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Monellin
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== Protein composition == Monellin's molecular weight is 10.7 [[Atomic mass unit|kDa]]. It has two noncovalently associated polypeptide chains: an A chain sequence with 44 amino acid residues, and a B chain with 50 residues.<ref name="jbc-248-534"/><ref name="pmid3614382">{{cite journal |vauthors=Ogata C, Hatada M, Tomlinson G, Shin WC, Kim SH | title = Crystal structure of the intensely sweet protein monellin | journal = Nature | volume = 328 | issue = 6132 | pages = 739β42 | year = 1987 | pmid = 3614382 | doi = 10.1038/328739a0 | bibcode = 1987Natur.328..739O | s2cid = 4323370 }}</ref> <blockquote> Monellin chain A (44 AA): <br /> {{mono|REIKGYEYQL YVYASDKLFR ADISEDYKTR GRKLLRFNGP VPPP}}<br /> Monellin chain B (50 AA): <br /> {{mono|GEWEIIDIGP FTQNLGKFAV DEENKIGQYG RLTFNKVIRP CMKKTIYEEN}}<br /> [[Amino acid#Table of standard amino acid abbreviations and properties|Amino acid]] sequence of the sweet protein monellin adapted from Swiss-Prot biological database of protein.<ref>[http://www.expasy.org/uniprot/P02881 UniProtKB/Swiss-Prot database entry #P02881]</ref><ref>[http://www.expasy.org/uniprot/P02882 UniProtKB/Swiss-Prot database entry #P02882]</ref></blockquote> Monellin has a secondary structure consisting of five beta-strands that form an antiparallel beta-sheet and a 17-residue alpha-helix.<ref name="pmid17329805"/> In its natural form, monellin is composed of the two chains shown above ({{PDB|3MON}}), but the protein is unstable at high temperatures or at extremes of pH.<ref name="pmid17329805"/> To enhance its stability, single-chain monellin proteins were created in which the two natural chains are joined via a Gly-Phe dipeptide linker.<ref name="pmid17329805"/> 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.<ref name="pmid17329805"/> 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.<ref name="pmid17329805"/>
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