The Structure of Biological Membrane - Essay Example

The Structure of Biological Membrane Outline Introduction 2. Membrane transportation 2 General principles 2.2. Passive transportation 2.3. Active transportation 2.4. Transporters grouping 3. Membrane channeling 3.1. Voltage-gated ion channels 3.2. Ligand-gated ion channels 3.3. Aquaporins and hydrophilic transmembrane channels 4. Conclusion 1. Introduction It is well known that biologic membranes define the external boundaries of cells and regulate the molecular interchange across these boundaries. The biological activities of membranes are strongly connected with their physical properties, e.g. self-sealing, flexibility, and selective permeability to polar solutes. For example, the cell's outer membrane is a selective barrier. It is responsible for the transportation of certain chemical substances from a relatively disorganized environment into the more orderly cellular interior (McKee 2004, p.22). Because membranes are selectively permeable, they retain certain compounds and ions within cells and within specific cellular compartments, while excluding others. However, membranes are not only passive barriers. They include a set of proteins specialized for promoting or catalyzing various cellular processes (Nelson & Cox 2004, p.369). Molecular transportation is the most important function of membranes facilitated by carrier and channel proteins (McKee 2004, p.62). 2. Membrane transportation General questions of transportation and channeling across biologic membranes are well considered in the encompassing textbooks of Lodish (2003, p.245-300), Nelson & Cox (2004, p.369-420), Kuchel & Ralston (1997, p.171-184), McKee (2004, p.353-366), Garrett & Grisham (1999, p.259-326), etc., in special monographs, e.g. Keizer (2000) and also in a variety of articles. All cells acquire from its environs the raw materials for biosynthesis and for energy production, and also release to its environment the byproducts of metabolism. Only some nonpolar compounds can cross the membrane unassisted. However, for polar or charged compounds or ions, a membrane protein is essential for transmembrane movement. 2.1. General principles Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be regulated to meet each cell's metabolic needs. For example, a cell's plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers of membranes are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes (McKee 2004, p.372). In the simplest cases a membrane protein facilitates the diffusion of a solute down its concentration gradient. However, transportation often occurs against a gradient of concentration, electrical charge, or both. In such cases, solutes must be "pumped" that requires energy. The necessary energy may come from ATP hydrolysis (i.e. directly), or may be supplied indirectly, e.g. in the form of movement of another solute down its electrochemical gradient with enough energy to carry another solute up its gradient (Nelson & Cox 2004, p.389; Garrett & Grisham 1999, p.296). Ions may also move across membranes via ion channels formed by proteins, or they may be carried across by ionophores, small molecules that mask the charge of the ions and allow them to diffuse through the lipid bilayer of membrane. With very few exceptions, the traffic of small molecules across the plasma membrane is mediated by proteins such as transmembrane channels, carriers, or pumps (Nelson & Cox 2004, p.391). So, ions cannot pass freely through the cell's phospholipid membrane. Instead, most ions flow through special channels built from multiple protein subunits. These subunits together form a pore across the membrane. Some channels are gated, fitted with proteins that act "as hinged doors, blocking the opening until stimulated to swing out of the way" (Robinson I 2003, p.109). For example, neurons have gated sodium channels that open to allow an electrical impulse to pass and then close to recharge the cell. Also molecules can be transported across the membrane by protein pumps that are ATP powered. Transportation of single molecules can also be powered indirectly, by coupling their movement to the flow of another substance. In addition to crossing the membrane directly, water passes through special channels formed by aquaporin (see section 3.3). 2.2. Passive transportation Mechanisms of transportation and channeling across membranes are classified according to whether they require energy. Following sections are based primarily upon considerations of Nelson & Cox (2004, p.389-391) and also Garrett & Grisham (1999, p.297-298). In general, the direction in which a charged solute tends to move spontaneously across a membrane depends on both the chemical gradient (the difference in solute concentration) and the electrical gradient across the membrane (so-called membrane potential). Together, these two factors are referred to as the electrochemical gradient or electrochemical potential. To pass through a lipid bilayer of membrane, a polar or charged solute must first give up its interactions with the water molecules in its hydration shell, then diffuse about 3 nm through a solvent (lipid) in which it is poorly soluble. The energy used to strip away the hydration shell and to move the polar compound from water into and through lipid is regained as the compound leaves the membrane on the other side and is hydrated. However, the intermediate stage of membrane passage is a high-energy state. An activation barrier must be overcome to reach the intermediate stage. The energy of activation for translocation of a polar solute across the bilayer is so large that pure lipid bilayers are virtually impermeable to polar and charged species. Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path through the bilayer for specific solutes. This facilitated diffusion across membrane is called passive transportation, and appropriate membrane proteins are called transporters or permeases. Transporters bind their substrates with stereochemical specificity through multiple weak, noncovalent interactions. The negative free-energy change associated with these weak interactions, counterbalances the positive free-energy change that accompanies loss of the water of hydration from the substrate, thereby lowering the energy of activation for transmembrane passage. Transporters span the lipid bilayer several times, forming a transmembrane channel lined with hydrophilic amino acid side chains. The channel provides an alternative path for a specific substrate to move across the lipid bilayer without its having to dissolve in the bilayer, further lowering the energy of activation for transmembrane diffusion. The result is an increase of several orders of magnitude in the rate of transmembrane passage of the substrate. 2.3. Active transportation Active transportation results in the accumulation of a solute above the equilibrium point. Active transport is thermodynamically unfavorable and takes place only when coupled (directly or indirectly) to an exergonic process, e.g. absorption of sunlight, an oxidation reaction, the breakdown of ATP, the concomitant flow of some other chemical species down its electrochemical gradient. In primary active transport, solute accumulation is coupled directly to an exergonic chemical reaction. Secondary active transport occurs when endergonic (uphill) transport of one solute is coupled to the exergonic (downhill) flow of a different solute that was originally pumped uphill by primary active transport (Kuchel & Ralston 1998, p.172-181). So, the Na+-K+ pump (also referred to as the Na+-K+ ATPase) is a prominent example of a primary transporter. Also, transmembrane ATP-hydrolyzing enzymes use the energy derived from ATP to drive the transport of ions or molecules. In secondary active transport, concentration gradients generated by primary active transport are harnessed to move substances across membranes. For example, the Na+ gradient created by the Na+-K+ ATPase pump can be used to transport D-glucose. The Na+-K+ pump uses at least one-third of available energy to pump Na+ out of and K+ into the cell. For every molecule of ATP hydrolyzed, three Na+ ions are pumped out of the cell and two K+ ions are pumped into the cell (McKee 2004, p.22, 381). Movement of ion without an accompanying counterion results in the endergonic separation of positive and negative charges, producing an electrical potential. This is so-called electrogenic process of transportation. The energetic cost of moving an ion depends on the electrochemical potential, the sum of the chemical and electrical gradients (see section 2.2). 2.4. Transporters grouping Transporters are very significant part of proteins. Examination of the many transporter genes reveals obvious sequence similarities among subsets of transporters. Similar amino acid sequences in proteins generally reflect similar three-dimensional structures and, often, similar mechanisms of action. A graph tree in which proteins are grouped together based on sequence homologies has the potential to disclose the transport properties of individual proteins on that tree. According to Nelson & Cox (2004, p.392-393), when "this tree is combined with knowledge of structure, specificity, or mechanism, we have a very useful and relatively simple representation of the huge group of transporters". Transporters can be classified into super-families, whose members have considerable similarity of sequence and might therefore be expected to share structural and functional properties. There are two very broad categories of membrane transporters: carriers and channels. Carriers bind their substrates with high stereospecificity, catalyze transport at rates well below the limits of free diffusion, and are saturable, i.e. there is some substrate concentration above which further increases will not produce a greater rate of activity. Channels generally allow transmembrane movement at rates several orders of magnitude greater than those typical of carriers, rates approaching the limit of unhindered diffusion. Channels typically show less stereospecificity than carriers and are usually not saturable. Most channels are oligomeric complexes of several, often identical, subunits, whereas many carriers function as monomeric proteins. Within each of these categories are superfamilies of various types, defined not only by their primary sequences but by their secondary structures. Some channels are constructed primarily of helical transmembrane segments, others have -barrel structures. Among the carriers, some simply facilitate diffusion down a concentration gradient; they are the uniporter superfamily. Others (active transporters) can drive substrates across the membrane against a concentration gradient, some using energy provided directly by a chemical reaction (primary active transporters) and some coupling uphill transport of one substrate with the downhill transport of another (secondary active transporters). 3. Membrane channeling Ion-selective channels provide specific mechanism for moving inorganic ions across membranes. Ion channels, together with ion pumps such as the Na+-K+ ATPase (see section 2.3), determine a plasma membrane's permeability to specific ions and regulate the concentration of ions and the membrane potential. For example, very rapid changes in the activity of ion channels in neurons cause the changes in membrane potential (the action potentials) that carry signals from one end of a neuron to the other. In myocytes, rapid opening of Ca2+ channels releases the Ca2+ that triggers muscle contraction (Nelson & Cox 2004, p.408; Lodish 2003, p.287). Ion channels are differentiating from ion transporters. Indeed, the rate of flux through ion channels (107 ions per second) can be several orders of magnitude greater than the turnover number for a transporter. Then, ion channels are not saturable (see section 2.4). And finally, they are "gated", i.e. opened or closed in response to some cellular event. So, biological systems control transfer of charges through membranes by providing high dielectric channels (e.g. membrane proteins or ion carriers). The channel, with its permanent dipoles can change the barrier for ion transfer to virtually any value required. 3.1. Voltage-gated ion channels In voltage-gated ion channels, a change in transmembrane electrical potential causes a charged protein domain to move relative to the membrane, opening or closing the ion channel. Both types of gating can be very fast. A channel typically opens in a fraction of a millisecond and may remain open for only milliseconds, making these molecular devices effective for very fast signal transmission (Lodish 2003, p.276-279). The voltage-gated calcium channel is found in all eukaryotes and in a wide variety of cell types. The cells in multicellular animals have a second, voltage-gated channel that is selective for sodium instead of calcium ions. Principles of its operations are similar. For example, let's consider the voltage-gated sodium channel (Lodish 2003, p.254). When the transmembrane voltage is -70 mV, the voltage-gated sodium channel is gated shut. When the plasma membrane is depolarized, the channel opens rapidly and then, after about 1 ms, nactivates. After the channel has gone through this cycle, it must spend at least 1 ms with the transmembrane voltage at the resting voltage before it can be opened by a second depolarization (Bolsover et al 2004, p.293-301). Although enough sodium ions move into the cell to dramatically change the transmembrane voltage, the concentration of sodium ions inside the cell is increased only very slightly (Lodish 2003, p.275). 3.2. Ligand-gated ion channels In ligand-gated channels (which are generally oligomeric), binding of an extracellular or intracellular small molecule forces an allosteric transition in the protein, which opens or closes the channel. For instance, nicotinic acetylcholine receptors (nAChRs) are well known ligand-gated ion channels, and their activation, by acetylcholine (ACh) or other agonists, causes a rapid change in ion permeability of the membrane in which they are embedded (Creighton 1999, p.44) Studies on the nAChR revealed a number of noncompetitive antagonists, both natural and synthetic, that blocked the change in permeability elicited by ACh without inhibiting the binding of ACh itself. It was, therefore, suggested that at least some of these compounds act by entering the ion channel and sterically blocking ion movement through it. The opening in the center of this ion channel is about 20 at its entrance, narrowing sharply at the membrane surface (Creighton 1999, p.48). 3.3. Aquaporins and hydrophilic transmembrane channels A family of integral proteins, the aquaporins (AQPs), provide channels for rapid movement of water molecules across all plasma membranes (Fu et al 2000). Water molecules flow through an AQP-1 channel at the rate of about 109 per second. For comparison, the highest known turnover number for an enzyme is that for catalase, 4107 per second. The low activation energy for passage of water through aquaporin channels (less then 15 kJ/mol) suggests that water moves through the channels in a continuous stream, in the direction dictated by the osmotic gradient. Aquaporins not allow passage of protons (hydronium ions), which would collapse membrane electrochemical potentials (Sui et al, 2001). AQP-1 has four monomers associated in a tetramer, each monomer forming a transmembrane pore with a diameter sufficient to allow passage of water molecules in single file. Each monomer consists of six transmembrane helical segments and two shorter helices, each of which contains the sequence Asn-Pro-Ala (NPA). The NPA-containing short helices extend toward the middle of the bilayer from opposite sides, with their NPA regions overlapping in the middle of the membrane to form part of the specificity filter - the structure that allows only water to pass. The residues that line the channel of each AQP-1 monomer are generally nonpolar, but carbonyl oxygens in the peptide backbone, projecting into the narrow part of the channel at intervals, can form hydrogen bonds with individual water molecules as they pass through; the two Asn residues in the NPA loops also hydrogen-bond with the water. The structure does not admit closely spaced water molecules that might form a chain to allow proton hopping, which would effectively move protons across the membrane. Critical Arg and His residues and electric dipoles formed by the short helices of the NPA loops provide positive charges in positions that repel any protons that might leak through the pore. 4. Conclusion So, the cell membranes define the external boundaries of cells and control the molecular interchange across these boundaries. Membranes contain proteins specialized for catalyzing various cellular processes. Nonpolar compounds can cross the membrane unassisted, but for transmembrane movement of charged compounds or ions a membrane protein is essential. Membrane transport mechanisms are classified as passive or active according to whether they require energy. Ions may move across membranes via ion channels formed by proteins, or they may be carried across by ionophores, small molecules that mask the charge of the ions and allow them to diffuse through the lipid bilayer. Transporters can be classified into so-called super-families, whose members have considerable similarity of sequence. There are two very broad categories of transporters: carriers and channels. Channels generally allow transmembrane movement at rates several orders of magnitude greater than those typical of carriers, rates approaching the limit of unhindered diffusion. Ion channels are distinguished from ion transporters. Ion channels provide hydrophilic pores through which select ions can diffuse, moving down their electrical or chemical concentration gradients; they are characteristically unsaturable and have very high flux rates. Many ion channels are highly specific for one ion, and most are gated by either voltage or a ligand. Membrane transport mechanisms are classified as passive or active according to whether they require energy. In passive transport, solutes moving across membranes move down their concentration gradient. In active transport, energy derived directly or indirectly from ATP hydrolysis or other energy sources is required to move an ion or molecule against its concentration gradient. Bibliography 1. Bolsover, SR, Hyams, JS, Shephard, EA, White, HA & Wiedemann, CG 2004, Cell Biology: a Short Course, 2nd edn, John Wiley & Sons, New York. 2. Creighton, TE 1999, Encyclopedia of Molecular Biology [4 vol.], John Wiley & Sons, New York. 3. Fu, D, Libson, A, Miercke, LJV, Weitzman, C, Nollert, P, Krucinski, J & Stroud, RM 2000, 'Structure of a Glycerol-Conducting Channel and the Basis for Its Selectivity', Science, vol. 290, pp. 481-486. 4. Garrett, RH & Grisham, CM 1999, Biochemistry, 2nd edn, Saunders College Publishing. 5. Keizer, J 2000, Computational cell biology, Springer Verlag, New York. 6. Kuchel, PW & Ralston, GB 1998, Schaum's Outline of Theory and Problems of Biochemistry, 2nd edn, McGraw-Hill, New York. 7. Lodish, H 2003, Molecular Cell Biology, 5th edn, Freeman, New York. 8. McKee, T & McKee, JR 2004, Biochemistry: the Molecular Basis of Life, 3rd edn, The McGraw-Hill, New York. 9. Nelson, DL & Cox, MM 2004, Lehninger Principles of Biochemistry, 4th edn, Worth Publishers, New York. 10. Robinson, R 2003, Genetics [4 vol.], Macmillan Reference USA, New York. 11. Schirmer, T 1998, 'General and Specific Porins from Bacterial Outer Membranes', Journal of Structural Biology, vol. 121, pp. 101-109. 12. Sui, H, Han, BG, Lee, JK, Walian, P & Jap, BK 2001, 'Structural Basis of Water-Specific Transport Through the AQP1 Water', Nature, vol. 414, pp. 872-878. 13. Unger, VM, Kumar, NM, Gilula, NB & Yeager, M 1999, 'Three-Dimensional Structure of a Recombinant Gap Junction Membrane Channel', Science, vol. 283, pp. 1176-1180. Read More