Amino acids are the fundamental molecular units that link together to form proteins, which perform nearly all functional and structural roles within the body. These molecules are categorized based on the chemical properties of their unique side chain, or R-group. The defining feature of an acidic amino acid is the presence of a second carboxyl group (-COOH) within its side chain. This extra carboxyl group easily releases a hydrogen ion (H+) into the surrounding solution. This proton release results in a net negative charge on the side chain at the body’s physiological pH, making the amino acid acidic.
The Two Acidic Amino Acids and Their Defining Characteristic
Only two of the 20 standard protein-building amino acids fall into the acidic category: aspartic acid (Asp) and glutamic acid (Glu). Each amino acid contains a central alpha-carbon atom bonded to an amino group, a single hydrogen atom, a main carboxyl group, and the unique R-group. For both Asp and Glu, the R-group contains an additional carboxyl group. Aspartic acid has a side chain that is one carbon shorter than glutamic acid.
At the neutral pH typical for human physiology (about 7.4), the acidic side chain carboxyl group is deprotonated, meaning it has lost its hydrogen ion. This leaves both molecules with a negative charge on their side chain. In this ionized state, they are referred to by their conjugate bases: Aspartate and Glutamate. This negative charge allows them to participate in electrostatic interactions, which are important for stabilizing protein structure and for enzyme function.
Key Physiological Functions
Aspartate and Glutamate play diverse roles that extend beyond being building blocks for protein synthesis. Both are heavily involved in the body’s metabolic processes, particularly nitrogen management. Aspartate is an important intermediate in the urea cycle, which detoxifies and excretes excess nitrogen in the form of urea.
Aspartate also plays a part in the malate-aspartate shuttle, a system that transports reducing equivalents into the mitochondria for energy production. Glutamate acts as a primary amino group donor or acceptor in many transamination reactions. It is central to amino acid metabolism, acting as a hub for nitrogen flow between different compounds.
Glutamate is recognized as the most abundant excitatory neurotransmitter in the central nervous system (CNS). It stimulates nerve cells to fire an electrical impulse, playing a role in brain functions such as learning and memory. Aspartate also functions as an excitatory neurotransmitter, though its effect is less pronounced than glutamate. Both molecules stimulate NMDA receptors, which are involved in synaptic plasticity.
Dietary Intake and Health Relevance
Aspartate and Glutamate are classified as non-essential amino acids, meaning the human body can synthesize them from other molecules. Despite this, they are routinely consumed as part of a normal diet, as they are naturally abundant in many protein-rich foods. Excellent dietary sources include aged cheeses, meat, poultry, fish, eggs, milk, and vegetables like tomatoes and mushrooms.
Glutamate is recognized for its contribution to the savory taste known as umami. This flavor profile is why it is often added to foods in the form of Monosodium Glutamate (MSG) as a flavor enhancer. The safety of consuming MSG has been subject to extensive scientific review.
Major international food safety organizations have concluded that glutamate, when consumed at typical levels, is safe for the general population. The body metabolizes free glutamate from food and protein-bound glutamate in the same way. Free glutamate simply enhances the palatability of foods without posing a health concern for most individuals.