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Protein is one of bio molecules present in living systems and formed by condensation polymerization of certain monomer units known as amino acids. In proteins, amino acids are bonded together by peptide bonds to form the fundamental structure of proteins. Apart from these peptide bonds, there are many other types of bonds formed by the side groups of amino acids between the amino acid units which twist and turn the protein into convoluted shapes. These orientations of proteins are unique and essential to perform certain functions within the human body.

Amino acids are organic compounds which composed of two functional groups: an amino group (NH2) and a carboxylic acid group (COOH) with a side chain and hydrogen atom. The presence various functional groups in side chain give each amino acid a unique set of characteristics.

Excluding "glycine" all other amino acids re optically active and form left and right handed isomers which are known as dextrorotatory and levorotatory forms respectively. The orientation of amino and carboxyl group in molecule make two type of configurations in molecule, that is D and L. Generally L-form of amino acids are found in proteins present in living systems.

There are twenty amino acids also called as proteinogenic amino acids which are used to form proteins in the human body. Amino acids can be classified on different basis like their polarity polar and non-polar amino acids, their acidic and basic nature; acidic, basic and neutral amino acids, their needs; essential and non-essential amino acids etc.

Structure of Amino Acid


Proline Structure

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  1. Proline, also known as L-proline, is a non-essential amino acid exists in the form of transparent crystals. The molecular formula of proline is C5H9NO2 with the molecular mass 115.13 g mol-1.
  2. The melting point of proline is in the range of 478-501 K but it usually decomposed during its melting process.
  3. The codons for proline are CCU, CCG, CCC and CCA. Since it's a non-essential amino acid, therefore it can be synthesized by the body by the breakdown of L-glutamate and other biosynthesis processes.
  4. Proline is mainly involved in blood pressure maintenance, tissue repair, arteriosclerosis prevention and collagen formation.
  5. The IUPAC name of proline is Pyrrolidine-2-carboxylic acid hence it is a secondary amino group called as imino group which is a part of a five member ring in molecule.

Pyrrolidine-2-Carboxylic Acid

Since there is no alpha hydrogen atom on proline molecule, it cannot form hydrogen bonds to stabilize the secondary structures of proteins and it cannot present in the secondary structure like alpha helix and beta sheet form of proteins, as it acts as hydrogen acceptors only. If this amino acid present in helix form, it shows some bend on molecule during folding.

It is generally found as the first residue of an alpha helix, at the edge strands of beta sheets and in turns, hence amino acid molecules are usually exposed by solvent molecules. The secondary structure of collagen protein consists of a polyproline helix which further stabilized by hydroxylation in the presence of prolyl hydroxylase and other additions of electron-withdrawing group likes fluorine.

The other structural feature of proline is their cis-trans configurations which exclusively shown by proline and play a vita role in the folding of proteins. Most peptide bonds adopt the trans-isomer due to less steric hindrance between amide hydrogen compare to cis-isomer. The cis-trans proline isomerization is a very slow process which can impede the progress of protein folding by trapping one or more proline residues which are necessary for folding in the non-native isomer.

Cis Trans Proline Configurations

All peptide bonds in protein structures are approximately co-planar and this rigidity of the peptide bond reduces the degrees of freedom of the polypeptide during folding. The trans-form of peptide bond is more favorable compare to cis-form, but sometimes cis-form is predominantly found with proline residues due to cyclic nature of the side chain both cis and trans configurations have more equivalent energies.

Hence proline is predominantly found in the cis configuration compare to other amino acids as the omega torsion angle of proline will be close to zero degrees for the cis configuration and 180 degrees for the trans configuration.
Trans Form of Peptide Bond

Just like other amino acids, proline can also exhibit D and L configuration, out of which L-form is S-stereoisomer is more common among different proteins.
D and L Configuration of Proline

Proline is a non-polar, aliphatic and hydrophobic cyclic amino acid in which the side chain composed of cyclic propyl group bonded with alpha carbon and a secondary amino group. The isoelectric point of proline is 6.30 as the pKa1 ($\alpha$-carboxyl group) for proline is 1.99 and pKa2 ($\alpha$-ammonium ion) is 10.60.
Isoelectric Point of Proline

Proline amino acid acts as a precursor of glutamic acid which involve on the formation of some important compounds like glutathione, glutamine, gamma aminobutyric acid, alpha ketoglutarate etc. Proline can also converted in hydroxyproline which has been incorporated into the protein molecule and act as an important component of the collagen which further involve in treatment of the damage cartilage and important for the vertebra and joints.

Proline also involves in the synthesis of Ornithine which leads to the formation of urea. Proline is non-essential amino acid which is very much essential for the normal health of the tendons joints and the muscles and also utilized medicine industries for the manufacturing of beneficial drugs.
Different uses of proline amino acid are as follow.

Uses of Proline Amino Acid

Proline Synthesis

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Since proline is a non-essential amino acid, hence it can be synthesized in living system by several biochemical processes which are catalyzed by various enzymes. Glutamate amino acid is the main precursor for biosynthesis of proline amino acid. The process involves the cyclization and reduction of glutamate to form proline amino acid.

All these biochemical reactions are catalyzed by enzymes. Main enzymes involve in this biosynthesis are
  • Glutamate dehydrogenase enzyme
  • Glutamate 5 kinase
  • Pyrrolin 5 carboxylate reductase
  • Glutamat 1 kinase
The $\gamma$  carboxyl group of glutamate gets reduced to form an aldehyde called as glutamate semi aldehyde which also acts as a precursor of Ornithine amino acid. This aldehyde further shows elimination reaction to form the Schiff base by elimination of water molecule. The reduction of Schiff base yields proline amino acid.

Overall reaction is a four step process involves one ATP and two NADPH+ H+ for per proline molecule.

Proline Synthesis

Another method for the synthesis of proline is by using diethyl malonate and acrylonitrile, which mainly form a racemic mixture of proline.
Synthesis of Proline
Proline can be synthesised by other amino acids like arginine. Arginine converts in Ornithine in the presence of arginase enzyme which further forms $\alpha $-keto-$\gamma $-aminovalerate. $\alpha $-keto-$\gamma $-aminovalerate is spontaneously converts in L-proline.
Amino Acid Proline
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