Curculin

Curculin
A curculin homodimer. From PDB: 2DPF​.
Identifiers
Symbol	CURC_CURLA
PDB	2PDF More Structures
UniProt	P19667

Curculin is a sweet protein that was discovered and isolated in 1990 from the fruit of Curculigo latifolia (Hypoxidaceae), a plant from Malaysia. Like miraculin, curculin exhibits taste-modifying activity; however, unlike miraculin, it also exhibits a sweet taste by itself. After consumption of curculin, water and sour solutions taste sweet. The plant is referred to locally as 'Lumbah' or 'Lemba'.

The active form of 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. While each of the two isoforms is capable of forming a homodimer, these do not possess the sweet taste nor the taste-modifying activity of the heterodimeric form. To avoid confusion, the heterodimeric form is sometimes referred to as "neoculin".

Amino acid sequence of sweet protein curculin adapted from Swiss-Prot biological database of protein sequences.

Curculin is considered to be a high-intensity sweetener, with a reported relative sweetness of 430-2070 times sweeter than sucrose on a weight basis.

A sweet taste, equivalent to a 6.8% or 12% sucrose solution, was observed after holding curculin in the mouth in combination with clear water or acidified water (citric acid), respectively. The sweet taste lasts for 5 minutes with water and 10 minutes with an acidic solution.
Sweetness was also observed with other acids such as ascorbic acid (vitamin C) and acetic acid.

The taste-modifying activity of curculin is reduced in the presence of ions with two positive charges (such as Ca²⁺ and Mg²⁺) in neutral pH solutions, although these ions have no effect in acidic solutions. In the same way, monovalent ions (such as Na⁺ and Cl⁻) have no effect in solutions with either neutral or acidic pH.

Although the "sweet-inducing" mechanism is unknown, it is believed that one active site of curculin strongly binds to the taste receptor membranes while a second active site fits into the sweet receptor site. The latter site is thought to be responsible for the induction of sweetness. Presence of Ca²⁺ and/or Mg²⁺, water and acids tune the binding of the active site of curculin to the receptor site and therefore modify perceived sweetness.