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Set: Tuesday 11th Biology of Cells November 2014 Illustrate with examples how proteins mediate different types of transport across biological membranes. Generally smaller non-polar hydrophobic molecules with high lipid solubility are able to diffuse through the phospholipid bilayer passively without aid from transport proteins, as long as a concentration gradient is present. For other molecules, including those that are hydrophilic, charged, large or polar, transport through membranes must be mediated by proteins. The main categories of protein mediated transmembrane transport are passive and active transport, each of which have subtypes. Facilitated diffusion is protein mediated passive transport. Protein mediated transport can be passive when there is a electrical or concentration gradient present, and the net movement of solutes from a higher to lower concentration is driven by the chemical (or electrochemical in the case of charged particles) potential gradient of the molecules. This movement across membranes occurs spontaneously since the free energy change is negative, so the process is exergonic. There are two main types of facilitated diffusion; that mediated by carrier proteins and that mediated by channel proteins. The general mechanism of action of carrier proteins to transport molecules passively across membranes is the solute binds its carrier at specific binding sites on the side of greater concentration, which induces a series of conformational change in shape. Then the carrier releases the solute on the other side of the membrane, and returns to its original state. An example of passive transmembrane transport by carrier proteins is by the plasma membrane protein GLUT 1, which is used to facilitate the uptake glucose into cells. GLUT 1 can also act as a vitamin C carrier protein; this illustrates that although transmembrane transport proteins can be very specific, as in the following example of a channel protein, the specificity of these transport proteins can vary. Channel proteins, which largely mediate the transport of ions across membranes, function by providing a hydrophilic pore through which molecules can move. Their selectivity for particular molecules is determined by their surface charge, the diameter of the selectivity filters, and which amino acid residues line the interior of the channels, meaning only molecules of the right size and charge may pass through. Solutes exit the pores through a ‘gate’, which may be opened and closed in response to a particular stimulus, such as voltage, mechanical stress or the binding of a ligand. For example, voltage gated K+ ion channels, which are sensitive to the cell membrane potential. Their selectivity filter, present at the most narrow part of the pore, has a specific amino acid sequence which interacts with the K+ ions, allowing them to pass through the channel down their electrochemical gradient, after causing them to disassociate with water molecules. Smaller positive ions cannot pass through, as the amino acids are unable to interact with them in a way to cause water to disassociate. They open and close in response to changes in membrane potential, triggered by amino acids that act as voltage sensors. Ion channels like these are used when rapid ion transport is required, such as in muscle contraction, and transmission of action potentials in nerve fibres. Set: Tuesday 11th Biology of Cells November 2014 While passive transport moves molecules down their concentration or electrochemical gradient, active transport occurs when proteins are required to move solutes through membranes against a gradient (chemical or electrochemical), a process with a positive free energy change, meaning energy input is required to drive these endergonic processes. All active transport across membranes is protein mediated. The two main types of active transport are primary and secondary. In primary active transport the input of energy used to move solutes across the membranes comes directly from hydrolysis of energy donors, most commonly ATP. In general, the transport protein catalyses the hydrolysis of ATP, GTP or PPi and the transport of the solutes across the membrane; the processes are coupled. Each energy donor has one or more unstable energy rich phosphoanhydride bond, which when hydrolysed release energy used to move solutes against their concentration gradient. An example of a protein that carries out this type of transport is the Na+-K+ translocating ATPase, which is present in plasma membranes. It pumps Na+ out of the cytoplasm, and K+ in. Per ATP molecule hydrolysed, 3Na+ ions are pumped out and 2K+ are pumped in, both moving against their electrochemical potential gradients. When Na+ binds to the protein, and it is phosphorylated using a phosphate group from an ATP molecule, a conformational change in shape is triggered, transferring Na+ across the membrane. It returns to its original shape by the binding of K+ from the extracellular fluid, and dephosphorylation, resulting in transfer of K+ into the cytoplasm. The electrochemical potential gradient set up by the Na+-K+ ATPase can be used in secondary active transport, also known as solute coupled transport. This type of protein mediated transmembrane transport uses an energy input from the movement of ions down their electrochemical potential gradient, which may have been set up by primary active transport, to transport other solutes against a gradient. If the solute is transported in the same direction as the ions that are moving down their electrochemical gradient the protein that mediates this is described as a symporter, while in opposite directions it is described as an antiporter. An example of a symporter is in Na+-couple glucose transport in the enterocytes of the small intestine. The electrochemical potential gradient established by Na+-K+ ATPases in primary active transport results in a higher concentration of Na+ ions in the lumen of the small intestine than in the cells. This results in Na+ entering the cell coupled with glucose, through a Na+-glucose symporter protein. The glucose is moving against its concentration gradient. This component of secondary active transport mediated by proteins is very similar to facilitated diffusion, as there is spontaneous movement of molecules down their chemical potential gradient. However this gradient is created by a process that uses ATP, so overall is defined as a type of active transport. The main types of protein-mediated transport across membranes include facilitated diffusion mediated by carrier and channel proteins, and primary and secondary active transport. However, there are other types of transmembrane transport by proteins of importance, such as the F-type ATPases, used in ATP Set: Tuesday 11th Biology of Cells November 2014 synthesis, that do not fit into these categories. Proteins are responsible for the transport of a range of molecules, including ions and polar molecules, across membranes. These types of transport are vital for cell and organism function, as they are used in production of ATP, homeostasis, cell volume regulation and communication, among other essential functions. Bibliography Julia Davie Lecture notes ‘Membranes - molecular superstructures’ Alberts, B. et al (2008) Molecular Biology of the Cell, 5th Edition (Garland) Berg, J., Tymoczko, J. and Stryer, L. (2011) Biochemistry, 7th Edition (Freeman) Sagun et al. (2005) ‘Vitamin C enters mitochondria via facilitative glucose transporter 1 (Glut1) and confers mitochondrial protection against oxidative injury’ FASEB J