Aspartate, often referred to as aspartic acid, is an amino acid that plays a widespread role in human metabolism and cellular communication. It is considered a non-essential amino acid, meaning the body can synthesize it from other compounds without strict reliance on dietary intake. Aspartate is one of the 20 common amino acids that serve as the fundamental building blocks for all proteins. Its chemical structure includes a negatively charged side chain at physiological pH, allowing it to participate in numerous biochemical reactions beyond protein assembly.
Aspartate: A Building Block and Signaling Molecule
Aspartate’s status as a non-essential amino acid means the body produces it internally, primarily through the transamination of oxaloacetate, an intermediate molecule in the citric acid cycle. The molecule exists in two forms: aspartic acid is the protonated form, while aspartate is the ionized, negatively charged form that predominates in the neutral environment of the body’s cells.
The primary purpose of aspartate is its incorporation into polypeptide chains during protein synthesis, where it contributes to the final three-dimensional structure of proteins. Its charged side chain is often positioned on the exterior of proteins, where it can interact with the cellular environment or participate in ionic bonding. Aspartate also acts as a precursor, serving as the starting material for synthesizing other biological molecules, including the non-essential amino acid asparagine.
Aspartate is also involved in the construction of nucleotides, the structural units of DNA and RNA. Specifically, it provides key carbon and nitrogen atoms necessary for building the heterocyclic rings of pyrimidines, such as cytosine and thymine. This role links aspartate directly to processes of cell division, growth, and the transmission of genetic information.
Key Roles in Energy and Nitrogen Metabolism
One of aspartate’s primary functions is its involvement in the urea cycle, a pathway that handles the detoxification of ammonia, a toxic byproduct of amino acid metabolism. In this process, aspartate contributes one of the two nitrogen atoms required to form urea, the compound excreted by the kidneys. This contribution occurs when aspartate condenses with citrulline, forming argininosuccinate in a reaction that drives the cycle forward.
The urea cycle is functionally linked to the citric acid cycle, and aspartate acts as a bridge between these two metabolic pathways. The carbon skeleton it leaves behind after donating its nitrogen atom is fumarate, which is recycled into the citric acid cycle to generate energy. This connection highlights aspartate’s role in maintaining the body’s nitrogen balance.
Aspartate is also a central player in the malate-aspartate shuttle, a system responsible for moving reducing equivalents into the mitochondria for energy production. The inner mitochondrial membrane is impermeable to NADH, the electron carrier produced during glycolysis in the cell’s cytosol. The shuttle indirectly transports the electrons from cytosolic NADH into the mitochondrial matrix.
Aspartate and its metabolic partner, malate, are exchanged across the mitochondrial membrane, carrying the necessary reducing power. This process ultimately regenerates NADH inside the mitochondria, where it delivers its electrons to the electron transport chain, resulting in the generation of adenosine triphosphate (ATP), the cell’s main energy currency. The efficiency of this shuttle is important in tissues with high energy demands, such as the liver, heart, and brain.
Aspartate’s Function in the Nervous System
In the central nervous system, aspartate functions as an excitatory neurotransmitter, much like its counterpart, glutamate. Excitatory neurotransmitters increase the electrical activity of neurons, making them more likely to fire an action potential and transmit a signal. Aspartate interacts with specific receptors on nerve cells, particularly the N-methyl-D-aspartate (NMDA) receptors.
The NMDA receptor is a type of ion channel that, when activated by a neurotransmitter like aspartate, allows positively charged ions, such as calcium, to flow into the neuron. This influx of ions is fundamental to several neurological processes, including synaptic plasticity, the cellular mechanism believed to underlie learning and memory formation. Aspartate acts as a selective agonist for these receptors, promoting their opening and subsequent signaling.
The balance of aspartate and glutamate signaling is tightly controlled within the brain, as excessive stimulation of NMDA receptors can lead to a phenomenon known as excitotoxicity. This overstimulation can cause the sustained influx of calcium ions, which, in high concentrations, can damage or kill the neuron. Its ability to activate NMDA receptors and modulate neuronal signaling is consistently recognized.
Dietary Sources and Considerations
Since aspartate is a non-essential amino acid, the body can produce all that it requires from metabolic intermediates, meaning a dietary deficiency is unlikely under normal circumstances. Nevertheless, aspartate is widely available in the diet because it is a component of almost all dietary proteins. Common food sources rich in aspartate include poultry, meat, eggs, and dairy products.
Plant-based sources also provide ample amounts of the amino acid, such as legumes, whole grains, nuts, and seeds.
A common consideration related to aspartate is its association with the artificial sweetener aspartame. Aspartame is a dipeptide composed of aspartate and phenylalanine, and upon digestion, it is broken down into these two amino acids and a small amount of methanol. Regulatory bodies, including the Food and Drug Administration (FDA), have extensively reviewed the safety of aspartame. The aspartate ingested from aspartame constitutes a very small portion of the total aspartate consumed daily from other food sources and is considered safe at typical consumption levels.