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#symporters

2 public questions tagged with this topic.

Membrane Transportation

Which of the following describes Na+/amino acid symporters?

Sodium-amino acid symporters belong to SLC6, SLC1 and SLC38 families mediating concentrative uptake of neutral, acidic and basic amino acids in intestinal epithelium, kidney proximal tubule, neurons and astrocytes. They co-transport one to two Na+ ions with one amino acid molecule, using sodium electrochemical gradient generated by Na+/K+ ATPase to drive amino acid accumulation against gradient up to hundred-fold, essential for protein synthesis, neurotransmitter precursor supply, osmolyte production and mTOR signaling. Transport cycle involves alternating access where Na+ binding increases affinity for amino acid, conformational switch inward, release of both solutes. Unlike primary P-type Ca2+ or H+ pumps they do not hydrolyze ATP nor reside exclusively in lysosomes, and they are not passive uniporters like GLUT. Some systems exchange intracellular K+ or H+ for additional regulation, but primary energetic hallmark remains sodium coupling that can be abolished by removing extracellular Na+ or inhibiting Na+/K+ pump, demonstrating secondary active concentrative mechanism distinct from facilitated diffusion.

Ref: Broer, Physiol Rev 2008, Amino acid transporters; Alberts et al., Chapter 11 nutrient symport.

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Membrane Transportation

Lactose permease functions as a:

Lactose permease LacY of Escherichia coli, characterized extensively by Kaback, is a twelve-transmembrane helix member of major facilitator superfamily and textbook exemplar of secondary active symport. LacY itself does not possess ATPase activity nor nucleoside-binding motifs; energy comes indirectly from electrochemical proton gradient across inner membrane maintained by respiratory chain H+ extrusion. In outward-open state protonation of Glu325 increases affinity for lactose at the central cavity; coupled binding induces rocker-switch movement of N and C terminal six-helix bundles to inward-open conformation releasing both solutes to cytoplasm where lactose is cleaved to glucose and galactose by beta-galactosidase. This co-transport allows concentration of lactose thousand-fold over medium when environmental sugar is scarce, supporting growth on lactose as sole carbon source. Analogous sodium-coupled SGLT and amino acid transporters in humans utilize identical chemiosmotic principle substituting Na+ for H+ as driving ion, demonstrating conserved energetics across prokaryotes and eukaryotes. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Kaback et al., Nat Rev Mol Cell Biol 2001, LacY mechanism; Alberts, Chapter 11 carriers.

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