this desirable reactivity, we determined the crystal structure of ObiH bound to PLP as the Schiff's base at 1.66 Å resolution. This high-resolution model revealed a unique active site configuration wherein the evolutionarily conserved Asp that traditionally H-bonds to the cofactor is swapped for a neighboring Glu. Molecular dynamics simulations combined with mutagenesis studies indicate that a structural rearrangement is associated with l-threonine entry into the catalytic cycle. Together, these data explain the basis for the unique reactivity of lTTA enzymes and provide a foundation for future engineering and mechanistic analysis.

"It's a Feeling That One Is Not Worth Food": A Qualitative Study Exploring the Psychosocial Experience and Academic Consequences of Food Insecurity Among College Students.

BACKGROUND: The issue of food insecurity is one of growing concern among institutions of higher learning in the United States. In addition to studies on the prevalence and risk factors, research is needed to better understand the mechanisms by which food insecurity affects students' health and well-being. OBJECTIVE: To critically explore the experience of food insecurity among college students and its impact on psychosocial health and academic performance. DESIGN: In-depth qualitative interviews were conducted with college students to gain a better understanding of the mechanisms by which food insecurity affects their psychosocial health and academic performance. PARTICIPANTS/SETTING: Twenty-five undergraduate students from a large public university in California who were recruited from a campus food pantry. RESULTS: Students discussed several themes related to the psychosocial effects of food insecurity: the stress of food insecurity interfering with daily life, a fear of disappointing family, resentment of students in more stable food and financial situations, an inability to develop meaningful social relationships, sadness from reflecting on food insecurity, feeling hopeless or undeserving of help, and frustration directed at the academic institution for not providing enough support. Students also discussed how food insecurity affected their academic performance through physical manifestations of hunger and the mental trade-off between focusing on food and focusing on academics. CONCLUSION: These findings contribute to the understanding of what it means to experience food insecurity in higher education and can inform how universities support students' basic needs.