Proline is a cyclic imino acid. α-imino acid is neutral, with an isoelectric point of 6.30, and its solubility in water is greater than that of any amino acid. At 25°C, it can dissolve about 162 g in 100 g of water. It is easy to deliquescence and difficult to crystallize, and has a sweet taste. Proline is co-heated with ninhydrin solution to generate a yellow compound. Once in the peptide chain, hydroxylation can occur to form 4-hydroxyproline, which is an important component of animal collagen. Hydroxyproline is also present in a variety of plant proteins and is especially involved in the formation of cell walls. Plants often have significant accumulation of proline under various adversities such as drought, high temperature, low temperature, and salinity. It is widely used in clinic, biomaterials, industry and so on.
Proline is an organic acid that is classified as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain an amino-NH2 but is a secondary amine. The secondary amine nitrogen is in the form of protonated NH2+ under biological conditions, while the carboxyl group is in the form of deprotonated -COO-. The "side chain" of the alpha carbon is attached to the nitrogen to form a pyrrolidine ring, classifying it as an aliphatic amino acid. It is non-essential in humans, which means the body can synthesize it from the non-essential amino acid L-glutamate. It is encoded by all codons beginning with CC (CCU, CCC, CCA, and CCG). Proline is a protein secondary amino acid that acts as a secondary amine because the nitrogen atom is attached both to the α-carbon and to a chain of three carbons that together form a five-membered ring.
Proline is biosynthesized from the amino acid L-glutamate. Glutamate-5-semialdehyde is first formed by glutamate 5-kinase (ATP-dependent) and glutamate-5-semialdehyde dehydrogenase (NADH or NADPH-requiring). It can then cyclize spontaneously to form 1-pyrroline-5-carboxylic acid, which is reduced to proline by pyrroline-5-carboxylic acid reductase (using NADH or NADPH), or converted to ornithine by ornithine aminotransferase, followed by cyclization to proline by ornithine ring deaminase.
L-Proline has been found to act as a weak agonist at glycine receptors as well as NMDA and non-NMDA (AMPA/kainic acid) ionotropic glutamate receptors. It is considered a potential endogenous excitotoxin. In plants, proline accumulation is a common physiological response to various stresses, but is also part of the developmental program of reproductive tissues such as pollen.