Stick structure of the amino acid residues in segment B54-B64 in Mabinlin II. Constructed in PyMOL (PDB 2DS2).
Mabinlins are sweet-tastingproteins 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[1] and characterised in 1993,[2] and is the most extensively studied of the four. The other variants of mabinlin-1, -3 and -4 were discovered and characterised in 1994.[3]
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 acid sequence of Mabinlins homologues are adapted from Swiss-Prot biological database of protein.[4][5][6][7]
The molecular weights of Mabinlin-1, Mabinlin-3 and Mabinlin-4 are 12.3 kDa, 12.3 kDa and 11.9 kDa, respectively.[3]
With a molecular weight of 10.4kDa, mabinlin-2 is lighter than mabinlin-1. It is a heterodimer consisting of two different chains A and B produced by post-translational cleavage. 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 disulfide bonds and is connected to the A chain through two intermolecular disulfide bridges.[2][8]
Mabinlin-2 is the sweet-tasting protein with the highest known thermostability,[9] which is due to the presence of the four disulfide bridges.[10] 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.[3]
The B54-B64 segment of the beta chain in Mabinlin II is the primary functional domain that enables its function as a sweet protein.The segment contains the [NL/I] tetralit motif,a unique structural arrangement that contains four Asn-Leu/lle dipole units. The motif is stabilized by Pro56, Pro62, and a cysteine that is involved in a disulfide bridge, which includes CysB59-CysB11. The stabilization ensures a rigid and defined conformation.The region is one of the primary binding sites for receptor interaction such as the sweet receptor nTR2/T1R3.[11]
Mabinlins sweetness were estimated to be about 100–400 times that of sucrose on molar basis, 10 times sucrose on a weight basis,[2][3] which make them less sweet than thaumatin (3000 times) but elicit a similar sweetness profile.[12]
The sweetness of mabinlin-2 is unchanged after 48 hours incubation at 80°C.[2]
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.[3][13]
As a sweetener
Mabinlins, as proteins, are readily soluble in water and found to be highly sweet; however, mabinlin-2 with its high heat stability has the best chance to be used as a sweetener.
During the past decade, attempts have been made to produce mabinlin-2 industrially. 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.[8]
Mabinlin-2 has been expressed in transgenicpotato tubers, but no explicit results have been reported yet.[14] However, patents to protect production of recombinant mabinlin by cloning and DNA sequencing have been issued.[15]
↑ Hu Z, He M (1983). "Studies on mabinlin, a sweet protein from the seeds of Capparis masaikai levl. I. extraction, purification and certain characteristics". Acta Botan. Yunnan. (5): 207–212.
1 2 3 4 Liu X, Maeda S, Hu Z, Aiuchi T, Nakaya K, Kurihara Y (January 1993). "Purification, complete amino acid sequence and structural characterization of the heat-stable sweet protein, mabinlin II". European Journal of Biochemistry. 211 (1–2): 281–7. doi:10.1111/j.1432-1033.1993.tb19896.x. PMID8425538.
↑ Guan RJ, Zheng JM, Hu Z, Wang DC (July 2000). "Crystallization and preliminary X-ray analysis of the thermostable sweet protein mabinlin II". Acta Crystallographica Section D. 56 (Pt 7): 918–9. doi:10.1107/S0907444900005850. PMID10930844.
↑ Nirasawa S, Liu X, Nishino T, Kurihara Y (October 1993). "Disulfide bridge structure of the heat-stable sweet protein mabinlin II". Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology. 1202 (2): 277–80. doi:10.1016/0167-4838(93)90016-K. PMID8399391.
↑ Kurihara Y (1992). "Characteristics of antisweet substances, sweet proteins, and sweetness-inducing proteins". Critical Reviews in Food Science and Nutrition. 32 (3): 231–52. doi:10.1080/10408399209527598. PMID1418601.
↑ Xiong LW, Sun S (1996). "Molecular cloning and transgenic expression of the sweet protein mabinlin in potato tubers". Plant Physiology. 111 (2): 147.
↑ USpatent 6051758,Sun, Samuel S.M.; Xiong, Liwen& Hu, Zhonget al.,"Recombinant Sweet protein Mabinlin",issued 2000-04-18, assigned to University of Hawaii
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