Secondary Active Transport

• Uses energy from ATP hydrolysis or ion flow to transport ions against electrochemical gradients or uncharged molecules against concentration gradients.

Secondary Active Transporters

• Differ from primary active transport by not being directly coupled to ATP hydrolysis.
• Use the electrochemical potential difference of an ion.
• Example: Glucose-Na+ symport and amino-acid-Na+ symport in intestinal epithelia facilitate glucose and amino acid uptake using the sodium gradient.
• Electrochemical gradient maintained by the Na+/K+ ATPase, powered by ATP hydrolysis.

Na+/glucose co-transport

• It is a secondary active transport mechanism.
• The Na+/glucose symporter transports 2 Na+ ions and one glucose molecule simultaneously into cells at the apical surface of small intestine epithelial cells.
• Sodium electrochemical potential drives glucose transport against its concentration gradient.
• Na+/K+-ATPase pump maintains the necessary Na+ electrochemical gradient.

V-type transporters

• Located in membranes of organelles and transport protons into these organelles.
• Responsible for the low pH in lysosomes and synaptic vesicles.
• Found in the plasma membrane of cells that acidify their environment like osteoclasts, tumor cells, macrophages, and sperm.
• Do not become covalently phosphorylated during ATP hydrolysis, unlike P-type transporters.

P-type transporters

• Phosphorylated transiently during operation, leading to conformational changes that transport ions.
• Na+/K+-ATPase and plasma membrane Ca2+-ATPase establish ion gradients required for cell functions and are present in all cell types.

ABC proteins

• Contain 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 mechanism are consistent among all ABC proteins.
• Categorized as: channel-type proteins (CFTR), channel regulators (SUR1), and active pumps (Pgp=ABCB1, ABCG2, TAP1/TAP2).

Multidrug resistance

• Occurs when cancer cells resist multiple anticancer agents.
• ABC transporters pump out the agents, preventing them from reaching lethal intracellular concentrations.
• Certain ABC transporters cause this, including P-glycoprotein (Pgp=ABCB1=MDR1), multidrug resistance proteins (MRP1=ABCC1), and BCRP (=ABCG2).

P-glycoprotein (Pgp, MDR1, ABCB1)

• An active pump type human ABC protein.
• It is present in the plasma membrane of cells in tissues/organs with barrier functions (intestinal epithelium, kidney, liver, blood-brain barrier, blood-testis barrier, and placenta).
• Protects the body from toxic amphiphilic or lipophilic compounds from external (xenobiotics) and internal (toxic metabolic side-products) sources.
• Often found in stem cells, tumor stem cells, and cancer cells.
• Contributes to chemotherapy resistance in tumors, like ABCG2 (BCRP) and MRP1(ABCC1).

ABCG2 (Breast Cancer Resistance Protein, BCRP)

• An active transporter type ABC protein.
• Has a broad substrate range (overlapping with Pgp), which includes xenobiotics and anticancer agents.
• Expressed in tumor cells, barrier regions, and stem cells.
• Transports uric acid and is involved in uric acid elimination.
• Mutations in ABCG2 can increase the risk of gout.

Cystic Fibrosis Transmembrane Conductance Regulator (= CFTR=ABCC7)

• Is a channel type ABC protein.
• Chloride channel in the apical membrane of epithelial cells.
• ATP binding to the nucleotide binding domain (NBD) and regulatory (R) domain phosphorylation by protein kinase A induces channel opening.
• Open CFTR channels allow Cl- to exit the epithelial cell, followed by passive Na+ movement, increasing osmotic pressure and water movement out of the cell.
• Inactivating mutations cause cystic fibrosis (CF), a multiorgan hereditary disease.
• Causes viscous mucus, leading to lung, gastro-intestinal and reproductive system problems.
• Serious recurrent lung infections from mucus retention can be fatal.

TAP1/TAP2 oligopeptide transporter

• A heterodimeric transporter formed by TAP1 and TAP2 half-transporter molecules.
• Expressed in the endoplasmic reticulum (ER) membrane.
• Pumps oligopeptides (from proteasomal degradation of cellular and viral proteins) into the ER lumen where they bind to MHC I protein.
• The complex is transported to the cell surface to present to cytotoxic T-cells.
• Inactivating mutations can cause immunodeficiency.

Sulfonylurea receptor 1 (SUR1), KATP channel

• SUR1 (ABCC8) is a channel regulator type ABC protein.
• SUR1 molecules and Kir6.2 subunits form an ATP-sensitive KATP potassium channel in the plasma membrane of pancreatic β cells.
• Involved in insulin secretion regulation.
• Increased ATP/ADP ratio (caused by elevated blood glucose) closes KATP channels, depolarizes the β-cell, and opens voltage-gated Ca2+ channels.
• Increased Ca2+ entry and cytosolic Ca2+ concentration cause insulin release from vesicles.

SLC (Solute Carrier) proteins

• It is a large group of transporter proteins, involving secondary active transporters (co-transporters) and passive transporters.
• Transport inorganic ions or water-soluble small molecules (amino acids, oligopeptides, nucleotides, vitamins, hexoses, drugs and drug metabolites) through the plasma membrane or organelle membranes.
• Secondary active transporters: Na+-glucose symporter, Na+-amino acid symporter, and Na+/Ca2+ exchanger.
• Passive transporters: glucose uniporters (GLUT transporters) and amino acid uniporters.

Glucose and amino acid uptake through the intestinal epithelium

• Directional glucose transport through intestinal epithelium to the extracellular fluid/blood is controlled by the distribution of transporters in the plasma membrane.
• Active glucose uptake from the gut lumen into enterocytes is mediated by the Na+-glucose symporter in the apical membrane, building a glucose concentration gradient.
• Accumulated glucose diffuses across the basal membrane of enterocytes through GLUT2 into the extracellular fluid based on its concentration gradient.
• The Na+ gradient, which drives glucose uptake, is maintained by the Na+/K+-pump in the basolateral membrane of enterocytes.
• A similar mechanism exists for amino acid transport, involving the Na+-amino acid symporter for active uptake in the apical membrane and amino acid uniporters for facilitated transport through the basal membrane.

Plasma membrane Na+/Ca2+ antiport (NCX, Na/Ca coupled transport)

• An electrogenic, secondary active transporter in the cytoplasm membrane.
• Transports 3 Na+ ions into the cell (down their electrochemical gradient) while transporting 1 Ca2+ ion out of the cytosol (against its electrochemical gradient).
• Powered by the electrochemical gradient of Na+.
• Important in cardiac myocytes, where it restores resting Ca2+ concentration after a Ca2+ signal.
• The Na+ electrochemical gradient required for Ca2+ export is maintained by the Na+/K+-ATPase.

Plasma membrane Ca2+ ATPase (PMCA)

• A primary active transporter (P-type ATPase) in the cytoplasm membrane.
• Transports Ca2+ from the cytosol to the extracellular space against its electrochemical gradient with the transport of 2 H+ into the cytosol, making the electroneutral.
• Maintains the resting cytosolic free Ca2+ concentration in mammalian cells.

SERCA

• Sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) is a primary active, P-type ATPase transporter in the sarcoplasmic and endoplasmic reticulum membrane.
• Transports Ca2+ from the cytosol into the ER/SR lumen using ATP hydrolysis.
• Contributes to maintaining and restoring resting cytosolic Ca2+ concentrations following a Ca2+ signal.

Ryanodine receptor (RYR)

• Intracellular ligand-gated Ca2+ channel in the sarcoplasmic and endoplasmic reticulum membrane.
• Activated by a part of the DHP receptor (conformational coupling) in skeletal muscle cells, or by Ca2+ (Ca2+-induced Ca2+ release, CICR) in cardiac myocytes and neurons.
• Upon opening, Ca2+ is released from the ER/SR into the cytosol, increasing Ca2+ concentration and leading to cell-specific responses (e.g., muscle contraction).

IP3 receptor (IP3-R)

• Intracellular ligand-gated Ca2+ channel in the endoplasmic reticulum membrane.
• Activated by IP3 (generated in the cell membrane upon receptor-ligand interaction).
• Upon opening, Ca2+ is released from the ER into the cytosol, increasing Ca2+ concentration and leading to cell-specific responses (e.g., hormone secretion).

Calmodulin

• Cytosolic Ca2+-binding protein with 4 Ca2+ binding pockets (EF-hand structure) that bind Ca2+ cooperatively.
• Conformation changes based on Ca2+ saturation, enabling interaction with and activation of target proteins.
• Activates PMCA (plasma membrane Ca2+-ATPase) or cytosolic protein kinases (CAM-kinase-II and myosin light chain kinase, MLC).

Pump-leak model of osmo- and volume regulation

• Homeostatic regulation of cell volume in isotonic medium.
• Tendency of ions to reach thermodynamic equilibrium results in net influx (Donnan effect), counterbalanced by the Na+/K+-pump.
• Na+/K+-pump reduces ion content by 1 ion per duty cycle.
• This is an osmotic equilibrium which is characterized by 0 net ion flow and zero net water movement.
• Inhibition of the Na+/K+-pump leads to ion and water accumulation, causing cell swelling, even in isotonic solutions.

RVD (Regulatory Volume Decrease)

• Cell volume regulation mechanism induced by cell swelling in hypotonic medium, reducing cell volume and water loss even under hypotonic conditions.
• Short-term RVD - net loss of inorganic ions, followed by water loss.
• Long-term RVD - reduction of cytosolic osmolality by reducing metabolite concentrations via efflux through transporters (e.g., taurine transporter) or favoring anabolic processes.

RVI (Regulatory Volume Increase)

• A cell volume regulation mechanism induced by cell shrinkage in hypertonic medium, increasing cell volume by gaining water under hypertonic conditions.
• Short-term RVI - net accumulation of inorganic ions, followed by water gain.
• Long-term RVI - increase of cytosolic osmolality by increasing metabolite concentrations via metabolite influx through transporters (e.g., taurine transporter) or favoring catabolic processes.

Steady-state pH of the cytosol

• Cytosolic pH remains by having the rate of base efflux (e.g., the Cl-/HCO3- antiport) equals the rate of acid efflux (e.g., the Na+/H+ antiport).
• At this pH, the graph of the pH-dependence of the acid efflux rate intercepts the pH-dependence of the base efflux rate.

Na+/H+ antiport

• It is an electroneutral exchanger in the cytoplasm membrane that mediates Na+ influx and H+ efflux from the cytosol.
• Regulates cytosolic pH, it works fast for acidic cytosolic pH by removing excess H+.
• Transporter participates in regulatory volume increase (RVI) by accumulating Na+ in the cytosol, increasing intracellular osmolality.

Cl-/HCO3- antiport

• An electroneutral exchanger in the cytoplasm membrane that mediates Cl- influx and HCO3- efflux from the cytosol.
• It regulates cytosolic pH, it works fast for alkaline pH by removing excess base (HCO3-).
• This transporter participates in regulatory volume increase by accumulating Cl- in the cytosol increasing intracellular osmolality.