Transcript Summary
PROTEINS AND MEMBRANES Proteins: Recall protein secondary structures: -helices and ß-sheets Proteins in membranes Integral: membrane-spanning (generally Nterminal out in plasma membrane) Peripheral: associated with membrane surface (often associated with membranespanning protein or membraneintercalating lipid) Proteins in membranes Bacteriorhodopsin: note the seven membrane-spanning -helices Maltoporin: a ß-barrel protein, with membrane-spanning ß sheets Although they are not common, there are several ß-barrel proteins known-primarily trans-membrane transport proteins. Unilateral membrane embedding may occur through hydrophobic amino acids Or connection to glycolipids Folding of membrane protein during synthesis is complex: Folding of lactose permease depends on the lipid composition of the membrane. Folding of aquaporin involves changes in the orientation of alphahelices. (Science 339:398, 25 January 2013) Membranes are heterogeneous: Membrane “rafts” are accumulations of proteins, stabilized by glycosphingolipids and cholesterol. (Science 327:46, 1 January 2010) The purpose of membranes is to control transport into and out of the cell or from one cell compartment to another. How molecules cross membranes 1. Dissolving in lipid layer Small non-polar molecules (benzene, ethanol, O2, CO2) Works for artificial lipid bilayers (i.e., no proteins) 2. Pores in lipid layer Small polar molecules (H2O, NH3) Also works for lipid bylayers Postulate transient pores 3. Channels, Carriers, Pumps Specific materials transported Specific membranes Regulated in time and sometimes direction Proteins Channels Protein complex forming controlled hole for rapid flow "DownhillУ(along free-energy gradient) ТGatedУ opens, ( closes in response to stimulus) Channels known for Na+, K+, Cl-, Ca2+, and others, including H2O (aquaporins)! Example of a channel: A nerve impulse involves depolarization, followed by re-polarization. Nerve cells’ electrical polarity results from coupled Na+ efflux and K+ influx. Depolarization results from Na+ influx (opened Na+ channels). Re-polarization results from K+ efflux (opened K+ channels). Potassium channel: responsible for re-polarizing nerve cells after a nerve impulse. Note the 3-angstrom constriction with negatively charged groups. How does this channel control K+ movement, and why is it specific for K+? This is the slide used to explain the ability of enzymes to catalyze chemical reactions: does this apply to the operation of channels? Enzymes bind to substrates, so G(ES) < G(E+S). However, if all they did was to bind, then G for the reaction would not be reduced. So when they bind the substrate, they stress It in some way, raising G(ES) and reducing G(ES*)(=Ea). The specificity of the K+ channel for K+, relative to Na+, depends on desolvation and resolvation energy. What would explain the specificity of the Na+ channel? From Science, 3/12/2010: “Pain’s in the genes”: “Subtle changes to a certain gene seem to determine how sensitive people are to pain, according to new research. In the past 5 years, researchers have discovered that three rare but serious pain disorders are caused by mutations in a gene called SCN9A. In nerve cells that relay painful sensations in the body's tissues to the central nervous system,…” How many of you are particularly sensitive to pain? …particularly insensitive? What do you think the mutation affected? (a) A membrane lipid (b) A channel (c) A carrier (d) A pump In nerve cells that relay painful sensations in the body's tissues to the central nervous system, SCN9A encodes instructions for sodium channels that help the cells fire. In two of the disorders, people carry faulty versions of the gene and suffer crippling pain because their sodium channels open too easily or can't close. In the third disorder, which leaves patients unable to feel pain at all, SCN9A produces a protein that can't function. "We wondered if more common, apparently harmless [changes] in the gene might give rise to an altered degree of pain threshold," says Geoffrey Woods, a medical geneticist at Cambridge University in the U.K., who discovered the genetic reason for this third disorder. One [genetic variant], found in 10% of the subjects, caused the greatest increase in reported pain between those who had it and those who didn't. When the team applied heat stimuli to 186 healthy women, they found that those with the rare version were more likely to have lower pain thresholds. It was as if the normal subjects had taken an ibuprofen, but the subjects with the rare SNP hadn't. Mechanosensitive channels in bacteria: these open in response to high tension (Science 321:1166, 29 August 2008) Summary •Intrinsic membrane proteins generally cross the membrane with an -helix •Some membrane pore proteins use a ß-barrel structure •Membrane proteins can associate in “rafts” stabilized by sterols and sphingolipids •Hydrophilic molecules, including ions, cross membranes through channels and pumps •Nerve function (and other functions) depend on control of channel opening •The specificity of the potassium channel depends on solvationdesolvation energy •Mutations in channel proteins influence ease of nerve stimulation