Editing Monellin

{{short description|Protein}}
{{infobox nonhuman protein
| Name = '''Monellin Chain B'''
| caption = X-ray crystal structure of an engineered single-chain monellin protein.<ref name="pmid17329805">{{PDB|2O9U}}; {{cite journal |vauthors=Hobbs JR, Munger SD, Conn GL | title = Monellin (MNEI) at 1.15 A resolution | journal = Acta Crystallographica Section F | volume = 63 | issue = Pt 3 | pages = 162–7 |date=March 2007 | pmid = 17329805 | pmc = 2330190 | doi = 10.1107/S1744309107005271 }}</ref>
| image = hmonellin.png
| Symbol = MonB
| AltSymbols = 
| RefSeq = 
| UniProt = P02882
| PDB = 2O9U
| PDB_supplemental = 
| Organism=Dioscoreophyllum cumminsii
}}
{{infobox nonhuman protein
| Name = '''Monellin Chain A'''
| Symbol = MonA
| AltSymbols = 
| RefSeq = 
| UniProt = P02881
| PDB = 1IV7
| PDB_supplemental = 
| Organism=Dioscoreophyllum cumminsii
}}
{{Infobox protein family
|Symbol=Monellin
|InterPro=IPR015283
|Pfam=PF09200
}}
'''Monellin''', a sweet [[protein]], was discovered in 1969 in the fruit of the West African shrub known as [[serendipity berry]] (''[[Dioscoreophyllum cumminsii]]''); it was first reported as a [[carbohydrate]].<ref name="jfs-34-408">GE Inglett, JF May. Serendipity berries - Source of a new intense sweetener. J Food Sci 1969, 34:408-411.</ref> The protein was named in 1972 after the [[Monell Chemical Senses Center]] in [[Philadelphia]], U.S.A., where it was isolated and characterized.<ref name="jbc-248-534">{{cite journal |vauthors=Morris JA, Martenson R, Deibler G, Cagan RH | title = Characterization of monellin, a protein that tastes sweet | journal = J. Biol. Chem. | volume = 248 | issue = 2 | pages = 534–9 |date=January 1973 | doi = 10.1016/S0021-9258(19)44407-4 | pmid = 4684691 | doi-access = free }}</ref>

== Protein composition ==
Monellin's molecular weight is 10.7 [[Atomic mass unit|kDa]]. It has two noncovalently associated polypeptide chains: an A chain sequence with 44 amino acid residues, and a B chain with 50 residues.<ref name="jbc-248-534"/><ref name="pmid3614382">{{cite journal |vauthors=Ogata C, Hatada M, Tomlinson G, Shin WC, Kim SH | title = Crystal structure of the intensely sweet protein monellin | journal = Nature | volume = 328 | issue = 6132 | pages = 739–42 | year = 1987 | pmid = 3614382 | doi = 10.1038/328739a0 | bibcode = 1987Natur.328..739O | s2cid = 4323370 }}</ref>
<blockquote>
Monellin chain A (44 AA): <br />
{{mono|REIKGYEYQL YVYASDKLFR ADISEDYKTR GRKLLRFNGP VPPP}}<br />
Monellin chain B (50 AA): <br />
{{mono|GEWEIIDIGP FTQNLGKFAV DEENKIGQYG RLTFNKVIRP CMKKTIYEEN}}<br />
[[Amino acid#Table of standard amino acid abbreviations and properties|Amino acid]] sequence of the sweet protein monellin adapted from Swiss-Prot biological database of protein.<ref>[http://www.expasy.org/uniprot/P02881  UniProtKB/Swiss-Prot database entry #P02881]</ref><ref>[http://www.expasy.org/uniprot/P02882  UniProtKB/Swiss-Prot database entry #P02882]</ref></blockquote>

Monellin has a secondary structure consisting of five beta-strands that form an antiparallel beta-sheet and a 17-residue alpha-helix.<ref name="pmid17329805"/>

In its natural form, monellin is composed of the two chains shown above ({{PDB|3MON}}), but the protein is unstable at high temperatures or at extremes of pH.<ref name="pmid17329805"/> To enhance its stability, single-chain monellin proteins were created in which the two natural chains are joined via a Gly-Phe dipeptide linker.<ref name="pmid17329805"/> This modified version of the protein (MNEI) has been studied using NMR and X-ray diffraction.

In addition to its secondary structure, four stably bound sulfate ions were located on the monellin protein, three on the concave face of the protein and one on the convex face of the protein.<ref name="pmid17329805"/> The sulfate ion on the convex face of the protein is of particular interest because it lies adjacent to a patch of positive surface potential, which may be important in electrostatic interactions with the negative T1R2-T1R3 sweet taste protein receptor.<ref name="pmid17329805"/>

== Sweetness properties ==
Monellin is perceived as sweet by humans and some Old World primates, but is not preferred by other mammals.<ref name="pmid17329805"/> The relative sweetness of monellin varies from 800 to 2000 times sweeter than sucrose, depending on the sweet reference against which it is assessed. It is reported to be 1500-2000 times sweeter than a 7% sucrose solution on a weight basis<ref name="pmid12510821">{{cite journal |vauthors=Kim NC, Kinghorn AD | title = Highly sweet compounds of plant origin | journal = Arch. Pharm. Res. | volume = 25 | issue = 6 | pages = 725–46 |date=December 2002 | pmid = 12510821 | doi = 10.1007/BF02976987| s2cid = 2265958 }}</ref><ref name="isbn0-8247-8475-8">{{cite book |author1=Gelardi, Robert C. |author2=Nabors, Lyn O'Brien | title = Alternative sweeteners | publisher = M. Dekker | location = New York | year = 1991 | isbn = 0-8247-8475-8 }}</ref>
<ref name="Kinghorn-book2">AD kinghorn and CM Compadre. Less common high-potency sweeteners. In Alternative Sweeteners: Second Edition, Revised and Expanded, L O'Brien Nabors, Ed., New York, 1991. {{ISBN|0-8247-8475-8}}.</ref> and 800 times sweeter than sucrose when compared with a 5% sucrose solution on a weight basis.<ref name="patent-US4122205">{{US patent reference  | number = 4122205  | y = 1978  | m = 10  | d =  24 | inventor =  Burge MLE, Nechutny Z | title = Sweetening compositions containing protein sweeteners }}</ref>

Monellin has a slow onset of sweetness and lingering aftertaste. Like [[miraculin]], monellin's sweetness is pH-dependent; the protein is tasteless below pH 2 and above pH 9. Blending the sweet protein with bulk and/or intense sweeteners reduces the persistent sweetness and shows a synergistic sweet effect.<ref name="isbn0-8247-0437-1">{{cite book |author1=Nabors, Lyn O'Brien |author2=Lyn O'Brien-Nabors | title = Alternative sweeteners / edited by Lyn O'Brien Nabors | publisher = Marcel Dekker | location = New York, N.Y | year = 2001 | isbn = 0-8247-0437-1 }}</ref><br /> Heat over 50&nbsp;°C at low pH denatures monellin proteins, causing a loss of the sweetness.<ref name="isbn0-8247-0437-1"/>

So far, five high-intensity sweet proteins have been reported: monellin (1969), [[thaumatin]] (1972), [[pentadin]] (1989), [[mabinlin]] (1983) and [[brazzein]] (1994).<ref name="2004-biopolymers">[http://www.wiley-vch.de/books/biopoly/pdf_v08/vol08_08.pdf  Biopolymers. Volume 8. Polyamides and Complex Proteinaceous Materials II. Sweet-tasting Proteins. I Faus and H Sisniega. p203-209. 2004. Eds. Wiley-VCH.] {{ISBN|3-527-30223-9}}.</ref>

===As a sweetener ===
Monellin can be useful for [[Sugar substitute|sweetening]] some foods and drinks, as it is a protein readily soluble in water due to its hydrophilic properties. However, it may have limited application because it denatures under high temperature conditions, which makes it unsuitable for processed food. It may be relevant as noncarbohydrate tabletop sweetener, especially for individuals such as diabetics who must control their sugar intake.<ref name="pmid17329805"/><br />
In addition, monellin is costly to extract from the fruit and the plant is difficult to grow. Alternative production such as chemical synthesis and expression in [[micro-organisms]] are being investigated. For instance, monellin has been expressed successfully in [[yeast]] (''[[Candida utilis]]'')<ref name="joceoj-35-654">XL Zhang, T Ito, K Kondo, T Kobayashi and H Honda. Production of single chain recombinant monellin by high cell density culture of genetically engineered Candida utilis using limited feeding of sodium ions. Journal of Chemical Engineering of Japan 2002. 35: 654-659.</ref> and synthesised by solid-phase method.<ref name="paac-74-1235">[http://www.iupac.org/publications/pac/2002/pdf/7407x1235.pdf M Kohmura, T Mizukoshi, N Nio, EI Suzuki and Y Ariyoshi. Structure–taste relationships of the sweet protein monellin. Pure Appl. Chem., Vol. 74, 1235-1242, 2002.]</ref> The synthetic monellin produce by yeast was found to be 4000 times sweeter than sucrose when compared to 0.6% sugar solution.<br />
Legal issues are the main barrier in its widespread use as a sweetener, as monellin has no legal status in the [[European Union]] or the [[United States]]. However, it is approved in [[Japan]] as a harmless [[food additive|additive]], according to the List of Existing Food Additives issued by the Ministry of Health and Welfare (published in English by [[Japan External Trade Organization|JETRO]]).

== See also ==
*[[Curculin]]
*[[Thaumatin]]
*[[Miraculin]]
*[[Stevia]]

==References==
{{Reflist}}

==External links==
*{{Commons category-inline}}
*[https://pdb101.rcsb.org/motm/199 Monellin], PDB Molecule of the Month
* {{PDBe-KB2|P02881|Monellin chain A}}
* {{PDBe-KB2|P02882|Monellin chain B}}

[[Category:Sugar substitutes]]
[[Category:Plant proteins]]