Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU,
CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body
can synthesize it. It is unique among the 20 protein-forming amino acids in that the
amine nitrogen is bound to not one but two alkyl groups, thus making it a secondary amine.
The more common L form has S stereochemistry.
Biosynthesis Proline is biosynthetically derived from the
amino acid L-glutamate. Glutamate-5-semialdehyde is first formed by glutamate 5-kinase and
glutamate-5-semialdehyde dehydrogenase. This can then either spontaneously cyclize to form
1-pyrroline-5-carboxylic acid, which is reduced to proline by pyrroline-5-carboxylate reductase,
or turned into ornithine by ornithine aminotransferase, followed by cyclisation by ornithine cyclodeaminase
to form proline.
Properties in protein structure The distinctive cyclic structure of proline's
side chain gives proline an exceptional conformational rigidity compared to other amino acids. It
also affects the rate of peptide bond formation between proline and other amino acids. When
proline is bound as an amide in a peptide bond, its nitrogen is not bound to any hydrogen,
meaning it cannot act as a hydrogen bond donor, but can be a hydrogen bond acceptor.
Peptide bond formation with incoming Pro-tRNAPro is considerably slower than with any other
tRNAs, which is a general feature of N-alkylamino acids. Peptide bond formation is also slow
between an incoming tRNA and a chain ending in proline; with the creation of proline-proline
bonds slowest of all. The exceptional conformational rigidity of
proline affects the secondary structure of proteins near a proline residue and may account
for proline's higher prevalence in the proteins of thermophilic organisms. Protein secondary
structure can be described in terms of the dihedral angles φ, ψ and ω of the protein
backbone. The cyclic structure of proline's side chain locks the angle φ at approximately
−60°. Proline acts as a structural disruptor in
the middle of regular secondary structure elements such as alpha helices and beta sheets;
however, proline is commonly found as the first residue of an alpha helix and also in
the edge strands of beta sheets. Proline is also commonly found in turns, and aids in
the formation of beta turns. This may account for the curious fact that proline is usually
solvent-exposed, despite having a completely aliphatic side chain.
Multiple prolines and/or hydroxyprolines in a row can create a polyproline helix, the
predominant secondary structure in collagen. The hydroxylation of proline by prolyl hydroxylase
increases the conformational stability of collagen significantly. Hence, the hydroxylation
of proline is a critical biochemical process for maintaining the connective tissue of higher
organisms. Severe diseases such as scurvy can result from defects in this hydroxylation,
e.g., mutations in the enzyme prolyl hydroxylase or lack of the necessary ascorbate cofactor.
Sequences of proline and 2-aminoisobutyric acid also form a helical turn structure.
Cis-trans isomerization Peptide bonds to proline, and to other N-substituted
amino acids, are able to populate both the cis and trans isomers. Most peptide bonds
overwhelmingly adopt the trans isomer, chiefly because the amide hydrogen offers less steric
repulsion to the preceding Cα atom than does the following Cα atom. By contrast, the cis
and trans isomers of the X-Pro peptide bond both experience steric clashes with the neighboring
substitution and are nearly equal energetically. Hence, the fraction of X-Pro peptide bonds
in the cis isomer under unstrained conditions ranges from 10-40%; the fraction depends slightly
on the preceding amino acid, with aromatic residues favoring the cis isomer slightly.
From a kinetic standpoint, cis-trans proline isomerization is a very slow process that
can impede the progress of protein folding by trapping one or more proline residues crucial
for folding in the non-native isomer, especially when the native protein requires the cis isomer.
This is because proline residues are exclusively synthesized in the ribosome as the trans isomer
form. All organisms possess prolyl isomerase enzymes to catalyze this isomerization, and
some bacteria have specialized prolyl isomerases associated with the ribosome. However, not
all prolines are essential for folding, and protein folding may proceed at a normal rate
despite having non-native conformers of many X-Pro peptide bonds.
Uses Proline and its derivatives are often used
as asymmetric catalysts in organic reactions. The CBS reduction and proline catalysed aldol
condensation are prominent examples. L-Proline is an osmoprotectant and therefore
is used in many pharmaceutical, biotechnological applications.
In brewing, proteins rich in proline combine with polyphenols to produce haze.
Specialties Proline is one of the two amino acids that
do not follow along with the typical Ramachandran plot, along with glycine. Due to the ring
formation connected to the beta carbon, the ψ and φ angles about the peptide bond have
fewer allowable degrees of rotation. As a result it is often found in "turns" of proteins
as its free entropy is not as comparatively large to other amino acids and thus in a folded
form vs. unfolded form, the change in entropy is less. Furthermore, proline is rarely found
in α and β structures as it would reduce the stability of such structures, because
its side chain α-N can only form one hydrogen bond.
Additionally, proline is the only amino acid that does not form a blue/purple colour when
developed by spraying with ninhydrin for uses in chromatography. Proline, instead, produces
an orange/yellow colour. History
Richard Willstätter synthesized proline by the reaction of sodium salt of diethyl malonate
with 1,3-dibromopropane in 1900. In 1901, Hermann Emil Fischer isolated proline from
casein and the decomposition products of γ-phthalimido-propylmalonic ester.
See also Hyperprolinemia
Inborn error of metabolism Prolidase deficiency
Prolinol Synthesis
Racemic proline can be synthesized from diethyl malonate and acrylonitrile:
References
Further reading Balbach, J.; Schmid, F. X., "Proline isomerization
and its catalysis in protein folding", in Pain, R. H., Mechanisms of Protein Folding,
Oxford University Press, pp. 212–49, ISBN 0-19-963788-1 . For a thorough scientific overview of disorders
of proline and hydroxyproline metabolism, one can consult chapter 81 of OMMBID Charles
Scriver, Beaudet, A.L., Valle, D., Sly, W.S., Vogelstein, B., Childs, B., Kinzler, K.W..
The Online Metabolic and Molecular Bases of Inherited Disease. New York: McGraw-Hill.
- Summaries of 255 chapters, full text through many universities. There is also the OMMBID
blog. External links
Proline MS Spectrum Proline biosynthesis
Proline biosynthesis