Active Transport and Na+/Glucose Symport

• Active transport involves moving ions against electrochemical potential gradients or uncharged molecules against concentration gradients.
• It uses energy either directly from ATP hydrolysis or indirectly through ion flow from an existing ion gradient.

Secondary Active Transporters

• These do not directly use ATP hydrolysis; they utilize the electrochemical potential difference of an ion.
• Examples include glucose-Na+ symport and amino acid-Na+ symport.
• Symporters take up glucose and amino acids from the small intestine lumen into the intestinal epithelia using sodium uptake.
• The sodium electrochemical gradient is maintained by the Na+/K+ ATPase, which uses ATP hydrolysis.

Na+/Glucose Coupled Transport

• This is a secondary active transport mechanism.
• The Na+/glucose symporter transports two Na+ ions and one glucose molecule into cells simultaneously, in the same direction, at the apical surface of small intestine epithelial cells, facing the intestinal lumen.
• The electrochemical potential of Na+ provides the energy for glucose transport against its concentration gradient.
• The Na+/K+ ATPase pump maintains the necessary Na+ electrochemical gradient.

V-Type Transporters

• These are vacuolar-type proton transporters in organelle membranes, transporting protons into the organelles.
• They maintain the low pH in lysosomes and synaptic vesicles.
• They are also found in the plasma membrane of cells like osteoclasts, tumor cells, macrophages, and sperm, acidifying their environment.
• Unlike P-type transporters, they do not become covalently phosphorylated during ATP hydrolysis.

P-Type Transporters

• These transporters are transiently phosphorylated during their operation.
• Conformation changes caused by phosphorylation lead to ion transport.
• The Na+/K+ -ATPase and plasma membrane Ca2+-ATPase create ion gradients essential for cell operations and are present in all cell types.

ABC Proteins

• They have two ATP-binding sites (NBDs) for ATP binding and hydrolysis, and two transmembrane domains (TMDs) that form substrate-binding sites.
• NBD structure and ATP hydrolysis mechanisms are consistent across all ABC proteins.
• ABC proteins are categorized as channel-type proteins (e.g., CFTR), channel regulators (e.g., SUR1), and active pumps (e.g., Pgp=ABCB1, ABCG2, TAP1/TAP2).

Multidrug Resistance

• This is the resistance of cancer cells to multiple anticancer agents, which are expelled by ABC transporters, preventing them from reaching lethal intracellular concentrations.
• It is caused by the expression of ABC transporters like P-glycoprotein (Pgp=ABCB1=MDR1), multidrug resistance proteins (MRP1=ABCC1), and BCRP (=ABCG2) in tumor cell plasma membranes.

P-Glycoprotein (Pgp, MDR1, ABCB1)

• It is an active transporter (active pump) type human ABC protein.
• It is expressed in the plasma membrane of cells in tissues with barrier functions, including intestinal epithelium, kidney, liver, blood-brain barrier, blood-testis barrier, and placenta.
• It protects the body from amphiphilic or lipophilic toxic compounds of external (xenobiotics) and internal origin (toxic metabolic byproducts).
• It is often expressed in stem cells, tumor stem cells, and cancer cells.
• Like ABCG2 (BCRP) and MRP1(ABCC1), it is involved in tumor chemotherapy resistance.

ABCG2 (Breast Cancer Resistance Protein, BCRP)

• It is an active transporter type ABC protein.
• It has a wide substrate spectrum, overlapping with Pgp, that includes xenobiotics and various anticancer agents.
• Besides tumor cells, it is expressed in barrier regions of the body and in stem cells.
• Its physiological substrate is uric acid, so it is involved in uric acid elimination.
• Mutations affecting ABCG2 function increase the risk of gout.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR=ABCC7)

• It is a channel type ABC protein.
• This chloride channel is expressed in the apical membrane of epithelial cells.
• Channel opening is induced by ATP binding to the nucleotide-binding domain (NBD) and phosphorylation of the regulatory (R) domain by protein kinase A.
• Open CFTR channels allow Cl- to exit the epithelial cell into the mucus, followed by passive movement of Na+ ions, increasing osmotic pressure and drawing water out of the cell.
• Inactivating mutations of the CFTR Cl- ion channel cause cystic fibrosis (CF), a multiorgan hereditary disease.
• High viscosity of secreted mucus causes symptoms affecting the lungs, gastrointestinal system, and reproductive system.
• Patients often die from severe recurrent lung infections due to mucus retention in the lungs.

TAP1/TAP2 Oligopeptide Transporter

• This is a heterodimeric transporter formed by half-transporter molecules TAP1 and TAP2.
• The TAP1/TAP2 transporter is expressed in the endoplasmic reticulum (ER) membrane.
• It pumps oligopeptides, produced from proteasomal degradation of cellular and viral proteins, into the ER lumen.
• In the ER lumen, oligopeptides bind to MHC I proteins, and the complex is transported to the plasma membrane for presentation to cytotoxic T-cells.
• Inactivating mutations of TAP1/TAP2 may lead to immunodeficiency.

Sulfonylurea Receptor 1 (SUR1), KATP Channel

• SUR1 (ABCC8) is a channel regulator type ABC protein.
• SUR1 molecules, along with Kir6.2 subunits, form an ATP-sensitive KATP potassium channel in the plasma membrane of pancreatic β cells.
• It regulates insulin secretion.
• High blood glucose increases the ATP/ADP ratio, closing KATP channels, depolarizing the β-cell, and opening voltage-gated Ca2+ channels.
• Increased Ca2+ entry and cytosolic Ca2+ concentration cause the release of stored insulin.

SLC (Solute Carrier) Proteins

• This is a large and diverse group of transporter proteins that includes secondary active transporters (coupled transporters) and passive transporters.
• They transport inorganic ions or water-soluble small molecules like amino acids, oligopeptides, nucleotides, vitamins, hexoses, drugs, and drug metabolites through the plasma membrane or intracellular organelle membranes.
• Secondary active transporters in the SLC family include Na+-glucose symporters, Na+-amino acid symporters, and Na+/Ca2+ exchangers.
• Passive transporter type SLC proteins include glucose uniporters (GLUT transporters) and amino acid uniporters.

Glucose and Amino Acid Uptake Through the Intestinal Epithelium

• Directional transport of glucose through the intestinal epithelium to the extracellular fluid/blood is facilitated by the segregation of involved transporter proteins in the plasma membrane.
• Active glucose uptake from the gut lumen into enterocytes, mediated by the Na+-glucose symporter in the apical membrane, creates a glucose concentration gradient.
• Accumulated glucose diffuses across the basal membrane of enterocytes through GLUT2 into the extracellular fluid, following its concentration gradient.
• The Na+ gradient driving glucose uptake is maintained by the Na+/K+-pump, also expressed in the basolateral membrane of enterocytes.
• A similar mechanism occurs for directional transport of amino acids, involving the Na+-amino acid symporter for active amino acid uptake in the apical membrane, and amino acid uniporters for facilitated transport through the basal membrane.

Plasma Membrane Na+/Ca2+ Antiport (NCX, Na+/Ca2+ Coupled Transport)

• It is an electrogenic, secondary active transporter in the cytoplasm membrane.
• Three Na+ ions are transported into the cell following their electrochemical gradient, while one Ca2+ ion is transported out of the cytosol against its electrochemical gradient.
• The transporter is powered by the electrochemical gradient of Na+.
• NCX is vital in cardiac myocytes for restoring resting Ca2+ concentration after a Ca2+ signal.
• The Na+ electrochemical gradient necessary for Ca2+ export is maintained by the Na+/K+-ATPase in the cell membrane.

Plasma Membrane Ca2+ ATPase (PMCA)

• It is a primary active transporter, P-type ATPase, in the cytoplasm membrane.
• Ca2+ is transported from the cytosol to the extracellular space against its electrochemical gradient, accompanied by the transport of two H+ ions into the cytosol making the process electroneutral.
• It maintains the resting cytosolic free Ca2+ concentration in mammalian cells.

SERCA

• Sarco-endoplasmic reticulum Ca2+ ATPase is a primary active, P-type ATPase transporter in the sarcoplasmic and endoplasmic reticulum membrane.
• The transporter moves Ca2+ from the cytosol into the ER/SR lumen using ATP hydrolysis.
• SERCA helps maintain the resting cytosolic Ca2+ concentration and restores it after a Ca2+ signal.

Ryanodine Receptor (RYR)

• It is an intracellular ligand-gated Ca2+ channel in the sarcoplasmic and endoplasmic reticulum membrane.
• In skeletal muscle cells, the channel is activated by a part of the DHP receptor (conformational coupling), while in cardiac myocytes and neurons, it is activated by Ca2+ (Ca2+-induced Ca2+ release, CICR).
• Channel opening releases Ca2+ from the ER/SR store into the cytosol, raising cytosolic Ca2+ concentration and triggering specific cellular responses (e.g., muscle cell contraction).

IP3 Receptor (IP3-R)

• It is an intracellular ligand-gated Ca2+ channel in the endoplasmic reticulum membrane.
• The channel is activated by IP3, generated in the cell membrane upon receptor-ligand interaction.
• Channel opening releases Ca2+ from the ER store into the cytosol, increasing cytosolic Ca2+ concentration and eliciting specific cellular responses (e.g., hormone secretion).

Calmodulin

• It is a cytosolic Ca2+-binding protein with four Ca2+-binding pockets having an EF-hand structure that cooperatively binds Ca2+.
• Depending on Ca2+ saturation, calmodulin changes conformation, enabling it to interact with and activate target proteins, such as PMCA or cytosolic protein kinases, CAM-kinase-II, and MLC (myosin light chain kinase).

Pump-Leak Model of Osmo- and Volume Regulation

• This homeostatic regulation maintains cell volume in an isotonic medium.
• The tendency of inorganic ions to reach thermodynamic equilibrium results in a net influx of ions (Donnan effect), which is counterbalanced by the Na+/K+ -pump.
• The Na+/K+ -pump reduces the cell's ion content by one ion per duty cycle.
• The resulting osmotic equilibrium features zero net ion flow and zero net water movement.
• Inhibition of the Na+/K+ -pump results in ion accumulation and water accumulation, even in isotonic conditions, leading to cell swelling.

RVD (Regulatory Volume Decrease)

• This cell volume regulatory mechanism is induced by cell swelling in hypotonic medium.
• It reduces cell volume and leads to water loss, even if the hypotonic condition persists.
• Short-term RVD involves the net loss of inorganic ions, followed by water loss.
• Long-term RVD decreases cytosolic osmolality by reducing metabolite concentrations (metabolic regulation) through efflux via transporters (e.g., taurine transporter) or by favoring anabolic processes.

RVI (Regulatory Volume Increase)

• This cell volume regulatory mechanism is induced by cell shrinkage in hypertonic medium.
• It increases cell volume by gaining water, even if the hypertonic condition is maintained.
• Short-term RVI involves the net accumulation of inorganic ions, followed by water gain.
• Long-term RVI increases cytosolic osmolality by increasing metabolite concentrations (metabolic regulation) through metabolite influx via transporters (e.g., taurine transporter) or by favoring catabolic processes (converting polymers to monomers).

Steady-State pH of the Cytosol

• At this pH, the rate of base efflux (e.g., Cl-/HCO3- antiport) equals the rate of acid efflux (e.g., Na+/H+ antiport).
• Both classes of transporters work at the same rate, maintaining a constant cytosolic pH.
• At this pH, the pH-dependence graph of the acid efflux rate intersects the pH-dependence graph of the base efflux rate.

Na+/H+ Antiport

• It is an electroneutral exchanger in the cytoplasm membrane that moves Na+ in and H+ out of the cytosol.
• The main function is cytosolic pH regulation, running quickly at acidic pH to remove excess H+.
• The transport rate decreases at alkaline pH.
• It also participates in regulatory volume increase (RVI) by accumulating Na+ in the cytosol and increasing intracellular osmolality.

Cl-/HCO3- Antiport

• It is an electroneutral exchanger in the cytoplasm membrane that moves one Cl- in and one HCO3- out of the cytosol.
• The main function of the transporter is cytosolic pH regulation, running quickly at alkaline pH to remove excess base (HCO3-).
• The transport rate decreases when cytosolic pH decreases.
• It also participates in regulatory volume increase by accumulating Cl- in the cytosol, thereby increasing intracellular osmolality.