Intensely sweet proteins as an alternative to sugar

Intensely sweet proteins as an alternative to sugar.

Photo: Pentadiplandra

There are a number of intensely sweet proteins which in the near future will come handy to those diabetics and dieters who crave for sweet dishes. The proteins are brazzein, pentadin, mabilinin, monellin, thaumatin, curculin and a sweet inducing miraculin. (Images given are of brazzein and its source plant Pentadiplandra)

Brazzein and Pentadin are proteins isolated from the fruits of Oubli (Pentadiplandra brazzeana Bail. –Capparaceae) a climbing shrub growing in some tropical countries of Africa. African locals call them "Oubli" (French for "forgot") in their vernacular language because their taste is said to encourage nursing infants to forget their mother's milk Both these are found in the extracellular region, in the pulp tissue surrounding the seeds. Pentadin was discovered earlier in 1989 and brazzein later in 1994

Brazzein, 500 to 2000 times sweeter than sugar, consists of 54 amino acid residues arranged in one alpha helix and three strands of anti-parallel beta sheets and a molecular weight of 6.5 kDa. It has four evenly spaced disulphide bonds. Residues 29–33 and 39–43, plus residue 36, as well as the C-terminus were found to be involved in the sweet taste of the protein. Brazzein is stable over a broad pH range from 2.5 to 8 and heat stable at 98°C for 2 hours. Brazzein is the smallest, most heat-stable and pH-stable member of the set of proteins known to have intrinsic sweetness.It is also very soluble in water (>50 mg/mL). It imparts a clean sweet taste and a lingering aftertaste.

It can be synthesised in laboratory and recombinant proteins were successfully produced via E. coli. It is also genetically engineered into maize and can be commercially extracted from the maize through ordinary milling. Approximately one ton of maize yields 1-2 kilograms of brazzein. It can also be engineered into plants like wheat to make pre-sweetened grains, e.g. for cereals. A synthetically improved brazzein, called pGlu-1-brazzein, was reported to be twice as sweet as the natural counterpart.

Pentadin is the second protein from fruits which was discovered first and isolated in 1989, from the same Oubli (Pentadiplandra brazzeana). It is a dimer with 33 aminoacids in alpha chain and 72 aminoacids in beta chain and molecular weight estimated to be 12kDa. It also consists of subunits coupled by disulfide bonds, but detailed structure is not available. It is reported to be 500 times sweeter than sucrose, with its sweetness having a slow onset and decline similar to monellin and thaumatin. However, pentadin's sweetness profile is closer to monellin than to thaumatin.

Mabilinin is a sweet protein obtained from the seeds of “Mabinlang” (Capparis masaikai Levi) a Chinese plant growing in Yunnan province and its sweetness is estimated to be around 400 times that of sucrose. It consists of an A chain with 33 amino acid residues and a B chain composed of 72 residues. The B chain contains two intramolecular disulfide bonds and is connected to the A chain through two intermolecular disulfide bridges. This is highly stable at higher temperature due to four disulfide bridges present.There are four homologues. Mabinlin-2 was first isolated in 1983 and characterised in 1993, and is the most extensively studied of the four. The other variants of mabinlin-1, -3 and -4 were discovered and characterised in 1994.The sweetness of Mabinlin-2 is unchanged after 48 hr incubation at boiling point]and of Mabinlin-3 and -4 are unchanged after 1 hr at 80°C.

Monellin (named after the Monell Chemical Senses Centre, Pannsylvania, U.S.A.) is obtained from the fruits of Discoreophyllum cumminsii Diels (Menispermaceae), a perennial dioecious twiner indigenous to tropical W. Africa, which produces, small red berries in clusters of hundreds. The mucilage within small red berries are intensely sweet, so much so that the fruit is named as serendipity berry (Refers to a Persian story, “The three Princes of serendip”, in which the princes make fortunate discoveries accidentally).

Monellin the sweetest natural product known—is about 3,000 times (weight) or 90,000 times (molar) sweeter than sugar. It has a mol. wt. of 10,700 and consists of a single polypeptide chain of about 91 amino acids. Monellin is strongly cationic, the isoelectric point being 9.3, is intensely sweet with a persistent sweet after-taste. The tertiary structure is important for the sweetness and unfolding of protein structure causes the loss of activity.

Thaumatin is obtained from the fruits of Thaumatococcus danielli Benth. (Marantaceae), a perennial herb of tropical Nigeria having a fleshy trigonal dark red fruit containing 3 black hard seeds with a pale yellow basal aril. The seeds are very sweet and for an hour or so afterwards cause sour materials eaten or drunk to taste very sweet. The fruit is named miraculous fruit.

Thaumatin, also a single polypeptide chain of M.W. 21,000; is very strongly cationic having an isoelectric point 11.7. It is 750-1600 times (wt) or 30,000-100,000 times (molar) sweeter than sugar. This protein leaves a licorice-like after taste in the mouth and consists of about 165 aminoacids of which 28 are sulphur containing. The basic aminoacid residues are 19 and acidic 20. The percentage of ammonia liberated indicates that most of the acidic residues are in the amide form (similar to monellin).

It is reported that thaumatin loses its sweet taste once the disulphide bridges are cleaved indicating that the tertiary structure is a determinant of sweetness.

Curculin or neoculin is a sweet glycoprotein isolated in 1990 from the fruit of Curculigo latifolia (Hypoxidaceae) a plant of from Malaysia. Apart from being sweet it a taste modifier too. After consumption of curculin, water and sour solutions taste sweet. The plant is referred to locally as 'Lumbah' or 'Lemba'. There is no other protein currently available with both sweet taste and taste modifying abilities curculin is a heterodimer consisting of two monomeric units connected through two disulfide bridges. The mature monomers each consist of a sequence of 114 amino acids, weighing 12.5 kDa (curculin 1) and 12.7 kDa (curculin 2), respectively. Curculin is 430-2070 times sweeter than sucrose on a weight basis

Taste modifiers

Miraculin is a taste modifying glycoprotein obtained from Synsepalum dulcifolium Daniel (Sapotaceae). This plant, indigenous to tropical Africa, is a shrub of 2-5 m height with ripe red fruits of long ellipsoidal shape and a thin layer of pulp surrounding a single large seed. The fruits are called miracle fruits.

Miraculin is a glycoprotein which changes the normal taste sensation of acids from sour to sweet after the tongue has been treated with the protein. This effect lasted for 3 hours after a 2.3 micromolar solution of miraculin was held in the mouth for 5 mm. A solution of this protein does not possess any detectable sweetness, but when activated by the acidic substances of mouth, a sweetness of excellent quality is produced. Miraculin on hydrolysis yields a polypeptide of about 100 amino acids and 6- 7% sugars like L-arabinose and D-xylose. The rnol. wt of the polypeptide is 44,000 and isoelectric point is 8.3-9. Only one residue of methionine is present and tryptophan is entirely lacking. This protein is believed to possess quaternary structure and form larger aggregates of 80,000 to l,00,000,

References

1. Wikipedia

2. Ravi Kant, (2005) Sweet proteins – Potential replacement for artificial low calorie sweeteners, Nutr J. 2005; 4: 5.

3. M.Daniel (2012) Useful Herbs of the Planet Earth, Scientific Publishers, Jodhpur,

Mammen Daniel