What Is Proline?

Also known as L-proline, it is an imino acid or a molecule that comprises both the carboxyl and imine functional groups. It is also a part of the twenty most crucial amino acids as humans and other animals biosynthesize it. The primary amine present on the carbon of the glutamate semialdehyde generally forms a Schiff base from which the aldehyde reduces, thus generating proline. The proline contains a secondary amine group (the only natural amino acid having a secondary amine), giving its unique helix rings in the structure. The proteins synthesized from proline also have discrete secondary structures and, therefore, appear different from that of the proteins synthesized from open-chain proteins.  

It was in 1900 that Nobel-laureate Richard M. Willstätter came up with the D, L-racemate synthesized from N-methylproline. Generally considered as an asymmetrical catalyst in numerous protein synthesis, Harvard University researchers often referred to proline as the 'simplest enzyme,' which was then elaborated as proline being one the few catalysts enabling prebiotic evolution. 

Proline Structure

Considered highly unusual for an amino acid to be cyclic in its structure (because of the secondary amine), proline forms a peptide bond that does not contain hydrogen on the α amino group. This is why prolines cannot give away its Hydrogen bond to balance the α helix or the β sheet. For prolines found at the end of the α helix, the absence of hydrogen atom creates a bend in the helix structure and can exist in isoenergetic cis and trans variations. 

For the biological systems in animals and humans, the amino acids are crucial components, because of its life-giving features and attributes. Found diversely across cells, muscles, eggs, and other animal processes, these can help in regulating the insulin generation, heal muscle tissues as well as contribute to our body metabolism. Often represented as one-lettered format P, these amino acids are made from an amino group and a carboxyl group, as well as an R-group, bonded with the central Carbon atom to form a helix structure. 

The L-proline belongs from the pyrrolidine (a cyclic amine) where the pro-S Hydrogen atom gets replaced by a carboxyl group. It has a structure that makes it an enantiomer to D-proline and has the conjugate acid of that of the L-prolinate. 

As the R-group in a proline gets bonded with the Nitrogen atom of the amino group, this is what gives proline its unique structure, distinct from other amino acids. Here’s how proline chemical structure appears in shape:

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Proline Synthesis 

Proline requires a carbon-nitrogen double bond (as it is an imino acid) and can be easily synthesized from Glutamic acid. The γ-carboxyl group gets reduced into an aldehyde, forming glutamate semialdehyde, which then further reacts to the α-amino group, producing water and Schiff base (a sub-class of imines). It is the Schiff base that gets also reduced to generate proline. 

Proline Mass (molar)


Chemical Formula


Melting Point

205–228 ºC 

Appearance in forms

White Crystals

The proline formula for the same is:

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Properties of Proline

The nitrogen atom in a peptide proline doesn't remain attached to the hydrogen and therefore makes a considerable influence in the susceptibility of peptide bonds and stimulates activity. The proline can form aggregation due to hydrophobic interaction of the pyrrolidine ring. Here are some properties of proline:

  • Prolines are aliphatic and hydrophobic, contributing to the intracellular signalling in biosystems.

  • The peptidases that get formed in the process of proline-involved biological processes, help in regulating the generation of proline throughout its lifetime, in its aminopeptidase P, prolidase forms. 

  • The HIV-I protease enzyme can lead to the generation of a Xaa-Pro peptide bond formation in the process that can be critical components in several immunological processes. 

  • The proline can display a variety of properties that may not be similar to other lighter molecular weight compounds in its aqueous form in terms of its solubility, density, and viscosity. There is strong hydrogen bonding with water that is observed in proline solutions. 

  • The proline solutions can enhance the solubility of other proteins by enabling a hydrophobic interaction with the protein surface, leading to the rise in the hydrophilic area.

Uses of Proline

There are several uses of L-Proline. Some of them are:

  • L-Proline is popularly used as a precursor to the L-glutamate and therefore known for its energy fuel pertaining capabilities. 

  • It is one of the primary amino acids present in human cartilage that helps in achieving youthful skin, and facilitate faster muscle repairs, crucial to the all-round working of the human joints and tendons, including strengthening of heart muscles and connective tissues. 

  • Sometimes L-Proline can also get oxidized in our kidney from Glycogenic, which on further oxidation yields L-Glutamic acid that plays an essential role in fighting the conditions of arthrosis and chordae.

  • L-Proline can also be found in several dietary supplements in the form of crystalline Proline, which can contribute to body metabolism.

FAQ (Frequently Asked Questions)

1. Why is Proline an Amino Acid?

The regular amino acids contain only one central carbon atom, which is also bonded to the amine nitrogen. In Proline, the secondary amine has a sidechain bonding with that of the central Carbon atom (the C-N bond). It gives Proline a certain rigidity that prevents it from forming any secondary alpha-helix structures but can build stable polyproline helix structures. 

2. Are Proline Neutral in its Charge?

With an aliphatic and hydrophobic nature, proline exists in neutral pH type in the environment, like in the case of amino acids. The side-chain reactions can govern an amino acid's overall functionality with water and classify it as hydrophobic, or polarity. As amino acids are chiral, they can exist in isomers and, therefore, commonly exist in neutral dipolar ions known as zwitterions. In such forms, the amino group present gets added with protons. In contrast, its subsequent carboxyl group loses protons and therefore yields polypeptide proline, which is unable to undergo a replacement of the Hydrogen bond in its α helix structure.