Definition
L-aspartate oxidase (EC 1.4.3.16) is a flavoprotein enzyme that catalyzes the oxidation of L-aspartate to iminosuccinate (also called oxaloacetate imine) with the concomitant reduction of molecular oxygen to hydrogen peroxide. The reaction constitutes the first step in the de novo biosynthetic pathway for nicotinamide adenine dinucleotide (NAD) in many bacteria and archaea.
Overview
L-aspartate oxidase (also abbreviated AspOx or NadB) is a key enzyme in the quinolate pathway of NAD⁺ biosynthesis. In this pathway, the enzyme converts L‑aspartate and O₂ into iminosuccinate, which is subsequently cyclized by quinolinate synthase (NadA) to produce quinolinate, a precursor of nicotinic acid mononucleotide (NaMN). The enzyme is typically found in prokaryotes such as Escherichia coli, Bacillus subtilis, and various thermophilic archaea. In some organisms, alternative pathways for NAD⁺ synthesis exist, and the presence of L‑aspartate oxidase can be dispensable.
The enzyme uses flavin adenine dinucleotide (FAD) as a prosthetic group. The catalytic mechanism proceeds via a two‑electron oxidation of the α‑amino group of L‑aspartate, generating an imine intermediate that is hydrolyzed spontaneously to oxaloacetate and ammonia in downstream steps.
Etymology / Origin
The name combines the substrate (“L‑aspartate”) with the suffix “oxidase,” indicating an enzyme that oxidizes its substrate using molecular oxygen as the electron acceptor. The systematic name, L‑aspartate:oxygen oxidoreductase (deaminating), reflects its functional activity. The gene encoding the enzyme is commonly designated nadB in bacterial genomes.
Characteristics
- Molecular weight: Approximately 48–55 kDa per monomer, depending on the organism.
- Cofactor: Non‑covalently bound FAD (flavin adenine dinucleotide).
- Quaternary structure: Typically functions as a homodimer; some archaeal homologs form higher‑order oligomers.
- Optimal conditions: Activity is generally maximal at neutral to slightly alkaline pH (7.0–8.5) and at temperatures reflecting the organism’s growth range (e.g., 37 °C for mesophiles, up to 80 °C for thermophiles).
- Kinetic parameters: Reported Kₘ values for L‑aspartate range from 0.05 to 0.5 mM, with V_max values varying according to assay conditions.
- Substrate specificity: Primarily specific for L‑aspartate; D‑aspartate and related amino acids are poor substrates.
- Cellular localization: Cytoplasmic enzyme in prokaryotes; no signal peptide or transmembrane regions are present.
Related Topics
- NAD⁺ biosynthesis – the metabolic pathway that generates the essential cofactor NAD⁺.
- NadB gene – the gene encoding L‑aspartate oxidase in bacterial genomes.
- Quinolinate synthase (NadA) – the enzyme that uses the product of L‑aspartate oxidase to form quinolinate.
- Flavoproteins – a broad class of proteins that contain flavin moieties (FAD or FMN) as prosthetic groups.
- Oxidative deamination – the chemical process of removing an amino group from an amino acid with concomitant oxidation, exemplified by the reaction catalyzed by L‑aspartate oxidase.
- Metabolic engineering – manipulation of NAD⁺ biosynthetic genes, including nadB, for biotechnological applications such as increased production of NAD⁺-dependent metabolites.