Proline, also known as pro, is a unique amino acid that plays a critical role in various biological processes. It is classified as a non-essential amino acid due to its endogenous synthesis within the human body. Proline is characterized by its cyclic structure, with the amino group attached to the side chain, creating a five-membered ring.
One key characteristic of proline is its rigid and inflexible structure, which sets it apart from other amino acids. This unique structure enables it to confer specific properties and functions in biological systems. Proline serves as a fundamental building block for peptides and proteins, contributing significantly to their stability and structure.
Application of Proline in Vaccines
Proline as a Stabilizer
One crucial aspect of vaccine development is maintaining vaccine stability, especially during storage and transportation. Proline has been utilized as a stabilizer in vaccines due to its unique structure, which helps prevent protein denaturation and aggregation. By acting as a "molecular chaperone," proline aids in maintaining the proper folding of proteins, ensuring vaccine integrity throughout its shelf life.
For example, for immunoglobulin Fab fragments used as vaccines, L-proline can preserve the monomeric form of the Fab by inhibiting conformational formation leading to bivalent complexation. Furthermore, L-proline inhibits Fab aggregation by restoring bivalent Fab to monovalent Fab, which means that L-proline can be used for long-term storage of Fab.
Additionally, proline penetrants have a higher ability to flocculate viral particles than salt. Viral particles flocculate efficiently at lower concentrations of osmotic agent (0.3 M proline), but 3.0 M is required in the presence of salt, Mg2SO4. 
Proline in Immunogenicity Enhancement
Proline's contribution to immunogenicity enhancement is another crucial aspect of its application in vaccines. Immunogenicity refers to the ability of a vaccine to induce a robust immune response. Proline possesses a significant influence on antigen presentation, resulting in enhanced immune recognition and subsequent activation of immune cells, such as T cells and B cells. In addition, studies have confirmed the immunostimulatory effect of dietary proline supplementation on mice immunized with inactivated vaccines. 
Proline Stabilization Method for Viral Vaccine Development
A method to stabilize the spike protein through appropriately positioned proline amino acid substitutions has been extensively studied and applied during the development of vaccines for HIV-1 Env and other viruses, including SARS-CoV-2. For optimal vaccine performance, proline residues were introduced at critical positions in the viral proteins to enhance the stability and presentation of these NAb epitopes to maintain the trimer in the prefusion configuration.
Position of proline substitutions in class I fusion proteins. 
- Tauheed Hasan, et al. Human Vaccines & Immunotherapeutics, 2019, 15(2): 514-525.
- Wenkai Ren, et al. Amino Acids. 2013, 45(3): 555-61.
- Rogier W. Sanders, et al. Cell Host Microbe. 2021, 10, 29(3): 327-333.
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