Nutrient Uptake in Bacterial Cells PDF

2-5: Transport in Bacterial Cells Lecture Overview: • How nutrients and other molecules get into bacterial cells • Textbook: Chapter 2.2 The cytoplasmic membrane: The gatekeeper of the cell o Few molecules move freely across the cytoplasmic membrane o Small uncharged, non-polar molecules o E.g. -...

2-5: Transport in Bacterial Cells Lecture Overview: • How nutrients and other molecules get into bacterial cells • Textbook: Chapter 2.2 The cytoplasmic membrane: The gatekeeper of the cell o Few molecules move freely across the cytoplasmic membrane o Small uncharged, non-polar molecules o E.g. - Dissolved O2, dissolved CO2, small alcohols/fatty acids Important for metabolism depends rate o Some molecules can cross at a meaningful rate (but significantly hindered by the membrane). E.g. H2O, glycerol, some amino acids… d I -Polar - ↳ some molec - for water a g naporins have o The membrane is essentially impermeable to many other molecules (large/charged molecules, Na+/K+) - ↳) ability a chargesreallykill o Cell dedicates significant resources to ensuring that molecules it needs (such as energy sources) get in, but potentially harmful substances do not How molecules enter bacterial cells: Passive transport (requires no energy from the cell) – molecules enter cell by moving down concentration gradient: o Simple diffusion o Facilitated diffusion Active transport (requires energy) – uptake against concentration gradient: o Simple transport o ABC transporters o Group transport Diffusion reminder Diffusion: The net movement of a chemical down it’s concentration gradient (from area of high concentration to area of low concentration). Entropically favorable! Diffusion requires no energy from the cell – concentration gradient represents a source of potential energy Osmosis is the diffusion of water through a selectively permeable membrane along its concentration gradient. (A low concentration of solutes = a high concentration of water) Some molecules (e.g. oxygen) can enter cell via simple diffusion Facilitated diffusion o Facilitated diffusion: Diffusion of molecules across the membrane via a membrane protein that acts as a channel. Porins of OM. o Can be specific -- recognition element for specific molecule(s) – only those molecules can pass – carrier proteins o Can be nonspecific (less specific) -- allow passage of molecules of a certain size/charge – channel proteins o Generally, the production and/or activity of these channels is usually regulated by the cell Textbook video screenshot Active transport: overview Any time a molecule is transported against its concentration PEP gradient, this requires energy Grove ~ Or -> in ansport This can come from stored chemical energy (e.g. ATP hydrolysis) or from dissipation of another concentration gradient (transporting another molecule along its concentration gradient) Textbook, Fig. 2.6 Simple transport: Symporters & antiporters Symporters & antiporters use the energy stored in chemical gradients (often, the proton motive force) to power the transport of a different molecule against its gradient Symport: Both molecules travel same direction (allowing protons in powers uptake of another chemical) Antiport: One molecule in, the other out (allowing protons in powers efflux of a molecule from the cell) Textbook, Fig. 2.6 Sodium proton antiporter Exchanges protons for Na+ ions. What would be the purpose of this?? Main function = pH (and Na+) homeostasis! Expel Na+ from cell under high salt conditions Lower pH of cell under alkaline conditions Transporters such as this are essential for maintaining homeostasis of cell. Textbook, Fig. 2.6 Lac permease symporter Proton motive force is exploited to drive the uptake of lactose and some related disaccharides (high energy food source) into the cell Example of how this type of transport can be used to acquire nutrients/energy Textbook, Fig. 2.6 Active transport: Group translocation Transported substance is bound by a transporter and is chemically modified during transport Classic example is glucose uptake using the phosphotransferase system. This system also transports other sugars Energy provided by hydrolysis of high energy phosphate bond in phosphoenolpyruvate (PEP) Phosphorylation of the sugar molecule is helpful metabolically (adding phosphate often first step in its metabolism) Textbook, Fig. 2.6 Active transport: ABC transporters ATP binding cassette (ABC transporters) use ATP to power the transport of substances across the cytoplasmic membrane One of largest/oldest gene families – found from bacteria to humans (all phyla on earth!). Very wide range of different molecules transported by different ABC transporters o 2 ATPase domains (proteins) provide energy o Transmembrane domain(s) (proteins) provides selective channel o Substrate binding protein binds molecule with high affinity and delivers it to the channel Related transporters involved in export of molecules Textbook, Fig. 2.6 Substrate binding proteins (periplasmic binding proteins) Prokaryotic ABC transporters are best studied in Gram-negative bacteria – which use “periplasmic binding proteins” to capture their ligand within the periplasm In archaea and Gram positive bacteria (lack OM) – the substrate binding protein is tethered to the cytoplasmic membrane Heide and Poolman, EMBO reports, 2002 ABC transporter example: Vitamin B12 Structure of vitamin B12 Vitamin B12 is a large, complicated, precious and scarce molecule Too large to diffuse through porins in OM Many bacteria (E. coli is model system) use transporters to efficiently take up B12 OM barrel protein BtuB binds B12 with high affinity, transports across OM using energy from TonB complex (via proton motive force) BtuCD-F = ABC transporter for uptake across cytoplasmic membrane – BtuF has very high affinity for B12 Above: Gruber etal, chemical society reviews, 2011 Below: Fowler etal, Chemistry & Biology, 2010 Vitamin B12 uptake – nicer picture Similar uptake systems are used for the uptake of other large/precious molecules Iron-binding siderophores (molecules secreted to capture precious iron) are taken up in a synonymous manner Gruber etal, chemical society reviews, 2011