Skip to content
Login Register

#ion transport

11 public questions tagged with this topic.

Membrane Transportation

The CFTR transporter functions as a:

Cystic fibrosis transmembrane conductance regulator CFTR, ABCC7, is an exceptional ABC protein that functions not as active pump but as cAMP-activated chloride channel gated by ATP. It contains two transmembrane domains each with six helices forming chloride-selective pore, two nucleotide-binding domains, and a unique cytosolic R domain with multiple PKA phosphorylation sites. Channel opening requires R domain phosphorylation by protein kinase A downstream of adenylate cyclase, followed by ATP binding and NBD dimerization stabilizing open state; ATP hydrolysis at NBD2 terminates open burst. In airway epithelium, sweat duct, pancreas and vas deferens, chloride secretion via CFTR drives water movement and keeps mucus hydrated and luminal pH regulated. Dysfunction leads to dehydrated airway surface liquid, thickened mucus, impaired mucociliary clearance, recurrent Pseudomonas infection, pancreatic insufficiency and elevated sweat chloride, hallmarks of cystic fibrosis. Its channel nature explains why modulators that improve gating rather than pumping restore function. 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: Gadsby et al., Nature 2006, CFTR gating cycle; Lodish, Molecular Cell Biology, CFTR chloride channel.

View Details
Membrane Transportation

The ABC transporter superfamily is involved in:

ABC transporter superfamily represents large gene family with 48 members in humans associated with inherited disorders and drug resistance, present also in bacteria where many function as importers. Core architecture comprises two transmembrane domains typically six helices each forming substrate translocation chamber with diverse selectivity filters, and two nucleotide binding domains located cytoplasmically containing Walker A phosphate binding P-loop, Walker B magnesium coordination hhhhhD, signature C motif LSGGQ characteristic of ABC plus H-loop and Q-loop coordinating ATP. Transport cycle involves ATP binding inducing tight dimerization of nucleotide binding domains sandwiching nucleotides, converting transmembrane domains from inward to outward facing releasing substrate, ATP hydrolysis and ADP plus phosphate release resetting to inward. This mechanism promiscuous for chemically diverse substrates: anionic chloride via CFTR, cationic lipids cholesterol via ABCA1 contributing to HDL formation, phosphatidylcholine via ABCB4, bile salts via ABCB11, peptides via TAP for MHC class I presentation, and myriad hydrophobic drugs including vinca alkaloids, anthracyclines exported by P-glycoprotein MDR1 conferring multidrug resistance in cancer and bacterial antibiotic resistance. Therefore transport of lipophilic molecules and multidrug resistance exemplifies family function.

Ref: Higgins, ABC Transporters Annual Review Cell Biol, Lipophilic Drug Export and MDR Phenotype.

View Details
Membrane Transportation

The P-class Ca2+ ATPase helps regulate calcium homeostasis by:

Calcium serves as ubiquitous intracellular second messenger regulating muscle contraction, neurotransmitter release, gene expression and apoptosis, requiring precise spatial temporal control. Resting cytosolic free calcium around 100 nanomolar maintained far below extracellular 1 to 2 millimolar and endoplasmic reticulum lumen 400 to 700 micromolar, creating 10,000 fold chemical gradient plus electrical component because membrane potential negative. P-type Ca2+ ATPases accomplish low cytosolic level by active extrusion. Plasma membrane Ca2+ ATPase PMCA ejects calcium outward, while SERCA pumps into endoplasmic and sarcoplasmic reticulum refilling stores. Each cycle hydrolyzes one ATP phosphorylating conserved aspartate in DKTGT motif generating E1P with two calcium occluded high affinity facing cytosol, then conformational transition to E2P low affinity facing lumen or extracellular space releasing calcium. Activity counteracts leak channels, store operated entry and voltage gated calcium influx. Binding without transport would not lower free concentration, facilitated or passive diffusion would equilibrate toward high outside leading to toxicity. Hence these pumps actively transport calcium out of cytoplasm preserving homeostasis and signaling competence.

Ref: Carafoli, Calcium Pump Review, P-Type Ca2+ ATPases Maintaining Cytosolic Calcium Homeostasis.

View Details
Membrane Transportation

Digitalis affects cardiac function by inhibiting:

Therapeutic use of foxglove extracts containing digoxin and digitoxin dates to Withering eighteenth century for dropsy. Molecular target identified as sodium potassium ATPase, P-type pump maintaining high potassium low sodium intracellular, essential for cardiac electrophysiology. Digitalis glycosides bind extracellular vestibule formed by transmembrane segments of alpha subunit in E2P conformation, competing with potassium and locking enzyme in phosphorylated outward open state, inhibiting cycle. Inhibition fraction modest at therapeutic concentrations about 20 to 40 percent raises submembrane sodium from 8 to approximately 15 millimolar. Reduced sodium gradient impairs forward mode of sodium calcium exchanger NCX1 that normally extrudes calcium using downhill sodium entry with stoichiometry three sodium per calcium. Less calcium extrusion elevates intracellular calcium stores in sarcoplasmic reticulum via SERCA loading, increasing calcium transient amplitude during systole enhancing actin myosin force generation, positive inotropic effect. V-type proton pumps acidifying lysosomes insensitive, calcium ATPases unaffected, CFTR chloride channel ABC family not target, specificity explaining cardiovascular action at nanomolar digoxin concentrations.

Ref: Withering and Hauptman, Digitalis and Na+/K+ ATPase Inhibition Cardiac Glycoside Mechanism.

View Details
Membrane Transportation

The E2 conformation of Na+/K+ ATPase has:

According to Post-Albers catalytic cycle Na+/K+ ATPase interconverts between E1 inward facing and E2 outward facing with phosphorylated intermediates. In E1 primary sites display high affinity for sodium, three ions bind cytosolic side coordinated by carboxyl side chains and backbone carbonyls accommodating sodium ionic radius 0.95 angstrom. ATP phosphorylates Asp369 generating E1P occluded trapping ions, isomerization to E2P opens outward, sodium affinity drops thousand fold due to rearrangement of M4-M6 helices widening pocket incompatible with high affinity sodium coordination, ions released extracellularly. Now pocket geometry reconfigured with different carbonyl spacing and glutamate residues favoring dehydrated potassium radius 1.33 angstrom, potassium binds from extracellular fluid where concentration about 4 millimolar with millimolar affinity, much higher than sodium affinity in same state. Binding triggers dephosphorylation via TGES motif, forming E2(K2) occluded then transitioning back to E1 releasing potassium inward low affinity environment. Hence E2 conformation characterized by high affinity for potassium and low for sodium opposite to E1.

Ref: Glynn, J Physiol, Post-Albers Cycle E2 High K+ Affinity Conformation of Na+/K+ ATPase.

View Details
Membrane Transportation

V-class ATPases differ from P-class ATPases in that:

Distinguishing pump families centers on mechanism of coupling ATP hydrolysis to ion translocation. P-class pumps such as Na+/K+ ATPase, SERCA, PMCA and H+/K+ ATPase share formation of covalent phosphoenzyme intermediate on conserved aspartate within DKTGT motif, inhibited by vanadate mimicking phosphate. They typically transport monovalent or divalent cations like sodium, potassium, calcium, heavy metals with fixed stoichiometry. V-class proton pumps acidifying vacuoles, lysosomes, endosomes and Golgi represent rotary ATPases composed of cytosolic V1 domain peripheral stalk and membrane Vo proteolipid ring. They hydrolyze ATP in V1 to drive rotation of central stalk and c-ring translocating protons without ever forming phosphoprotein, mechanism analogous to F-type synthase but operating exclusively as proton ATPase. Importantly substrate specificity limited to protons, not sodium, potassium or calcium. Claim they transport Na+ and K+ misassigns function; Na+/K+ exchange belongs to P-class. Therefore difference lies in absence of phosphorylated intermediate and proton specific rotary mechanism versus phosphoenzyme mediated cation exchange.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: V-Type vs P-Type ATPases - Phosphorylation Difference.

View Details
Membrane Transportation

The E1 conformation of Na+/K+ ATPase has:

Post-Albers scheme for P-type ATPases defines two principal conformers interconverting via phosphorylation. E1 state is outward closed, inward open, with N domain positioned to accept ATP and transmembrane domain exposing high affinity sodium sites to cytosol. Detailed structures from shark rectal gland enzyme show three sodium ions coordinated by oxygen atoms in pocket formed by M4, M5, M6 and M8 with micromolar affinity at cytosolic concentration about 10 millimolar, while potassium affinity submillimolar low because pocket incompatible with potassium dehydration geometry. Phosphorylation produces E1P occluded trapping ions, then isomerizes to E2P outward open releasing sodium due to affinity drop thousand fold. In E2, potassium binding sites with different carbonyl arrangement display high potassium affinity from extracellular side where potassium about 4 millimolar, sodium affinity negligible. Dephosphorylation triggers occlusion and transition back to E1. This reciprocal affinity switch ensures ordered antiport. Thus characteristic of E1 conformation is high affinity for sodium and low affinity for potassium as opposed to E2.

Ref: Albers and Post Model, P-Type Pump Cycle, E1 High Na+ Affinity and E2 High K+ Affinity States.

View Details
Membrane Transportation

The selectivity filter of an ion channel ensures:

Ion channels achieve selective conduction despite high throughput by incorporating a narrow selectivity filter that discriminates on basis of size, dehydration energy and electrostatic coordination. Pore domain S5, P-loop, S6 forms inverted teepee; P-loop folds as two short pore helices P1 and P2 scaffolding filter. In potassium channels carbonyl oxygens from TVGYG sequence replace hydration waters precisely matching potassium radius but requiring too large cavity for smaller sodium. In sodium channels DEKA motif and outer ring carboxylates create high field site preferring partially hydrated sodium with one water and lysine blocking divalents. Calcium channels use EEEE locus binding calcium with high affinity enabling double occupancy knock-off. Once optimal ion enters filter it is stabilized, non-matching ions face energetic barrier and are excluded. Gate opening controlled by voltage sensor S4 or ligand binding occurs elsewhere, filter itself remains rigid. No ATP hydrolysis occurs. Hence filter ensures only compatible ion type passes, establishing basis for electrical signaling, selective reabsorption and action potential generation.

Ref: Hille, Ion Channels of Excitable Membranes, Chapter 13: Selectivity Filter and Molecular Sieving.

View Details
Enzyme Kinetics

Which type of enzyme catalyzes the movement of ions across membranes?

Translocase is the accurate response regarding enzymatic activity or regulation described in this question. Enzymes are biological catalysts that accelerate reactions by lowering activation energy through specific substrate binding and transition state stabilization. In the context of Enzymes Basics, Translocase plays a specific catalytic or regulatory role determined by its active site configuration and mechanism of action. The other options (Oxidoreductase, Transferase, and Ligase) are either different enzymes with distinct substrate specificities, act through different mechanisms, or are involved in separate metabolic pathways.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 6

View Details