Significance: Phgdh is a crucial enzyme in the astrocytic synthesis of l -serine from glucose. Our study uncovers the role of glial-neuronal cross-talk in regulating NMDAR synaptic activation through the astrocytic export of Phgdh-derived l -serine to generate the coagonists d -serine and glycine. We clarify the relative roles of d -serine and glycine by showing that like d -serine, Phgdh-derived glycine mediates NMDAR synaptic activation in the mature hippocampus. We show that glycine is a significant regulator of d -serine synthesis and release, revealing a cross-talk between glycine and d -serine metabolism that fine tunes NMDAR activation. Our data reveal a multifaceted mechanism regulating NMDAR activity, which ultimately depends on a Phgdh-dependent serine shuttle.

Nutritional immunity is a form of innate immunity widespread in both vertebrates and invertebrates. The term refers to a rich repertoire of mechanisms set up by the host to inhibit bacterial proliferation by sequestering trace minerals (mainly iron, but also zinc and manganese). This strategy, selected by evolution, represents an effective front-line defense against pathogens and has thus inspired the exploitation of iron restriction in the development of innovative antimicrobials or enhancers of antimicrobial therapy. This review focuses on the mechanisms of nutritional immunity, the strategies adopted by opportunistic human pathogen Staphylococcus aureus to circumvent it, and the impact of deletion mutants on the fitness, infectivity, and persistence inside the host. This information finally converges in an overview of the current development of inhibitors targeting the different stages of iron uptake, an as-yet unexploited target in the field of antistaphylococcal drug discovery.

: Mutations in human genes might lead to the loss of functional proteins, causing diseases. Among these genetic disorders, a large class is associated with the deficiency in metabolic enzymes, resulting in both an increase in the concentration of substrates and a loss in the metabolites produced by the catalyzed reactions. The identification of therapeu-tic actions based on small molecules represents a challenge to medicinal chemists because the target is missing. Alternative approaches are biology-based, ranging from gene and stem cell therapy, CRISPR/Cas9 technology, distinct types of RNAs, and enzyme replace-ment therapy (ERT). This review will focus on the latter approach that since the 1990s has been successfully applied to cure many rare diseases, most of them being lysosomal stor-age diseases or metabolic diseases. So far, a dozen enzymes have been approved by FDA/EMA for lysosome storage disorders and only a few for metabolic diseases. Enzymes for replacement therapy are mainly produced in mammalian cells and some in plant cells and yeasts and are further processed to obtain active, highly bioavailable, less degradable products. Issues still under investigation for the increase in ERT efficacy are the optimiza-tion of the interaction of the enzymes with cell membrane and internalization, the reduction in immunogenicity, and the overcoming of blood-brain barrier limitations when neuronal cells need to be targeted. Overall, ERT has demonstrated its efficacy and safety in the treatment of many genetic rare diseases, both saving newborn lives and improving pa-tients’ life quality, and represents a very successful example of targeted biologics.

Human serine racemase is a pyridoxal 5′-phosphate (PLP)-dependent dimeric enzyme that catalyzes the reversible racemization of L-serine and D-serine and their dehydration to pyruvate and ammonia. As D-serine is the co-agonist of the N-methyl-D-aspartate receptors for glutamate, the most abundant excitatory neurotransmitter in the brain, the structure, dynamics, function, regulation and cellular localization of serine racemase have been investigated in detail. Serine racemase belongs to the fold-type II of the PLP-dependent enzyme family and structural models from several orthologs are available. The comparison of structures of serine racemase co-crystallized with or without ligands indicates the presence of at least one open and one closed conformation, suggesting that conformational flexibility plays a relevant role in enzyme regulation. ATP, Mg²⁺, Ca²⁺, anions, NADH and protein interactors, as well as the post-translational modifications nitrosylation and phosphorylation, finely tune the racemase and dehydratase activities and their relative reaction rates. Further information on serine racemase structure and dynamics resulted from the search for inhibitors with potential therapeutic applications. The cumulative knowledge on human serine racemase allowed obtaining insights into its conformational landscape and into the mechanisms of cross-talk between the effector binding sites and the active site.

Among multidrug-resistant bacteria, methicillin-resistant Staphylococcus aureus is emerging as one of the most threatening pathogens. S. aureus exploits different mechanisms for its iron supply, but the preferred one is acquisition of organic iron through the expression of hemoglobin (Hb) receptors. One of these, IsdB, belonging to the Isd (Iron-Regulated Surface Determinant) system, was shown to be essential for bacterial growth and virulence. Therefore, interaction of IsdB with Hb represents a promising target for the rational design of a new class of antibacterial molecules. However, despite recent investigations, many structural and mechanistic details of complex formation and heme extraction process are still elusive. By combining site-directed mutagenesis, absorption spectroscopy, surface plasmon resonance and molecular dynamics simulations, we tackled most of the so far unanswered questions: (i) the exact complex stoichiometry, (ii) the microscopic kinetic rates of complex formation, (iii) the IsdB selectivity for binding to, and extracting heme from, α and β subunits of Hb, iv) the role of specific amino acid residues and structural regions in driving complex formation and heme transfer, and (v) the structural/dynamic effect played by the hemophore on Hb.

The N‐methyl d‐aspartate (NMDA) receptors play a key role in excitatory neurotransmission, and control learning, memory and synaptic plasticity. Their activity is modulated by the agonist glutamate and by the co‐agonists d‐serine and glycine. In the human brain, d‐serine is synthesized from l‐serine by the dimeric pyridoxal 5′‐phosphate‐dependent enzyme serine racemase, which also degrades l‐ and d‐serine to pyruvate and ammonia. The dependence of l‐ and d‐serine β‐elimination and l‐serine racemization activities on ATP concentration was characterized, and was found to be strongly cooperative, with Hill coefficients close to 2 and apparent ATP dissociation constants ranging from 0.22 to 0.41 mm. ATP binding to the holo‐enzyme, monitored by the fluorescence changes of the coenzyme, was also determined to be cooperative, with an apparent dissociation constant of 0.24 mm. Glycine, an active‐site ligand, increased the serine racemase affinity for ATP by ~ 22‐fold, abolishing cooperativity. Conversely, ATP increased the non‐cooperative glycine binding15‐fold. These results indicate cross‐talk between allosteric and active sites, leading to the stabilization of two alternative protein conformations with ATP affinities of ~ 10 μM and 1.8 mm, as evaluated within the Monod, Wyman and Changeux model. Therefore, intracellular ATP and glycine control d‐serine homeostasis, and, indirectly, NMDA receptor activity. Because hyper‐ and hypo‐activation of NMDA receptors are associated with neuropathologies, the development of allosteric drugs modulating serine racemase activity is a promising therapeutic strategy.