Inhibition of human serine racemase, an emerging target for medicinal chemistry.

Proteins of glutamatergic NMDA receptor signaling pathways have been studied as targets for intervention in a variety of neuropathological conditions, including neurodegenerations, epilepsy, neuropathic pain, drug addiction, and schizophrenia. High activity NMDA-blocking agents have been designed to treat some of these disorders; however, their effect is often compromised by undesirable side effects. Therefore, alternative ways of modulating NMDA receptor function need to be sought after. The opening of the NMDA receptor ion channel requires occupation of two distinct binding sites, the glutamate site and the glycine site. It has been shown that D-serine, rather than glycine, can trigger the physiological NMDA receptor function. D-serine is a product of the activity of a specific enzyme, serine racemase (SR), which was identified a decade ago. SR has therefore emerged as a new potential target for the NMDA-receptor-based diseases. There is evidence linking increased levels of D-Ser to amyotrophic lateral sclerosis and Alzheimer's disease and decreased concentrations of D-serine to schizophrenia. SR is a pyridoxal-5'-phosphate dependent enzyme found in the cytosol of glial and neuronal cells. It is activated by ATP, divalent cations like Mg(2+) or Ca(2+), and reducing agents. This paper reviews the present literature on the activity and inhibition of mammalian SRs. It summarizes approaches that have been applied to design SR inhibitors and lists the known active compounds. Based on biochemical and docking analyses, i) we delineate for the first time the ATP binding site of human SR, and ii) we suggest possible mechanisms of action for the active compounds. In the end, we discuss the SR features that make the discovery of its inhibitors a challenging, yet very important, task of medicinal chemistry.

Hydroxamic acids as a novel family of serine racemase inhibitors: mechanistic analysis reveals different modes of interaction with the pyridoxal-5'-phosphate cofactor.

Mammalian serine racemase (SR) is a pyridoxal-5'-phosphate (PLP) dependent enzyme responsible for the biosynthesis of the neurotransmitter D-serine, which activates N-methyl-D-aspartate (NMDA) receptors in the CNS. Aberrant regulation of NMDA receptor signaling has been implicated in a variety of neuropathologies, and inhibitors of SR would therefore be a worthwhile tool for further investigation or treatment of such conditions. Here, we identify a series of small aliphatic hydroxamic acids (HAs) that act as potent SR inhibitors. However, specificity studies showed that some of these HAs can act as nonspecific inhibitors of PLP-dependent enzymes. We employed NMR, MS, and UV/vis spectroscopic techniques to reveal that the nonspecific effect is likely due to irreversible interaction of the HA moiety with PLP to form aldoxime species. We also characterize L-aspartic acid beta-hydroxamate as a competitive and selective SR inhibitor that could be used as a scaffold for further inhibitor development.

Recombinant human serine racemase: enzymologic characterization and comparison with its mouse ortholog.

D-serine plays a key role in glutamatergic neurotransmission in mammalian brain as a co-agonist of N-methyl-D-aspartate receptors. The enzyme responsible for D-serine biosynthesis, serine racemase (SR), is therefore a promising target for treatment of neuropathologies related to glutamate receptor excitotoxicity, such as stroke or Alzheimer's disease. Much of the experimental work to date has been performed on mouse serine racemase, which shares a high level of sequence identity with its human ortholog. In this work, we report the synthesis of a human SR gene variant optimized for heterologous expression in Escherichia coli and describe the expression and purification of active recombinant human SR. This strategy may be of general interest to researchers wishing to express mammalian proteins in a bacterial system. Furthermore, we conduct a thorough analysis of the kinetics and inhibitor-sensitivity of the recombinant enzyme, and we provide the first direct comparison of human and mouse SR based on our kinetic data. The orthologs behave similarly overall and exhibit identical inhibition profiles, validating the use of mouse models in SR research.

Engineering non-natural inhibitor sensitivity in protein tyrosine phosphatase H1.

Protein tyrosine phosphatase H1, a member of the ubiquitous protein tyrosine phosphatase (PTP) superfamily of enzymes, is an important signaling molecule, mutant forms of which have been found in human colorectal cancers. Selective PTPH1 inhibitors would be valuable tools for investigating PTPH1's roles in cellular regulation. However, no PTPH1-specific inhibitors are known. To identify target-selective inhibitors of human PTPH1, we have redesigned a PTPH1/inhibitor interface. Structure-based protein design was used to identify two amino-acid residues, isoleucine 846 and methionine 883, that control PTPH1's sensitivity to oxalylaminoindole PTP inhibitors. Mutation of residues 846 and 883 to alanine and glycine, respectively, conferred novel inhibitor sensitivity onto PTPH1. From a small panel of putative inhibitors, compounds that potently and selectively target the inhibitor-sensitized PTPH1 mutants were identified.

Allele-specific inhibitors of protein tyrosine phosphatases.

Protein tyrosine phosphatases (PTPs) are critical cell-signaling molecules. Inhibitors that are selective for individual PTPs would be valuable tools for dissecting complicated phosphorylation networks. However, the common architecture of PTP active sites impedes the discovery of such compounds. To achieve target selectivity, we have redesigned a PTP/inhibitor interface. Site-directed mutagenesis of a prototypical phosphatase, PTP1B, was used to generate "inhibitor-sensitized" PTPs. The PTP1B mutants were targeted by modifying a broad specificity PTP inhibitor with chemical groups that are sterically incompatible with wild-type PTP active sites. From a small panel of putative inhibitors, compounds that selectively inhibit Ile219Ala PTP1B over the wild-type enzyme were identified. Importantly, the corresponding mutation also conferred novel inhibitor sensitivity to T-cell PTP, suggesting that a readily identifiable point mutation can be used to generate a variety of inhibitor-sensitive PTPs.