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Sodium/substrate symport (or co-transport) is a widespread mechanism of solute transport across cytoplasmic membranes of pro- and eukaryotic cells. Thereby the energy stored in an inwardly directed electrochemical sodium gradient (sodium motive force, SMF) is used to drive solute accumulation against a concentration gradient. The SMF is generated by primary sodium pumps (e.g. sodium/potassium ATPases, sodium translocating respiratory chain complexes) or via the action of sodium/proton antiporters. Sodium/substrate transporters are grouped in different families based on sequence similarities. One of these families, known as the sodium:solute symporter family (SSSF), contains over a hundred members of pro- and eukaryotic origin. The average hydropathy plot for SSSF proteins predicts 11 to 15 putative transmembrane domains (TMs) in alpha-helical conformation. A secondary structure model of PutP from Escherichia coli suggests the protein contains 13 TMs with the N-terminus located on the periplasmic side of the membrane and the C-terminus facing the cytoplasm. The results support the idea of a common topological motif for members of the SSSF. Transporters with a C-terminal extension are proposed to have an additional 14th TM. An ordered binding model of sodium/substrate transport suggests that sodium binds to the empty transporter first, thereby inducing a conformational alteration which increases the affinity of the transporter for the solute. The formation of the ternary complex induces another structural change that exposes sodium and substrate to the other site of the membrane. Substrate and sodium are released and the empty transporter re-orientates in the membrane allowing the cycle to start again. ==Subfamilies== *Sodium/pantothenate symporter *Sodium/proline symporter *Cation/acetate symporter ActP 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sodium-solute symporter」の詳細全文を読む スポンサード リンク
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