Proline
From Wikipedia, the free encyclopedia
Proline | |
---|---|
Systematic name | (S)-Pyrrolidine- 2-carboxylic acid |
Abbreviations | Pro P |
Chemical formula | C5H9NO2 |
Molecular mass | 115.13 g mol-1 |
Melting point | 221 °C |
Density | ? g cm-3 |
Isoelectric point | 6.30 |
pKa | 1.95 10.47 |
PubChem | 614 |
CAS number | [147-85-3] |
EINECS number | 205-702-2 |
SMILES | C1CCNC1C(=O)O |
Disclaimer and references |
L-Proline is one of the twenty proteinogenic units which are used in living organisms as the building blocks of proteins. The other nineteen units are all primary amino acids, but due to the cyclic binding of the three-carbon side chain to the nitrogen of the backbone, proline lacks a primary amine group (−NH2). The nitrogen in proline is properly referred to as a secondary amine. Proline is sometimes called an imino acid, although the IUPAC definition of an imine requires a carbon-nitrogen double bond. In biological terminology, however, the category "amino acids" is generally taken to include proline.
Contents |
[edit] Structural properties
The distinctive cyclic structure of proline's side chain locks its φ backbone dihedral angle at approximately -75°, giving proline an exceptional conformational rigidity compared to other amino acids. Hence, proline loses less conformational entropy upon folding, which may account for its higher prevalence in the proteins of thermophilic organisms. 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, which may account for the curious fact that proline is usually solvent-exposed, despite having a completely aliphatic side chain. Because proline lacks a hydrogen on the amide group, it cannot act as a hydrogen bond donor, only as a hydrogen bond acceptor.
Multiple prolines and/or hydroxyprolines in a row can create a polyproline helix, the predominant secondary structure in collagen. The hydroxylation of proline (or other additions of electron-withdrawing substituents such as fluorine) 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 (vitamin C) cofactor.
Sequences of proline and 2-aminoisobutyric acid (Aib) also form a helical turn structure[citation needed].
[edit] Cis-trans isomerization
Peptide bonds to proline and other N-substituted amino acids (such as sarcosine) are able to populate both the cis and trans isomers. Most peptide bonds prefer overwhelmingly to adopt the trans isomer (typically 99.9% under unstrained conditions), chiefly because the amide hydrogen (trans isomer) offers less steric repulsion to the preceding Cα atom than does the following Cα atom (cis isomer). By contrast, the cis and trans isomers of the X-Pro peptide bond are nearly isosteric (i.e., equally bad energetically); the Cα (cis isomer) and Cδ atoms (trans isomer) of proline are roughly equivalent sterically. 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 X, with aromatic residues favoring the cis isomer slightly.
Cis-trans proline isomerization is a very slow process that can impede the progress of protein folding by trapping one or more prolines crucial for folding in the nonnative isomer, especially when the native isomer is the rarer cis. 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 isomers of many X-Pro peptide bonds.
[edit] Synthesis and usage
Proline is biosynthetically derived from the amino acid L-glutamate and its direct precursor is the real imino acid (S)-Δ1-pyrroline-5-carboxylate (P5C).
Proline and its derivatives are often used as asymmetric catalysts in organic reactions. The CBS reduction or proline catalysed aldol condensation are prominent examples.
Proline has a sweet flavor with a distinct aftertaste. Proline also causes slight irritation to the tongue like Sichuan Pepper[citation needed].
For unknown reasons, L-proline is used as an ingredient in energy drinks such as "Sobe power fruit punch".
[edit] See also
- Collagen
- Polyproline helix
- Peptide bond (for more discussion of cis-trans isomerization)
- For a thorough scientific overview of disorders of proline and hydroxyproline metabolism, one can consult chapter 81 of OMMBID[1]. For more online resources and references, see inborn errors of metabolism.
justis
[edit] External links
- Proline biosynthesis
- Computational Chemistry Wiki
- Proline biosynthesis
- Links to external chemical sources
[edit] References
- Balbach J, Schmid FX. (2000). Proline isomerization and its catalysis in protein folding. In Mechanisms of Protein Folding 2nd ed. Editor RH Pain. Oxford University Press.
- ^ Charles Scriver, Beaudet, A.L., Valle, D., Sly, W.S., Vogelstein, B., Childs, B., Kinzler, K.W. (Accessed 2007). 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.
Biochemicals | Major Families of||
Peptides | Amino acids | Nucleic acids | Carbohydrates | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides | ||
Analogues of nucleic acids: | The 20 Common Amino Acids | Analogues of nucleic acids: |
Alanine (dp) | Arginine (dp) | Asparagine (dp) | Aspartic acid (dp) | Cysteine (dp) | Glutamic acid (dp) | Glutamine (dp) | Glycine (dp) | Histidine (dp) | Isoleucine (dp) | Leucine (dp) | Lysine (dp) | Methionine (dp) | Phenylalanine (dp) | Proline (dp) | Serine (dp) | Threonine (dp) | Tryptophan (dp) | Tyrosine (dp) | Valine (dp) |