13.11
Transporte Ativo Secundário
Um exemplo de como as células utilizam a energia contida nos gradientes eletroquímicos é demonstrado pelo transporte de glicose para o interior das cé…
Ao contrário do transporte primário, o transporte ativo secundário utiliza energia armazenada nos gradientes eletroquímicos dos íons, para transportar solutos contra seus gradientes de concentração.
Uma proteína que exemplifica o transporte ativo secundário é o transportador ligado a sódio-glicose ou SGLT1. Inicialmente, esse transportador é posicionado de forma que o lado voltado para o citosol fique fechado, mas a extremidade extracelular esteja aberta. Isso expõe dois locais de ligação de sódio carregados negativamente ao meio ambiente, que são então ligados por íons de sódio carregados positivamente.
Como mais íons de sódio povoam o espaço extracelular do que o citoplasma, os íons de sódio ligados ao transportador descem seu gradiente eletroquímico.
O transportador ligado ao sódio tem uma alta afinidade pela glicose, presente em um nível baixo externo, mas em alta concentração dentro da célula.
Uma molécula de glicose então se liga ao transportador contra seu gradiente de concentração, e essa ligação simultânea de sódio e glicose faz com que a proteína feche sua região extracelular e abra o lado voltado para o citoplasma.
Os íons de sódio então se desprendem e entram no citoplasma. Isso diminui a afinidade da proteína pela glicose, liberando-a no citoplasma. Uma vez vazio, o transportador retorna à sua orientação inicial.
View the full transcript and gain access to JoVE Core videos
Q1: How does secondary active transport differ from primary active transport?
Secondary active transport uses energy stored in electrochemical gradients of ions like sodium, rather than directly consuming ATP. In this process, one solute moves down its gradient while another moves against its gradient simultaneously. Primary active transport, by contrast, directly uses ATP hydrolysis to power transport. Secondary active transport is therefore energetically dependent on the ion gradients established by primary active transport pumps.
Q2: What role does the sodium electrochemical gradient play in SGLT1 function?
The sodium electrochemical gradient—created by both chemical concentration differences and electrical charge—drives SGLT1 transport. Sodium ions are more concentrated outside the cell and move inward down this gradient, releasing energy the transporter uses to move glucose against its concentration gradient. This coupled movement of sodium and glucose is the fundamental mechanism enabling secondary active transport in intestinal and kidney cells.
Q3: How does SGLT1 bind and transport sodium and glucose simultaneously?
SGLT1 has two negatively charged sodium-binding sites and one glucose-binding site. When sodium ions and a glucose molecule bind together to the transporter, the protein undergoes a conformational change, closing its extracellular end and opening its cytoplasmic side. Sodium ions then detach and enter the cytoplasm, decreasing the transporter's glucose affinity and releasing glucose into the cell.
Q4: Why is glucose transport into cells important for understanding disease?
Glucose transport mechanisms are targets for treating metabolic diseases. In diabetes, excess blood glucose causes complications like nerve damage, making SGLT inhibition a potential therapeutic strategy. Cancer cells require more glucose than normal cells, so glucose transporters are being investigated as anti-cancer therapy targets. Understanding glucose absorption into the small intestine and cellular uptake mechanisms informs these therapeutic approaches.
Q5: What maintains the sodium concentration gradient that powers secondary active transport?
An ATP-driven pump embedded in the cell membrane actively expels sodium ions from the cytoplasm, maintaining higher extracellular sodium concentrations. This pump creates both a chemical gradient and an electrical gradient, since expelled sodium ions are positively charged. The resulting electrochemical gradient is directed inward and provides the energy source for secondary active transporters like SGLT1.
Q6: Where are sodium-glucose linked transporters primarily located in the body?
SGLTs are primarily located in the membranes of intestinal and kidney cells, where they facilitate glucose absorption from the organ lumen into the bloodstream. These transporters are essential for nutrient uptake and reabsorption in these tissues. Their strategic placement allows cells to harness electrochemical gradients for efficient glucose transport across epithelial barriers.
Q7: What happens to SGLT1 after sodium and glucose are released into the cytoplasm?
After sodium ions detach and glucose is released into the cytoplasm, SGLT1 returns to its initial orientation with its cytoplasm-facing side closed and extracellular end open. This reset allows the transporter to bind new sodium and glucose molecules and repeat the transport cycle. The conformational cycling is essential for continuous secondary active transport function.