Mabinlin

Four homologues sweet-tasting proteins mabinlin were extracted from the seed of Mabinlang (Capparis masaikai Levl.), a Chinese plant growing in Yunnan province. Mabinlin-2 was first isolated in 1983 and characterised in 1993, it is the most studied protein. The 3 other variants of mabinlin-1, -3 and -4 were discovered and characterised in 1994.

Protein structure
The 4 mabinlins are very similar in their Amino acids sequences (see below).

Chain A

M-1: EPLCRRQFQQ HQHLRACQRY IRRRAQRGGL VD

M-2: QLWRCQRQFL QHQRLRACQR FIHRRAQFGG QPD 

M-3: EPLCRRQFQQ HQHLRACQRY LRRRAQRGGL AD 

M-4: EPLCRRQFQQ HQHLRACQRY LRRRAQRG 

Chain B

M-1: EQRGPALRLC CNQLRQVNKP CVCPVLRQAA HQQLYQGQIE GPRQVRQLFR AARNLPNICK IPAVGRCQFT RW

M-2: QPRRPALRQC CNQLRQVDRP CVCPVLRQAA QQVLQRQIIQ GPQQLRRLFD AARNLPNICN IPNIGACPFR AW

M-3: EQRGPALRLC CNQLRQVNKP CVCPVLRQAA HQQLYQGQIE GPRQVRRLFR AARNLPNICK IPAVGRCQFT RW

M-4: EQRGPALRLC CNQLRQVNKP CVCPVLRQAA HQQLYQGQIE GPRQVRRLFR AARNLPNICK IPAVGRCQFT RW

''Amino acids sequence of the sweet-tasting proteins Mabinlins homologues adapted from Swiss-Prot biological database of protein.

The molecular weights of Mabinlin-1, Mabinlin-3 and Mabinlin-4 are 12.3 kDa. 12.3 kDa. and 11.9 kDa respectively.

With a weight of 10.4kDa, mabinlin-2 is lighter than mabinlin-1. It is a hetero dimmer consisting of two different chains A and B. The A chain is composed of 33 amino acid residues and the B chain is composed of 72 amino acid residues. The B chain contains two intramolecular disulphide bonds and is connected to the A chain through two intermolecular disulphide bridges.

Mabinlin-2 is the sweet-tasting protein with the highest known thermostablility, which is due to the presence of the four disulfide bridges. It has been suggested also that the difference in the heat stability of the different mabinlin homologues is due to the presence of an arginine residue (heat-stable homologue) or a glutamine (heat-unstable homologue) at position 47 in the B-chain.

Sweetness properties
Mabinlins sweetness were estimated to be about 100-400 times that of sucrose on weight basis, which make them less sweet than thaumatin (3000 times) but elicit a similar sweetness profile.

The sweetness of mabinlin-2 is unchanged after 48 hours incubation at boiling point.

Mabinlin-3 and -4 sweetness stayed unchanged after 1 hour at 80°C, while mabinlin-1 loses sweetness after 1 hour at the same condition.

As a sweetener
Mabinlins, as proteins, are readily soluble in water and found to be highly sweet, however mabilin-2 with its high heat stability as the best chance to be used as a sweetener.

Last decade, researches undertake to produce industrially mabinlin-2 through expression in micro-organisms and synthesis are more or less successful. The sweet-tasting protein has been successfully synthesised by a stepwise solid-phase method in 1998, however the synthetic protein had an astringent-sweet taste.

Mabinlin-2 has been expressed in transgenic potato tubers, but no explicit results have been reported yet.

However, patents to protect production of recombinant mabinlin by cloning and DNA sequencing has been issued.