Lecture 13-Transport across Biological Membranes IV (April 3) PDF

LECTURE 13 W24 Finish Module 4 Start Module 5 Secondary Active Transport in Prokaryotes Lactose permease in E. coli (LacY from lacY,) LacY is coupled to proton transport Prototype for proton-driven cotransporters e.g., lactose permease in E. coli 1:1 symport of lactose (up gradient) and H+ (down gra...

LECTURE 13 W24 Finish Module 4 Start Module 5 Secondary Active Transport in Prokaryotes Lactose permease in E. coli (LacY from lacY,) LacY is coupled to proton transport Prototype for proton-driven cotransporters e.g., lactose permease in E. coli 1:1 symport of lactose (up gradient) and H+ (down gradient) The e- flow through electron transport chain → H+ gradient across cytoplasmic membrane The H+ gradient drives the co-transport of lactose This gradient powers many active transporters, all H+ symporters “lac operon” Lactose permease in E. coli (LacY from lacY,) If cytochrome oxidase of the electron transport chain is inhibited by CN-, LacY acts as a passive transporter allowing equilibration of lactose across the membrane Secondary Active Transport in Eukaryotes The sodium-potassium ATPase generates ion gradients that drive the sodium-glucose symporter in animals Na+- glucose symporter (Apical Surface) Takes up glucose from the intestine Driven by flow of Na+ down chemical as well as electrical gradient (inside negative) 2Na+out + glucoseout 2 Na+in + glucosein strong thermodynamic tendency for Na+ to move into the cell provides the energy needed for the transport of glucose into the cell Glucose has net flow OUT of Glucose uniporter (GLUT 2) (Basal Surface) these cells! Glucose moves down its concentration NOTE: Na+ glucose symporter is an entirely gradient via GLUT2 from the cell into the different protein from the passive glucose blood at the basal surface transporter Na+ glucose symporter in kidney SGLT2 is a target for treatment of diabetes Gliflozins are inhibiters of SGLT2 The inhibit glucose reabsorption in the kidney and thereby lower blood [glucose]. Important in the treatment of type 2 diabetes SGLT2 inhibitors : Invokana (canagliflozin), Forxiga (dapagliflozin) and Jardiance (empagliflozin). First entered the Canadian market in 2014. https://www.sciencedirect.com/scien ce/article/pii/S008525381830125X CHANNELS AND PORES ▪ Bacterial porins are built as multi-stranded β-barrels ▪ Porins are found in the outer membrane of Gram-negative bacteria such as E. coli (similar pores are also found in mitochondria and chloroplasts) Minimum number of β-strands required to form a barrel is 8, but they can be bigger, & are usually 16. Eukaryotic Ion Channels Ligand-gated versus voltage-gated Figure 12.33, Page 444 Lehninger 8e Voltage-Gated Mammalian K+-channels Additional protein domains sense membrane potential Structural basis for voltage-gating S4 helix contains highly-conserved Arg residues. Normally, negative potential inside pulls S4 inside. Depolarization draws S4 to extracellular side. Although K+ is present in the closed channel, the pore closes on the bottom, near the cytosol, preventing K+ passage. Mammalian channels are organized similarly to the bacterial K+ channel, with extra domains that change conformation with membrane potential and open/close the channel Voltage-gated Na+ channels of neurons Variety of quaternary structures, but only principal subunit alpha is essential, with 4 homologous domains shown here. In each domain, Hlx4 has high-density of positivelycharged residues. Ligand-gated ion channels Acetylcholine-receptor ion channel 5 subunits, alpha2-beta-gamma-delta (α2βγδ), arranged in a ring, each with 4 membrane-spanning helices, M1-M4 transmembrane channel is lined by 5 M2 α-helices, one from each subunit Mechanism of ligand-gating Na+ selectivity dictated by rings of negative side chains at top and bottom entrance to the channel blocked by curare (poison arrows), bungarotoxin (snake venom) – competitive antagonists Acetylcholine & other neurotransmitters All of these neurotransmitters trigger the opening of cation (Na+, K+, Ca2+) channels. acetylcholine binding to receptor ion channel in membrane of post-synaptic nerve cell opens the channel Na+ ions flow down concentration gradient → reduction in membrane potential from −75 mV to 0 mV (depolarization) propagates electrical impulse Aquaporins Form Hydrophilic Transmembrane Channels for the Passage of Water Aquaporins (AQPs) = provide channels for movement of water molecules across plasma membranes – each protein has a specific location and role – low activation energy suggests that water moves in a continuous stream – do not allow passage of protons (hydronium ions, H3O+) The 11 Known Aquaporins in Mammals Table 11-3 Permeability Characteristics and Predominant Distribution of Known Mammalian Aquaporins Aquaporin Permeant (permeability) Tissue distribution Primary subcellular distribution AQP0 Water (low) Lens Plasma membrane AQP1 Water (high) Erythrocyte, kidney, lung, vascular endothelium, brain, eye Plasma membrane AQP2 Water (high) Kidney, vas deferens Apical plasma membrane, intracellular vesicles AQP3 Water (high), glycerol (high), urea (moderate) Kidney, skin, lung, eye, colon Basolateral plasma membrane AQP4 Water (high) Brain, muscle, kidney, lung, stomach, small intestine Basolateral plasma membrane AQP5 Water (high) Salivary gland, lacrimal gland, sweat gland, lung, cornea Apical plasma membrane AQP6 Water (low), anions (NO−3>Cl−) Kidney Intracellular vesicles AQP7 Water (high), glycerol (high), urea (high) Adipose tissue, kidney, testis Plasma membrane AQP8 Water (high) Testis, kidney, liver, pancreas, small intestine, colon Plasma membrane, intracellular vesicles AQP9 Water (low), glycerol (high), urea (high) Liver, leukocyte, brain, testis Plasma membrane AQP10 Water (low), glycerol (high), urea (high) Small intestine Intracellular vesicles