Serine Racemase Modulates Intracellular D-Serine Levels through an α,β-Elimination Activity

Abstract

D-Serine is a D-amino acid that occurs at high levels in the mammalian brain and is an endogenous ligand of the "glycine site" of N-methyl D-aspartate (NMDA) 1 receptors (1-4). NMDA receptors play key roles in excitatory synaptic transmission, plasticity, and learning and memory (5). Overactivation of the NMDA receptor and the resultant influx of calcium into cells is a major culprit in the cell death that occurs following stroke and neurodegenerative diseases. Blockers of the "glycine site" of the receptor are neuroprotective in animal models of stroke (5). Endogenous D-serine is required for NMDA receptor activation, and its removal markedly decreases NMDA receptor activity (3). In the vertebrate retina, endogenous D-serine may also mediate the light-dependent increase in neuronal activity by activating NMDA receptors (6). More recently, D-serine was suggested to play a role in the long term potentiation of synaptic transmission in the hippocampus, indicating a role of endogenous D-serine in long term synaptic plasticity (7).D-Serine is synthesized by serine racemase, a pyridoxal phosphate (PLP)-dependent enzyme enriched in the mammalian brain (8, 9). Serine racemase has high sequence homology with the fold-type II group of PLP enzymes, such as serine/threonine dehydratase and D-serine dehydratase (10, 11). In addition to converting L-to D-serine, serine racemase catalyzes the ␣,␤-elimination of water from L-serine to form pyruvate and ammonia (12). The initial rates of racemization and ␣,␤-elimination of L-serine by serine racemase are strongly stimulated by magnesium and ATP, indicating that the complex Mg⅐ATP is a physiological ligand of the enzyme (12).In accordance with accepted mechanisms of PLP-catalyzed reactions (13-16), a mechanism for racemization and ␣,␤-elimination catalyzed by serine racemase is depicted in Scheme 1. PLP, bound to the enzyme through an internal aldimine with The termination of signaling by a neurotransmitter in the brain normally requires its re-uptake and metabolism. D-Serine signaling is thought to involve its release from cells to

Nitric oxide S -nitrosylates serine racemase, mediating feedback inhibition of d -serine formation

“…Purification of SR was carried out as described in ref. 21. In brief, mouse full-length SR DNA was subcloned into pET-28cϩ, which encodes a 6x-histidine tag and introduced into BL21 codon plus bacteria (Stratagene).…”

Serine racemases from barley, Hordeum vulgare L., and other plant species represent a distinct eukaryotic group: Gene cloning and recombinant protein characterization☆

“…This effect occurs especially in brain areas, since a D-amino acid oxidase, which degrades D-amino acids in mammalian cells, is restricted to the cerebellum and brainstem and a dehydratase activity-deficient serine racemase mutant accumulates 1a (Foltyn et al, 2005). Several D-amino acids and D-amino acid-containing peptides have been identified in plants.…”

“…Mammalian serine racemase is an unique enzyme that catalyzes not only serine (1a, 1b) racemization but also dehydration of 1a and 1b to pyruvate (2) in the presence of divalent cations; this distinguishes it from bacterial amino acid racemases (Cook et al, 2002;De Miranda et al, 2002;Strisovsky et al, 2003;Foltyn et al, 2005) (Fig. 1).…”

Distribution of eukaryotic serine racemases in the bacterial domain and characterization of a representative protein in Roseobacter litoralis Och 149

“…These results suggest that RiSR catalysed serine racemization rather than dehydration. Foltyn et al (2005) reported that the serine dehydration-deficient mutant (Gln155Asp) of mouse SR exhibits enhanced D-serine synthesis compared with that of wild-type SR. Therefore, D-serine production may depend on the proportion of serine dehydration activity relative to that of racemization activity.…”

“…D-serine is synthesized from L-serine by serine racemases (SRs), which belong to a group of pyridoxal 59-phosphate (PLP)-dependent enzymes (Grishin et al, 1995;Yoshimura & Esak, 2003;Yoshimura & Goto, 2008). Currently, many genes encoding eukaryotic SRs in humans (De Miranda et al, 2000;Hoffman et al, 2009), mice (Foltyn et al, 2005;Strísovský et al, 2005), plants (Fujitani et al, 2006(Fujitani et al, , 2007Gogami et al, 2010), slime moulds (Ito et al, 2012) and fission yeasts (Goto et al, 2009) have been cloned, and the enzymatic properties of these proteins have been characterized.…”