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- picture The nicotinic acetylcholine receptor
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| bet | 8/13 | | Sana | 16.05.2024 | | Hajmi | 1,48 Mb. | | #237391 |
Bog'liq Structure and function of biological membranes.2.2. 3- picture The nicotinic acetylcholine receptor.
Active transport The transport of molecules across a membrane against a concentration gradient requires energy, and is referred to as active transport. This energy can be obtained from ATP hydrolysis (primary active transport), from light (as, for example, in the case of the bacterial proton pump bacteriorhodopsin), or from an electrochemical gradient of an ion such as Na+ or H+ (secondary active transport).
Calcium ions signal many events, including muscle contraction, neurotransmitter release and cellular motility. However, high cytoplasmic concentrations of Ca2+ are toxic to the cell. Therefore Ca2+ must be tightly regulated and removed from the cytoplasm either into internal stores (the ER, and the SR in muscle cells) or into the extracellular space. This Ca2+ removal is carried out by a family of Ca2+-ATPases, including the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), which hydrolyse ATP to move Ca2+ against its electrochemical gradient into the ER and SR (Figure 8). There are Ca2+-ATPases in the ER, Golgi and plasma membrane, and despite their sequence similarity, these proteins are differentially targeted to the appropriate membrane. These Ca2+ pumps are primary active transporters. SERCA moves two Ca2+ ions into the ER or SR for every ATP molecule that is hydrolysed. The pump undergoes a cycle of binding ATP and phosphorylation, and undergoes large conformational changes every time it transports a pair of Ca2+ ions. SERCA is a P-type ATPase (so called because it is phosphorylated during ion transport). There are many P-type ATPases, and they are conserved in evolution across many species. The Na+/K+-ATPase is one of these P-type ATPases, and it works in a similar way to SERCA to pump Na+ out of the cell and K+ into the cell using energy derived from the hydrolysis of ATP. We have now obtained three-dimensional structures of SERCA in a number of conformational states, which allow scientists to visualize the transport process. Secondary active transport requires an ion electrochemical gradient to drive the uphill transport of another solute. The downhill movement of one species drives the uphill movement of the other. This can be symport (in which both types of molecule or ion travel across the membrane in the same direction) or antiport (in which the two species travel in opposite directions).
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