Monellin, a sweet

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

, 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 A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

.GE Inglett, JF May. Serendipity berries - Source of a new intense sweetener. J Food Sci 1969, 34:408-411. The protein was named in 1972 after the

Monell Chemical Senses Center The Monell Chemical Senses Center is an independent, non-profit scientific research institute located at the University City Science Center campus in Philadelphia. Founded in 1968, it is dedicated to interdisciplinary basic research on the sen ...

Philadelphia Philadelphia ( ), colloquially referred to as Philly, is the List of municipalities in Pennsylvania, most populous city in the U.S. state of Pennsylvania and the List of United States cities by population, sixth-most populous city in the Unit ...

, U.S.A., where it was isolated and characterized.

Protein composition

Monellin's molecular weight is 10.7 kDa. It has two noncovalently associated polypeptide chains: an A chain sequence with 44 amino acid residues, and a B chain with 50 residues.

Monellin chain A (44 AA):

Monellin chain B (50 AA):

Amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
sequence of the sweet protein monellin adapted from Swiss-Prot biological database of protein. UniProtKB/Swiss-Prot database entry #P02882
/ref>

Monellin has a secondary structure consisting of five beta-strands that form an antiparallel beta-sheet and a 17-residue alpha-helix. In its natural form, monellin is composed of the two chains shown above (), but the protein is unstable at high temperatures or at extremes of pH. To enhance its stability, single-chain monellin proteins were created in which the two natural chains are joined via a Gly-Phe dipeptide linker. 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. 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.

Sweetness properties

Monellin is perceived as sweet by humans and some Old World primates, but is not preferred by other mammals. 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 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. . and 800 times sweeter than sucrose when compared with a 5% sucrose solution on a weight basis. Monellin has a slow onset of sweetness and lingering aftertaste. Like

miraculin Miraculin is a taste modifier, a glycoprotein extracted from the fruit of '' Synsepalum dulcificum''. The berry, also known as the miracle fruit, was documented by explorer Chevalier des Marchais, who searched for many different fruits during ...

, 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.
Heat over 50 °C at low pH denatures monellin proteins, causing a loss of the sweetness. So far, five high-intensity sweet proteins have been reported: monellin (1969),

thaumatin Thaumatin (also known as talin) is a low-calorie sweetener and taste modifier. The protein is often used primarily for its flavor-modifying properties and not exclusively as a sweetener. The thaumatins were first found as a mixture of protein ...

(1972), pentadin (1989), mabinlin (1983) and brazzein (1994). Biopolymers. Volume 8. Polyamides and Complex Proteinaceous Materials II. Sweet-tasting Proteins. I Faus and H Sisniega. p203-209. 2004. Eds. Wiley-VCH.
.

As a sweetener

Monellin can be useful for 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.
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 Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...

('' Candida utilis'')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. and synthesised by solid-phase method.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.
Legal issues are the main barrier in its widespread use as a sweetener, as monellin has no legal status in the

European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...

or the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. However, it is approved in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

as a harmless

additive Additive may refer to: Mathematics * Additive function, a function in number theory * Additive map, a function that preserves the addition operation * Additive set-function see Sigma additivity * Additive category, a preadditive category with fin ...

, according to the List of Existing Food Additives issued by the Ministry of Health and Welfare (published in English by

JETRO is an Independent Administrative Institution established by Japan Export Trade Research Organization as a nonprofit corporation in Osaka in February 1952, reorganized under the Ministry of International Trade and Industry (MITI) in 1958 (later th ...

References

External links

*
Monellin
PDB Molecule of the Month * * {{PDBe-KB2, P02882, Monellin chain B Sugar substitutes Plant proteins

Protein composition

Sweetness properties

As a sweetener

See also

References

External links