Deamidation is a fundamental chemical reaction where an amide group is removed from a molecule. In biological systems and manufactured products, this process occurs most often on the amino acids Asparagine (Asn) and Glutamine (Gln) within a protein chain. This modification is common in all biological organisms and is a factor in the long-term stability of manufactured substances. The reaction is a non-reversible chemical change that significantly alters the properties of the protein over time.
The Chemical Process of Deamidation
The deamidation reaction involves the hydrolysis of the amino acid side chain, resulting in the conversion of Asparagine to Aspartic acid and Glutamine to Glutamic acid. This transformation is a spontaneous, non-enzymatic reaction that occurs naturally under physiological conditions inside the body. The rate of the reaction is heavily influenced by environmental factors such as elevated temperature, specific pH levels, and the local protein sequence.
Asparagine residues are particularly susceptible to this change and often deamidate much faster than Glutamine. The Asparagine reaction proceeds through a five-membered ring intermediate called a succinimide. Hydrolysis of this intermediate can lead to two products: the expected Aspartic acid, or an altered form called isoaspartic acid, which changes the protein’s main structural chain. The glutamine reaction follows a similar path but forms a less-favored, six-membered glutarimide ring intermediate, which accounts for its slower conversion rate.
How Deamidation Changes Protein Function
This chemical change significantly impacts the protein’s physical behavior. The conversion from the neutral amide group to the carboxylic acid group introduces a negative electrical charge at a specific site on the protein. This change in charge distribution disrupts the electrostatic forces and hydrogen bonds that maintain the protein’s precise three-dimensional shape.
Altering the charge destabilizes the protein’s native structure, potentially leading to unfolding. This loss of shape reduces the protein’s solubility, making it prone to aggregation. Furthermore, when isoaspartic acid is formed, it adds an extra atom into the protein’s backbone, causing a kink in the chain that further alters the overall structure and function.
Deamidation in Cellular Aging and Disease
The accumulation of deamidated proteins contributes to cellular aging. Because deamidation is non-enzymatic and irreversible, proteins with long lifespans are susceptible to a gradual build-up of these modifications over decades. This slow chemical deterioration can eventually overwhelm the cell’s quality control mechanisms.
A well-studied example is found in crystallins, the proteins of the human eye lens. These proteins are among the longest-lived in the body, remaining stable for a person’s entire life. Over time, crystallins accumulate deamidation, particularly in the gamma- and alpha-crystallin types.
This modification causes the proteins to lose stability and aggregate into large, insoluble masses that scatter light, which is the underlying mechanism of age-related cataract formation. The accumulation of these modified, misfolded proteins also contributes to cellular senescence, where cells lose their ability to divide and function correctly. Deamidation-induced aggregation and structural changes interfere with normal cellular activities, leading to a decline in tissue function as the body ages. The presence of deamidated proteins is a hallmark of many age-related conditions.
Controlling Deamidation in Applied Sciences
Controlling the rate of deamidation is a significant concern in both the food and pharmaceutical industries, where product stability is paramount. In food science, deamidation alters the functional properties of proteins, affecting characteristics like texture, solubility, and the ability to form gels or emulsions. This is relevant for the shelf life and quality of processed foods.
For therapeutic proteins, such as monoclonal antibodies used as biopharmaceuticals, deamidation can be highly detrimental. A change in the protein’s structure or charge can lead to a loss of drug efficacy, reduced stability during storage, or trigger an unwanted immune response in a patient.
To mitigate this, manufacturers carefully control factors like storage temperature and solution pH, often keeping them low. This helps slow the spontaneous reaction rate and maintain product integrity over its intended lifespan.