Aspartate synthesis in humans involves the generation of aspartate from oxaloacetate via transamination by aspartate aminotransferase or amino acid oxidase. Once synthesized, aspartate can be coupled to aspartyl tRNA via aspartyl-tRNA synthetase and used by the body in protein synthesis. The aspartate content in human proteins is about 7%. Aspartate can be converted to another polar amino acid, asparagine, via asparagine synthase. Aspartate is a precursor to many other cofactors or compounds involved in cellular signaling including N-acetyl-aspartate, beta-alanine, adenylsuccinate, arginino-succinate and N-carbamoylaspartate. Aspartate is also a metabolite in the urea cycle and participates in gluconeogenesis. Additionally, aspartate carries the reducing equivalents in the mitochondrial malate-aspartate shuttle, which utilizes the ready interconversion of aspartate and oxaloacetate. The conjugate base of L-aspartic acid, aspartate, also acts as an excitatory neurotransmitter in the brain which activates NMDA receptors.