Biochem SG 10/11
Why are there so many different transporters for glucose? What seem to be the fundamental difference between them? It is certainly not their substrate. How do we characterize transport?
-12 transporters each with unique kinetic properties, patterns of tissue distribution, and function. -There are many different types of glucose transporters because every tissue in our body has different needs and each glucose transporter therefore has to be tissue specific. That is also why each one has a different Kt (affinity for glucose). For example, GLUT1, in the liver, has a large Kt and thus can respond to increased levels of intracellular glucose by increasing outward transport while GLUT4 activity increases when insulin signals a high blood glucose concentration. The fundamental difference: Na+ independent glucose transporters (mediated by 14 subtypes; GLUT1-14) act as facilitated diffusion and does no require energy. Na+ dependent transporters (in the epithelial of the intestine) require energy to transport glucose against its gradient and Na+ is a co-transporter (symport).
What are the sugar alcohols found in sphingolipids, glycerophospholipids? Glycosphingolipids?
-?
What are ABC transporters?
-ABC transporters are a large family of ATP-dependent transporters that pump amino acids, peptides, metal ions, various lipids, bile salts, and many hydrophobic compounds (including drugs) out of cells against their concentration gradient. ABC transporters have two nucleotide-binding domains (which acts as the conserved molecular motor) and two transmembrane domains containing multiple transmembrane helices. Many ABC transporters are in the plasma membrane while others are also in the ER and the membranes of mitochondria and lysosomes. MDR1 is an ABC transporter that pumps chemotherapeutic drugs (Adriamycin, doxorubicin, and vinblastine) out of the cell and thus blocks their therapeutic effects.
Describe the structure and function of Aquaporin.
-Aquaporins are a family of integral membrane proteins that provide channels for rapid movement of water molecules across all plasma membranes. Aquaporins are responsible for: -Swelling/shrinkage of erythtocytes in response to abrupt changes in osmolarity as blood travels through the renal medulla. -Water secretion by the endocrine glands that produce sweat, saliva, and tears. -Urine production and water retention in the nephron. Structure: They have a tetramer of identical subunits, each with a transmembrane pore. Each subunit in AQP1 has six transmembrane helical segments (which form the pore through the monomer) and 2 shorter helices, both of which contain the sequence NPA (which form part of the water channel). Three conserved residues lie in the middle of the channel (Asn, proline, alanine) and are responsible for the selective passage of water molecules (selectivity filter).
In the ER there are differences in the types of phospholipids found on the cytosolic and luminal surfaces. We have discussed this in previous chapters. But, it is know that the phospholipids can flip and flop between surfaces. How is this accomplished?
-At physiological temperature, transbilayer—or "flip- flop"—diffusion of a lipid molecule from one leaflet of the bilayer to the other (Fig. 11-16a) occurs very slowly if at all in most membranes. Transbilayer movement requires that a polar or charged head group leave its aqueous environment and move into the hydrophobic interior of the bilayer, a process with a large, positive free-energy change. There are, however, situations in which such movement is essential. For example, during synthesis of the bacterial plasma membrane, phospho- lipids are produced on the inside surface of the mem- brane and must undergo flip-flop diffusion to enter the outer leaflet of the bilayer. A family of proteins, the flippases, facilitates flip- flop diffusion, providing a transmembrane path that is energetically more favorable and much faster than the uncatalyzed movement. -Flippase (P-type ATPase) moves phosphatidylethanolamine and phosphatidylserine from the outer side of the membrane to the cytosolic leaflet. Also moves newly synthesized phospholipids from their site of synthesis (ER) to the luminal leaflet. Consumes about one ATP per molecule of phospholipid translocated. -Floppase (ABC transporter) moves phospholipids from cystolic to outer leaflet. It is also ATP-dependent. -Scramblase moves any membrane phospholipids in either direction, down its concentration gradient, toward equilibrium. Thus, it is ATP-independent. Activity rises sharply with an increase in cystolic Ca2+ concentration.
Previously we have discussed v-Snare and t-Snare. But another membrane of these events are mediated by fusion proteins. Discuss the fusion process.
-At the synapsis, intracellular vesicles loaded with neurotransmitters fuse with the plasma membrane. During fusion, a v-SNARE (receptor in the cytoplasmic face of the vesicle) and a t-SNARE bind to each other and undergo a structural change that produces a bundle of long, thin rods made up of helices from both SNARES and two helices from SNAP25. The two SNAREs initially interact at their ends, then zip up into the bundles of helices. This structural change pulls the two membranes into contact and initiates the fusion of their lipid bilayers.
Although we have only discussed α-helical transmembrane proteins there are a group of proteins that span the membrane in what is called a β-barrel, discuss these proteins and how they associate with the membrane lipids.
-B barrel: in which 20 or more transmembrane segments form B sheets that line a cylinder. The same factors that favor a-helix formation in the hydrophobic interior of a lipid bilayer also stabilize B barrels. When no water molecules are available to H-bond with the carbonyl O and N of the peptide bond, maximal interchain H bonding gives the most stable conformation. Planar B sheets do not maximize these interactions and are general not found in the membrane interior; B barrels do allow all possible H bonds and are apparently common among membrane proteins. A polypeptide is more extended in the B conformation than in an a-helix; just 7-9 residues of B conformation are needed to span a membrane. Recall that in the B conformation, alternating side chains project above and below the sheet. In B strands of membrane proteins, every second residue in the membrane-spanning segment is hydrophobic and interacts with the lipid bilayer; aromatic side chains are commonly found at the lipid-protein interface. The other residues ay or may not be hydrophilic.
What are bile salts?
-Bile salts are bile acids compounded with an ion and are polar derivatives of cholesterol. Bile acids are conjugated with an amino acid: glycine or taurine. They are excreted in the duodenum where they acts as detergents in the intestine, emulsifying dietary fats to make them more readily accessible to digestive lipases, and are also potent signaling molecules in both the liver and intestine. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Taurocholic acid (taurine + cholic acid) and glycocholic acid (glycine + cholic acid) represent about 80% of all bile salts in the human body.
What is the function of the protein caveolin? What process takes place in the structure formed by caveolin?
-Caveolin is an integral membrane protein with two globular domains connected by a hairpin-shaped hydrophobic domain, which binds the protein to the cytoplasmic leaflet of the plasma membrane. Three palmitol groups attached to the carboxyl-terminal globular domain further anchor it to the membrane. Caveolin binds cholesterol in the membrane, and the presence of caveolin forces the associated lipid bilayer to curve inward, forming caveolae in the surface of the cell (They form dimers that associate with cholesterol-rich regions which forces inward curvature of the membrane to form caveolae). The involve both the leaflets of the bilayer—the cytoplasmic leaflet, from which the caveolin globular domains project, and the exoplasmic leaflet, a typical sphingolipid/cholesterol raft with associated GPI-anchored proteins. Caveolae are implicated in a variety of cellular functions, including membrane trafficking within cells (endocytosis) and the transduction of external signals into cellular responses. The receptors for insulin and other growth factors, as well as certain GTP-binding proteins and protein kinases associated with transmembrane signaling, appear to be localized in rafts and perhaps in caveolae.
We have in this chapter a further discussion of lipid rafts. How are proteins anchored in the rafts?
-Cholesterol-spingolipid microdomains in the outer membrane of the plasma membrane are slightly thicker and more ordered than their neighboring microdomains rich in phospholipids; they behave like sphingolipid rafts. These rafts are classified into two types of intergral membrane proteins: 1. Those anchored to the membrane by two covalently attached long-chain saturated fatty acids attached through Cys residues. 2. GPI-anchored proteins. These lipid anchors form more stable associations with the cholesterol and long acyl groups in rafts than with the surrounding phospholipids. -These lipid anchors, like the acyl chains of sphingolipids, form more stable associations with the cholesterol and long acyl groups in rafts than with the surrounding phospholipids. The "raft" and "sea" domains of the plasma membrane are not rigidly separated; membrane proteins can move into and out of lipid rafts in seconds. But in the shorter time scale (microseconds) more relevant to many membrane-mediated biochemical processes, many of these proteins reside primarily in a raft
Describe the ecosinoids and what are the functions of the different ecosinoid groups?
-Eicosanoids are paracrine hormones and act only on cells near the point of hormone synthesis instead of being transported in the blood to act on cells in other tissues or organs. In other words, they carry messages to nearby cells. -There are three classes of eicosanoids: Prostroglandins: Regulate the synthesis of the intracellular messenger, cAMP. Stimulate contraction of the smooth muscle of the uterus during menstruation and labor. Others affect a) blood flow to specific organs, b) the wake-sleep cycle, and c) the responsiveness of certain tissues to hormones such as epinephrine and glucagon. Prostaglandins in a third group elevate body temperature (producing fever) and cause inflammation and pain. Thromboxanes: They are produced by platelets (also called thrombocytes) and act in the formation of blood clots and the reduction of blood flow to the site of a clot. Leukotrienes: They are powerful biological signals. For example, leukotriene D4, derived from leukotriene A4, induces contraction of the muscle lining the airways to the lung. (The strong contraction of the smooth muscles of the lung that occurs during anaphylactic shock is part of the potentially fatal allergic reaction).
What is the role of carbonic anhydrase in the chloride-bicarbonate exchange?
-Erthrocyte contains another facilitated diffusion system, an anion exchanger that is essential in CO2 transport to the lungs from tissues such as skeletal muscle and liver. Waste CO2 released from respiring tissues into the blood plasma enters the erythrocyte, where it is converted to bicarbonate (HCO3-) by the enzyme carbonic anhydrase. The HCO3- reenters the blood plasma for transport to the lungs. Because HCO3- is much more soluble in blood plasma than is CO2 this roundabout route increases the capacity of the blood to carry CO2 from the tissues to the lungs. IN the lungs, HCO3- reenters the erythrocyte and is converted to CO2, which is eventually released into the lung space and exhaled. To be effective, this shuttle requires very rapid movement of HCO3- across the erythrocyte membrane. -The chloride-bicarbonate exchanger (anion exchange protein) increases the permeability of the erythrocyte membrane to HCO3- more than a millionfold. It is an integral protein and mediates the simultaneous movement of two anions: for each HCO3- ion that moves in one direction, one Cl- ion moves in the opposite direction, with no net transfer of charge, the exchange is electroneutral (called a cotransport system...substates move in opposite direction is called antiport). Carbonic anhydrase is an enzyme that catalyzes the conversion of CO2 to bicarbonate and H+. It is only expressed in red blood cells. In active tissues CO2 diffuses in the RBC where it is converted into bicarbonate, which can then be exported from the cell in exchange for a Cl- (chloride shift). H+ then binds to hemoglobin. Back in the lung, O2 increases, H+ is released and the reaction goes the other way.
Describe the structure and function of the K+ Channel.
-In excitable cells such as neurons, potassium channels shape action potential and set the resting membrane potential. They also regulate cellular processes such as the secretion of hormones (ie. Insulin release from beta-cells in the pancreas). Potassium channels may also be involved in maintaining vascular tone. Structure: The K+ channel consists of four identical subunits that span the membrane and form a cone within a cone surrounding the ion channel, with the wide end of the double cone facing the extracellular space. Each subunit has two transmembrane alpha-helices as well as a third short helix that contributes to the pore region. The outer cone is formed by one of the transmembrane helices of each subunit while the inner cone (which forms the channel for K+; surrounds the ion channel and cradles the ion selectivity filter) is formed by the other four helices. The sequence, Thr-Val-Gly-X-Gly, acts as a selectivity filter. -At the inner and outer plasma membrane surfaces, the entryways to the channel have several negatively charged amino acid residues, which presumably increase the local concentration of cations such as K+ and Na+. The ion path through the membrane begins (on the inner surface) as a wide, water-filled channel in which the ion can retain its hydration sphere Furhter stabilization is provided by the short a-helices in the pore region of each subunit, with the partial negative charges of their electric dipoles poined at K+ in the channel. About 2/3 of the way through the membrane, this channel narrows in the region of the selectivity filter, forcing the ion to give up its hydrating water molecules. Carbonyl O atoms in the backbone of the selectivity filter replace the water molecules in the hydration sphere, forming a series of perfect coordination shells through which the K+ moves. This favorable interaction with the filter is not possible for Na+ which is too small to make contact with all the potential O ligands. The preferential stabilization of K+ is the basis for the ion selectivity of the filter, and mutations that change residues in this part reduce selectivity. There are four potential K+ binding sites along the selectivity filter, each composed of an O cage that provides ligands for the K+ ions. IN the crystal structure, two K+ ions are visible within the selectivity filter and two water molecules occupy the unfilled positions. K+ ions pass through the filter in single file; their mutual electrostatic repulsion most likely just balances the interaction of each ion with the selectivity filter and keeps them moving. Movement of the 2 K+ ions is concerted: first they occupy positions 1 and 3, then they hop to positions 2 and 4. The Energetic difference btwen the two configurations is very small. The selectivity pore is a flat surface, which is ideal for rapid movement through the channel
What is the difference between primary and secondary active transport? Give an example of both, and characterize the process of each transporter.
-In primary active transport, the transporter itself hydrolyzes ATP to drive solute movement against an electrochemical gradient. Na+/K+ ATPase (pump). It exports 3 Na+ out and 2 K+ inside simultaneously. -In secondary active transport, it does not use ATP directly. First a gradient of ion X (often Na+) is established by primary active transport. The movement of solute X down its concentration gradient now provides the energy to drive cotransport of the desired solute against its electrochemical gradient. An example is Na+ dependent glucose transport. It uses the concentration of Na+ outside of the cell to move inside with its gradient, which provides energy for the glucose to enter the cell.
Describe how protein associates with membranes.
-Membrane proteins may be divided operationally into two groups (Fig. 11-6). Integral proteins are very firmly associated with the membrane, removable only by agents that interfere with hydrophobic interactions, such as detergents, organic solvents, or denaturants. Peripheral proteins associate with the membrane through electrostatic interactions and hydrogen bond- ing with the hydrophilic domains of integral proteins and with the polar head groups of membrane lipids. They can be released by relatively mild treatments that interfere with electrostatic interactions or break hydrogen bonds; a commonly used agent is carbonate at high pH. Peripheral proteins may serve as regulators of membrane-bound enzymes or may limit the mobility of integral proteins by tethering them to intracellular structures. Integral proteins are asymmetric (one side on the interior and one side on the exterior) and the hydrophobic portions will have secondary structure a-helixes. -Some membrane proteins contain one or more covalently linked lipids of several types: long-chain fatty acids, isoprenoids, sterols, or glycosylated derivatives of phosphatidylinositol, GPI (Fig. 11-14). The attached lipid provides a hydrophobic anchor that inserts into the lipid bilayer and holds the protein at the membrane surface. The strength of the hydrophobic interaction between a bilayer and a single hydrocarbon chain linked to a protein is barely enough to anchor the protein securely, but many proteins have more than one attached lipid moiety. Other interactions, such as ionic attractions between positively charged Lys residues in the protein and negatively charged lipid head groups, probably contribute to the stability of the attachment. The association of these lipid-linked proteins with the membrane is certainly weaker than that for integral membrane proteins and is, in at least some cases, reversible. But treatment with alkaline carbonate does not release GPI-linked proteins, which are therefore, by the working definition, integral proteins.
Discuss the different types of cellular transporter families.
-Membrane proteins that speed up the movement of a solute across a membrane by facilitating diffusion are called transporters (or permeases). Transporters bind their substrates with stereochemical specificity through multiple weak, noncovalent interactions. The negative free-E change associated with these weak interactions, delta G binding, counterbalances the positive free-E change that accompanies loss of the water of hydration from the substrate, delta G dehydration, thereby lowering delta g ++ 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 alternate path for a specific substrate to move across the lipid bilayer without its having to dissolve in the bilayer, further loweing delta G ++ for transmembrane diffusion (increase in the rate of passage of the substrate). -Carriers bind their substrates with high stereospecificity, catalyze transport at rates well below the limits of free diffusion, and are saturable in the same sense as are enzymes: there is some substrate concen- tration 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 -Among the carriers, some sim- ply facilitate diffusion down a concentration gradient; they are the uniporter superfamily. Others (active trans- porters) can drive substrates across the membrane against a concentration gradient, some using energy provided directly by a chemical reaction (primary ac- tive transporters) and some coupling uphill transport of one substrate with the downhill transport of another (secondary active transporters). -Simple Diffusion: when nonpolar compounds move from a region of higher concentration to lower concentration, until the two compartments have equal solute concentrations. No energy is used. Facilitated Diffusion: involves a protein that acts as a pore to transport the molecule across the membrane in the direction of the gradient. No energy is used. Primary Active: the energy released by ATP hydrolysis drives solute movement against an electrochemical gradient. Secondasry Active: the movement of a solute X down its concentration gradient now provides the energy to drive cotransport of the desired solute against its electrochemical gradient Ion Channel: Ionophore-mediated Ion Transport:
Describe the difference between a lipid bilayer versus a micelle.
-Micelles are spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules. These molecules are arranged with their hydrophobic regions aggregated in the interior, where water is excluded, and their hydrophilic head groups at the surface, in contact with water. Micelle formation is favored when the cross-sectional area of the head group is greater than that of the acyl side chain(s), as in free fatty acids, lysophospholipids (phospholipids lacking one fatty acid), and detergents such as sodium dodecyl sulfate. -A bilayer is in which two lipid monolayers (leaflets) form a 2-D sheet. Bilayer formation occurs most readily when the cross-sectional areas of head group and acyl side chain(s) are similar, as in glycerophospholipids and sphingolipids. The hydrophobic portions in each monolayer, excuded fro water, interact with each other. The hydrophilic head groups interact with water at each surface of the bilayer. Because the hydrophobic regions at its edges are transiently in contact with water, the bilayer sheet is relatively unstable and spontaneously forms a third type of aggregate: it folds back on itself to form a hallow sphere, a vesicle or liposome. By forming vesicles, bilayers lose their hydrophobic edge regions, achieving maximal stability in their aqueous environment.
Discuss the differences between P type, V type and F type ATPases.
-P-type ATPase are cation active transporters that are reversibly phosphorylated by ATP as part of the transport cycle. N domain (nucleotide domain) binds ATP. The A domain phosphorylates the aspartate. Phosphorylation forces a conformational change that is central to movement of the cation across the membrane. P domain removes phosphate on aspartate. Wisespread in eukaryotes and bacteria to pump out toxic heavy metal ions such as Cd2+ and Cu2+. V-type ATPase is a class of proton-transporting ATPases and is responsible for acidifying intracellular compartments, such as vacuoles in plants and lysosomes, endosomes, Golgi, and secretory vesicles in animals. They reside within the membranes of vacuoles and lysosomes (and ect.) and pump protons into the vacuoles/lysosomes, creating their low internal pH. Structure(be detailed): has a transmembrane domain made of multiple identical c subunits that serves as a proton channel and a peripheral domain that contains the ATP hydrolysis sites located on three identical B subunits. F-type ATPase are active transporters that catalyze the uphill transmembrane passage of protons (both in and out) driven by ATP hydrolysis. It creates a sufficiently large proton gradient that can supply the energy to drive ATP synthesis. It resides in the outer membrane of the mitrochondria. Structure: has a transmembrane domain of multiple copies of the c subunit that serves as the proton channel and a peripheral domain consisting of three alpha subunits, 3 beta subunits, and a central shaft.
Explain the Fluid Mosaic Model.
-Phospholipids form a bilayer in which hydrophobic tails are pointed in toward each other and the hydrophilic heads are pointed outward. Mosaic refers to the wide variety of components that make up the membrane: lipids (cholesterol, phospholipids, glycoproteins, ect.) and proteins, which can be integral or peripheral. Integral proteins float in this sea of lipid, held by hydrophobic interactions with their non polar amino acid side chains. The membrane mosaic is fluid because most of the interactions among its components are noncovalent, leaving individual lipid and protein molecules free to change constantly and move laterally in the plane of the membrane (Brownian movement), which can be quite rapid. Movement from one face to the other is restricted.
Discuss the distribution of lipids in membranes, between the inner and outer face and the heterogeneity of the outer and inner face.
-Plasma membrane lipids are asymmetrically distributed between the two monolayers of the bilayer, al- though the asymmetry, unlike that of membrane pro- teins, is not absolute. In the plasma membrane of the erythrocyte, for example, choline-containing lipids (phosphatidylcholine and sphingomyelin) are typically found in the outer (extracellular or exoplasmic) leaflet, whereas phosphatidylserine, phosphatidylethanolamine, and the phosphatidylinositols are much more common in the inner (cytoplasmic) leaflet. Changes in the distribution of lipids between plasma membrane leaflets have biological consequences. For example, only when the phosphatidylserine in the plasma membrane moves into the outer leaflet is a platelet able to play its role in formation of a blood clot.
The external portion of a transmembranous protein and the cytosolic portion of that same protein are generally found to have major differences in the charge of the amino acids found in those domains, discuss why.
-Positive inside rule: positively charged residues (Lys, Arg, and His) are in contact with the cytoplasm. Insertion orientation determined by the regions flanking a hydrophobic stretch has the most + charges and will be on the inside. The side chains of the aromatic amino acids [phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp)] provide a surface of negative electrostatic potential than can bind to a wide range of cations through a predominantly electrostatic interaction.
Discuss the role of integrins, cadherins and selectins with the ECM and how they articulate with the P.M. and the cytoskeleton.
-Several families of integral protiens in the plasma membrane provide specific points of attachment between cells, or between a cell and extracellular matrix proteins -integrins: are heterodimeric proteins (two unlike subunits, a and B) anchored to the plasma membrane by a single hydrophobic transmembrane helix in each subunit. The large extracellular domains of the a and b subunits combine to form a specific binding site for extracellular proteins such as collagen and fibronectin, which promotes adhesion to the ECM. A wide variety of specificities may be generated from various combinations of a and b. They serve as receptors and signal transducers, conveying information across the plasma membrane in both directions. Integrins regulate many processes, including platelet aggregation at the site of a wound, tissue repain, the activity of immune cells, and the invasion of tissue by a tumor. -Cadherins are plasma membrane proteins that are also involved in surface adhesion and undergo hemophilic interactions with identical cadherins in an adjacent cell. -Selectins have extracellular domains that, in the presence of Ca2+, bind specific polysaccharides on the surface of an adjacent cell. They are present primarily in the various types of blood cells and in the endothelial cells that line blood vessels. They are an essential part of the blood-clotting process.
How do sphingolipids structural differ from glycerophospholipids? Glycosphingolipids?
-Sphingolipids, the fourth large class of membrane lipids, also have a polar head group and two nonpolar tails, but unlike glycerophospholipids and galactolipids they contain no glycerol. Sphingolipids are composed of one molecule of the long-chain amino alcohol sphingosine (also called 4-sphingenine) or one of its derivatives, one molecule of a long-chain fatty acid, and a polar head group that is joined by a glycosidic linkage in some cases and by a phosphodiester in others (Fig. 10-12).Carbons C-1, C-2, and C-3 of the sphingosine mol- ecule are structurally analogous to the three carbons of glycerol in glycerophospholipids. When a fatty acid is attached in amide linkage to the ONH2 on C-2, the re- sulting compound is a ceramide, which is structurally similar to a diacylglycerol. Ceramide is the structural parent of all sphingolipids. -glycerophospholipids: Attached to 2 fatty acids (long chain alkyl groups) and a polar head group composed of PO4 and alcohol, (two fatty acids attached in ester linkage to the first and second carbons of glycerol, and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon). The head group is joined to glycerol through a phosphodiester bond in which the phosphate group bears a negative charge at neutral pH. -glycosphingolipids: Occur largely in the outer face of plasma membranes, when a fatty acid is attached in amide linkage to the -NH2 on C-2, the resulting compound is a ceramide (structurally similar to diacylglycerol), have head groups with one or more sugars connected directly to the -OH at C-1 of the ceramine moiety; they do not contain phosphate.
How is the ECM associated with the plasma membrane?
-The Extracellular Matrix is composed of many different parts such as microfilaments, and receptor proteins (integrin) but is mainly made up of Glycoproteins (Collagen, Proteoglycans, and Fibronectins), all of which are assembled into an organized meshwork in close association with the surface of the cell that produced them. Fibronectin: dimeric glycoprotein that binds proteoglycans and collagen. On the outer surface of the plasma membrane integrins (alpha beta heterodimers) bind to fibronectin as well as collagen and laminin to anchor the cells to the ECM. Integrins also bind to actin filaments in the cytosolic surface of the plasma membrane to anchor the intracellular portion.
Discuss how the Na+- Glucose transporter functions. What other proteins function in conjunction with this transporter to make it function?
-The apical surface of the intestinal epithelial cell is covered with microvilli, long thin projections of the plasma membrane that greatly increase the surface area exposed to the intestinal contents. Na+-glucose symporters in the apical plasma membrane take up glucose from the intestine in a process driven by the downhill flow of Na+ -Na+-Glucose secondary active transporter in the plasma membrane that takes up glucose from the intestine in a process driven by the downhill flow of Na+. Both enter the cell in the same direct (symport). The energy required for this process comes from two sources: 1) the greater concentration of Na+ outside than inside and 2) the membrane potential, which is inside negative and therefore draws Na+ inward. As glucose is pumped from the intestine into the epithelial cell at the apical surface, it is simultaneously moved from the cell into the blood by passive transport through GLUT2 in the basal surface. The crucial role of Na+ in symport and antiport systems such as this requires the continued outward pumping of Na+ to maintain the transmembrane Na+ gradient.
Compare fatty acid chain interactions between saturated and unsaturated fatty acids that contain a single cis double bond.
-The nonpolar hydrocarbon chain accounts for the poor solubility of fatty acids in water. The carboxyl acid group is polar. -Saturated fatty acids have a waxy consistency, while unsaturated fatty acids of the same lengths are oily liquids. In the fully saturated compounds, free rotation around each carbon-carbon bond gives the hydrocarbon chain great flexibility. The most stable conformation is the fully extended form. These saturated molecules can pack together tightly in crystalline arrays, with atoms all along their lengths in van der Waals contact with the atoms of neighboring molecules (hydrophobic interactions). -In unsaturated fatty acids, a cis double bond forces a kink in the hydrocarbon chain (can have trans fatty acids-ie meat). Fatty acids with one or more kinks cannot pack together as tightly as fully saturated fatty acids, result in less stable aggregates and their interactions with each other are therefore weaker (Because it takes less thermal E to disorder these poorly ordered arrays of unsaturated fatty acids, this results in a lowered melting point).
Discuss the electrochemical gradient in relation to transport.
-There is an electrochemical voltage gradient; the outer side carries a net positive and the inside carries a net negative charge -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 (Vm) across the membrane. These two factors are called the electrochemical gradient. This behavior of solutes is in accordance with the second law of thermodynamics: molecules tend to spon- taneously assume the distribution of greatest random- ness and lowest energy. -When there is a membrane separating solutes, the direction of a charged solutes tends to move spontaneously across the membrane in the direction of the electrochemical gradient, from higher to lower concentration. Transport in the direction of the gradient is passive and does not require energy. Transport against the gradient requires energy and is called active transport (can be primary or secondary).
What are the unique lipids found in the archaea?
-They have membrane lipids containing long-chain (32 C's) branched hydrocarbons linked at each end to glycerol. These linkages are through ether bonds, which are much more stable to hydrolysis at low pH and high temp than are the ester bonds found in the lipids of eubacteria and eukaryotes. At each end of the extended molecule is a polar head consisting of glycerol linked to either phosphate or sugar residues. They are called glycerol dialkyl glycerol tetraethers (GDGTs). The central C is in the R configuration in arches.
Cell for the most part do not carry out passive transport. Instead they carry out facilitated transport. Discuss the differences.
-passive transport involves high rates of diffusion down a concentration gradient, saturability, and specificity; transported species always moves down its electrochemical gradient and is not accumulated above the equilibrium concentration. (which is not always possible). Facilitated transport involves a protein that acts as a pore to transport the molecule across the membrane in the direction of the gradient. No energy is used.
How do the transmembranous proteins associate with the membrane lipids? The book uses glycophorin as a model you should try another membrane protein.
A transmembrane protein is a protein which spans the entire length of the cell membrane. It is embedded between the phospholipids. A polypeptide chain surrounded by lipids (having no water molecules to hydrogen bond with) will tend to form alpha- helices or beta- sheets, which maximizes intrachain hydrogen bonding. -If the side chains of all amino acids in a helix are nonpolar, hydrophobic interactions with the surrounding lipids stabilize the helix. -Hydrophobic side chains are oriented toward the outside of the transmembrane protein, where they interact with membrane lipids, while the hydrophilic residues are within the core, making the passing of polar molecules possible. -Both the amino-terminal and carboxyl-terminal domains of transmembrane proteins, which protrude out of both sides of the bilayer, contain many polar or charged amino acid residues and are therefore quite hydrophilic. -The side chains of residues, Tyr and Trp, of the transmembrane proteins serve as membrane interface anchors, able to interact simultaneously with the central lipid phase and the aqueous phases on either side of the membrane. Bacteriorhodopsin is a transmembrane protein. The single polypeptide chain folds into seven hydrophobic alpha- helices, each of which traverses the lipid bilayer roughly perpendicular to the plane of the membrane. The seven transmembrane helices are clustered, and the space around and between them is filled with the acyl chains of membrane lipids.
Discuss the GLUT 4 cycle onto the cell membrane and it relationship with signal transduction.
Transport of glucose into a myocyte by GLUT4 (passive transporter) is regulated by insulin: 1. GLUT4 is stored within a cell in membrane vesicles. 2. When blood glucose levels rise after a meal, insulin (released from the pancrease) interacts with the cell's receptor. In turn, the vesicles with GLUT4 move to the cytosolic surface and fuse with the plasma membrane, increasing the number of GLUT4 in the membrane so they can transport more glucose inside the cell and lower the blood glucose. 3. When blood glucose levels return to normal, insulin levels drop and GLUT4 are removed from the plasma membrane by endocytosis, forming small vesicles back into the cytosol. 4. The smaller vesicles fuse with larger endosome. 5. Patches of the endosome enriched with GLUT4 bud off to become small vesicles, ready to return to the surface when insulin levels rise again.
