Monellin, a sweet

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

, 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 In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...

.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 a non-profit independent scientific institute located at the University City Science Center campus in Philadelphia, in Pennsylvania. Monell conducts and publishes interdisciplinary basic research on taste, Olfa ...

Philadelphia Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Sinc ...

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

Protein composition

Monellin's molecular weight is 10.7

kDa The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...

. 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 hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
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 modifiers, 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 ...

, 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 flavor 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 Pentadin, a sweet-tasting protein, was discovered and isolated in 1989, in the fruit of Oubli (''Pentadiplandra brazzeana'' ), a climbing shrub growing in some tropical countries of Africa. The fruit has been consumed by the apes and the natives ...

(1989),

mabinlin Mabinlins are sweet, sweet-tasting proteins extracted from the seed of mabinlang (''Capparis masaikai Levl.''), a plant growing in Yunnan province of China. There are four homologues. Mabinlin-2 was first isolated in 1983 and characterised in ...

(1983) and

brazzein Brazzein is a sweet-tasting protein extracted from the West African fruit of the climbing plant Oubli (''Pentadiplandra brazzeana'' Henri Ernest Baillon, Baillon). It was first isolated by the University of Wisconsin–Madison in 1994. 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 Sweetness is a basic taste most commonly perceived when eating foods rich in sugars. Sweet tastes are generally regarded as pleasurable. In addition to sugars like sucrose, many other chemical compounds are sweet, including aldehydes, keton ...

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 A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

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. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

(''

Candida utilis Torula (''Cyberlindnera jadinii'') is a species of yeast. Use Torula, in its inactive form (usually labeled as torula yeast), is widely used as a flavoring in processed foods and pet foods. It is often grown on wood liquor, a byproduct of paper ...

'')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

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or the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. However, it is approved in

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

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-functionn see Sigma additivity * Additive category, a preadditive category with f ...

, 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 the ...

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