StudyGuideCompilationforExam2.pdf LIFE 210
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This 11 page Study Guide was uploaded by Mikaela Maldonado on Monday October 5, 2015. The Study Guide belongs to LIFE 210 at Colorado State University taught by Paul J Laybourn in Fall 2015. Since its upload, it has received 262 views. For similar materials see Introductory Eukaryotic Cell Biology in Entomology at Colorado State University.
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Date Created: 10/05/15
ABC transporters normally function a to keep toxins from crossing the bloodb rain barrier b to protect the genomes of normal stem cells in our bodies c to keep toxins in the mother39s blood from affecting a developing fetus d all th ree of these fu nction s amino acids A all have planar structures B alphacarbons always have four different chemical groups attached C can lack a carboxyl group all together D found in cells are generally restricted to the 20 common amino acids Are plasma membrane lipids symmetrically or asymmetrically distributed and what does this mean Plasma membrane lipids are asymmetrically distributed This refers to the predominance of net negatively charged serine and inositol containing glycerophospholipids on the inner surface and uncharged phospholipids containing choline and sphigosinebased glycolipids on the outer surface It also refers to the lipid rafts which contain concentrations of glycolipids containing negatively charged sialic acids and cholesterol and proteins Blood and metastatic cancers A are often treated through surgery B are generally treated using chemotherapy C rarely develop drug resistance D none of these answers E B and C but not A The catalytic mechanism of serine proteases A involves covalent catalysis B involves transition state stabilization C involves acidbase catalysis D involves proximityorientation E involves all of the above Cell membrane lipid bilayer asymmetry a is unstable due to constant flip flop of membrane lipids between the innerand outer leaflets b refersto the uneven distribution of membrane lipid types between the innerand outer membrane leaflets c refersto the uneven distribution of membrane lipid types in the two dimensional plane ofthe outer cell surface d results in most ofthe glycosphingolipids being found on the surface ofthe plasma membrane facing the cystosol the Cells A and B are the same size shape and temperature but cell A is metabolically quiet and cell B is actively consuming oxygen More oxygen will diffuse into cell B because the concentration gradient into cell B is steeper Chemotherapy rather than surgical excision is used to cure a all cancers since 2005 b malignant metastatic and blood cancers c solid benign tumors d because cancers cannot become resistant Cholesterol is essential for lipid raft formation because Q Sphingolipids will pack togethertoo closely Q Glycerophospholipids will pack together too closely without cholesterol Q Sphingolipids have large head groupshuge sugar complex for the head leaves gaps between the tails that the cholesterol has to fill in Q Sphingolipids have small head groups Q Glycerophospholipids have large head groups Cholesterol is essential for lipid raft formation because A Sphingolipids will pack togethertoo closely without cholesterol B glycerophospholipids will pack togethertoo closely without cholesterol C Sphingolipids have large head groups D Sphingolipids have small head groups E glycerophospholipids have large head groups Compare passive channel and transporter primary active and secondary active transport Passive transport occurs from high to low concentrations Active transport requires energy to move molecules against their concentration gradient Primary active transport directly uses energy like ATP or light to create an electrochemical gradient charge differentials Secondary active transport uses the electrochemical gradient created by a primary active transporter Compare symport and antiport cotransport systems symport2 solutes in the same direction antiportZ solutes in opposite directions Concurrent acid and base catalysis A can easily be set up in a proteinfree system B involves making reactants better electrophiles and better nucleophiles C never occurs in the same enzyme D works through transition state stabilization The correct order of these molecules for the ability to cross lipid bilayers from most to least readily is COZ gtethanolgtH20gtglucosegt Ca2gt RNA The delta G for a molecule can be at equilibrium across a lipid bilayer and yet have different concentrations on one side than the other true Describe exceptions to the common polarity orientation of proteins identify important active sites proteinprotein interaction sites or intermembrane regions in transmembrane proteins Describe how lipid bilayers function as a permeability barrier for polar molecules Diagram or describe the relative abilities of gases small polar uncharged larger polar uncharged and charged molecules to diffuse through a membrane Lipid bilayers have a hydrophobiccoreswith polar headsthat form the outside ofthe mem braneThis makesa high activation energy of passage because polar molecules passage through the nonpolar region is not favorable Size and polarity ofthe molecule trying to passthrough the layer determinesthe activation energy Small non polar molecules like gasespassfreely through the membrane hydrophobiccore have very small activation energy of passageThe largerthe molecule and the greaterthe polarity ofthe molecule the slowerthe molecule will passthrough the hydrophobic core have greaterand greater activation energy of passage Transport proteins can catalyze this passage describe in one or two sentences what detergent does to destroy native protein structure In this description indicate whether the mechanism of denaturation involves primarily entropy or enthalpy or both form a quotcoatquot around hydrophobic functional groups that decrease the hydrophobic forces and affects entropy describe in one or two sentences what low pH does to destroy native protein structure In this description indicate whether the mechanism of denaturation involves primarily entropy or enthalpy or both results in protonation ofionizable functional groups COOH NH3 positive charge repulsion loss of ionic bonds affects enthalpy describe in one or two sentences what polar organic solvents do to destroy native protein structure In this description indicate whether the mechanism ofdenatu ration involves primarily entropy or enthalpy or both H bonds with water reducing order of water reduces polarity of solvent Decreases hydrophobic forces affects entropy describe in one or two sentences what urea does to destroy native protein structure In this description indicate whether the mechanism of denaturation involves primarily entropy or enthalpy or both polar molecule Hbonds with water reducing order in water around hydrophobic functional groups Decreases hydrophobic forces affects entropy Describe or diagram the mechanism of Serine 19539s participation in serine protease catalysis only that particular step As depicted above and to the right Serinei 95 initially replaces H20 in the hydrolysis ofthe peptide bond In doing so it forms a transient covalent bond with the substrate second image thus carrying out covalent catalysis Describe or diagram the two main mechanisms through which Histidine57 participates in serine protease catalysis Base In the unprotonated form it Hbonds with the OH group of Serinei 95 to make the O a better nucleophile Acid After accepting the proton Histidine57 Hbonds with the N of the peptide bond to make it better leaving group Describe the functions of Acidbase catalysis Acidbase catalysis involves Hbonding between a functional group and a portion ofthe substrate making it a better nucleophile base catalyst or a better electrophile acid catalyst more amenable to n ucleoph ilic attack Describe the functions of covalent catalysis covalent bond is formed between a functional group on the enzyme for example an amino acid R group and a substrate transition state intermediate that creates a catalytic mechanism Describe the functions of proximityorientation catalysis Proximityorientation catalysis interactionsbinding of substrates reactants in the active site of the enzyme brings them togethereffectively increasing their concentrations and holds them in the proper geometric orientation to favor the reaction for example bringing a water molecule at the right orientation to react with carbonyl carbon and displace the bond with the O of the Serine195 OH group Describe the functions of transition state stabilization Transition state stabilization occu rs when the active site of the enzyme binds the transition state intermediate Si with higher affinity than the substrate S For example the tetrahedral intermediate forming addition bonds with the quotoxyanion holequot in serine protease active sites Describe the orientation of the proteins during folding in respect to polarity aqueous soluble proteins fold with the hydrophobic amino acids on the inside away from water and the hydrophilic amino acids on the outside interacting with water Describe the three main levels of protein bonding primary structure involving covalent peptide bonds between amino acids secondary structure stabilized by Hbonds tertiary structure driven by hydrophobic forces and stabilized by all four types of noncovalent bonds Describe three of the important features of a B sheet polypeptide structure In the 3 sheet structure several extended polypeptides ortwo regions of the same polypeptide lie side by side and are stabilized by hydrogen bonding between adjacent chains Adjacent chains may be either parallel Same direction of movement along the chain with a repeat distance of about 65 A or antiparallel opposite direction of movement 7 A repeat The R groups are often small and alternately protrude from opposite faces of the b sheet Describe three of the important features of the dhelical polypeptide structure quotThe o helical structure ofa polypeptide is tightly wound around a long central axis each turn of the rightha nded helix contains 36 residues and stretches 54 A along the axis The peptide NH is hydrogenbonded to the carbonyl oxygen of the fourth amino acid along the sequence toward the amino terminus The R groups of the amino acid residues protrude outward from the helical backbonequot and can give polarity to the helix Diagram and describe the three primary structural components of membrane lipids using an actual membrane lipid as an example note a full chemical structure is not requiredjust the name for each portion polar head serine choline ethanolimine etc backbone glycerol or sphingosine nonpolar tails 2 typically fatty acyl groups Diagram the rotational lateral and transmembrane diffusion of membrane lipids How is this affected by temperature and membrane lipid composition The membrane uidity is decreased with decreased temperature until it solidi es like olive oil in a refrigerator Membrane lipids with unsaturated fatty acyl tails have kinks in them This interferes with Van der Waals interaction between the tails This lowers the temperature required to solidify the membrane keeping it uid at lower temperatures Diagram the structure of a membrane protein in the membrane Describe the difference between a hydrophobic ohelix and an amphipathic ohelix integral membrane proteins are within the membrane These proteins have hydrophobic surfaces that interact well with the hydrophobic core of the lipid bilayer that allow the interaction These are commonly singlepass within cell membranes A hydrophobic qhelix has only R groups that are hydrophobic An amphipathic qhelix has a pattern of polar and nonpolar regions that allow one side to be polar and the other nonpolar This protein has extramembrane parts that are hydrophillic making the hydrophobic qhelix amphipathic Multipass proteins form channels have amphipathic helices Diagram the structure ofthe bimolecular sheets micelles and liposomes formed by membrane lipids indicating the forces driving and stabilizingtheir formation Membrane lipids form into bilayers with the tails on the inside away from water and the head groups form an outer layer facing the water Formation of the bilayers is driven by hydrophobic forces and stabilized by Van der Waals interactions between the hydrocarbon tails Membrane lipids preferentially form bilayers over micelles as a result of having two hydrophobic tails making them cylindrical and less able to pack togetherin such atight curvature The difference between primary and secondary active transport is that a primary uses Na and secondary uses K b primary uses ATP and secondary uses GTP c they transport the same molecules but in opposite directions d primary uses energy form an energy source directly whereas secondary uses it indirectly Differential membrane permeability a requires the energy ofATP hydrolysis by carrier proteins to transport ions in and out of cells through secondary active transport b is overcome by active transport of ions through gated channel proteins c excludes the entry of most gases into cells d results in carrier protein transport having a kinetic response to a solute concentration gradient identical to other types ofenzymes Drug resistance in cancer cells A can arise through amino acid changes in the target oncoprotein B can arise through increased DNA repairand apoptosis inactivation C can develop through activation of normal detoxification systems D can involve pum psthat keep the drug out Eallofthe answersare correct Explain the advantage of using amphipathic d helices to create a membrane channel protein that has multiple membrane spanning domains Outside can be non polar inside polar This creates structure within the lipid bilayer as well as a channel for ions and such to pass th roughAmphipathic o helixbased pores have more versatility in pore size and flexibility as compared to Bbarrels Explain why a hydrophobic ohelix is used to create a single ohelical membranespanning domain The hydrophobic o helix wants to interact with the hydrophobic side chains of lipids By making the entire surface ofthe o helix contain hydrophobic residues this interaction can be optimized in an isolated membrane domain Five students in this class always sit together in the front row Question 2 If analogous to why lipid bilayers form the reason for this is A nobody else in the class wants to sit next to them B they really like each other The following forms of transmembrane transport require coupling to an energetically favorable reaction a secondary active passive b simple diffusion passive c primary active passive d primary active secondary active Histidine 57 in the active site of serine proteases a functions through covalent catalysis b functions though both acid and base catalysis c functions through transition state stabilization d functions only as an acid catalyst If an enzyme is added to a solution where its substrate and products are in equilibrium A additional product would be formed B additional substrate would be formed Cthe reaction would change from endergonic to exergonic Dthe free energy ofthe system would change E nothingthe reaction would stay at equilibrium Ifthe concentration of calcium in the cytoplasm is 20 mM and the concentration of calcium in the surrounding fluid is 01 mM the cell could increase the calcium concentration in the cytoplasm through a active transport b passive transport c any ofthese mechanisms d diffusion If the plasma membrane becomes permeable to NA and K the NAK pump would a Be completed inhibited b Begin to pump NA in both directions c Begin synthesizing ATP rather than hydrolyzing it d Continue to pump ions and hydrolyze ATP but only generate heat in the process e Continue to pum p ions without hydrolyzing ATP indicate the effect of an enzyme catalyst on Aern AGi Enzymes have no effect on the Aern Enzymes function through decreasing the AGi Indicate the AAGi the AG of Si and the AG of ESi delta delta G is the difference between the delta G with and without an enzyme delta G of s is the quothumpquot Delta g of ES is the delta G value when an enzyme is in the reaction In general enzymes increase the rate of reactions A by changing the G of the S and P B by coupling the reaction to ATP hydrolysis C through effects on the reaction thermodynamics D by reducing the energy of activation In most animal cells channel proteins allow a Amino acids into a cell b NA out of a cell c NA into a cell d Glucose into a starved cell e Glucose out of a starved cell In the process of catalyzing a reaction enzymes aaffect both the change in free energy of the reaction and the activation energy baffect only the change in free energy of the reaction but not the activation energy caffect only the activation free energy but not the change in free energy of the reaction daffect neither the change in free energy of the reaction nor the activation free energy Leventhal39s paradox a refers to protein folding occuring much more slowly than expected bthe essentially random nature of proteins folding c is solved by proteins folding in a stepwise non random fashion d explains why proteins can take years to fold ehas nothing to do with pulling socks from a drawer and matching them or monkeys typing Shakespeare verses The lipid bilayer of biological membranes a is held together primarily by covalent bonds b is relatively permeable to polar and charged molecules c is unaffected by detergents d is selfsealing in an aqueous environment The lipids in the lipid bilayer a are energetically favored to dissociate in an aqueous environment b are covalently crosslinked together to form a macromolecule c commonly undergo inter lipid bilayer leaflet diffusion flipflop d usually form a 2D fluid phase at physiological temperatures Membrane electrochemical gradients involve both charge and concentration components with regard to NA K exists as a quotsupportquot for this system Membrane fluidity is a single cell in response to changes in ambient temperature is regulated by a increasing the protein content b producing more heat from cellular metabolic reactions c decreasing the sphingolipid content overall dthe relative proportion of membrane lipidswith unsaturated and saturated fatty acid tails Membrane lipids A are classified first by backbone and second by head group b all have a net positive charge c by definition contain phosphate D that contain sphingosine always have a sugar in their head group Membrane lipids a contain amphipathic alpha helices b are negatively charged at one end and positively charged at the other end c can have either one or two fatty acyl tails d form into bilayers to satisfy the second law of thermodynamics Membrane transporter proteins have been imagined to function through a quotrevolving doorquot mechanism Is this likely to be the most plausible mechanism no Micelle formation is driven by a van der waals b hydrophillic forces c hydrophobic forces d hydrogen bonds The multidrug membrane transporter PGlycoprotein PGP is the beststudied ABC family transporterfound to function in multidrug resistance What are PGP39s normal quotgoodquot functions in our bodyWhat is the evidence that some of these functions are coopted by cancer cells to protect themselves from anticancer drugs P glycoprotein PG Pnormally functionsto keeptoxins out of sanctuaries in the body and out ofthe whole body Such sanctuaries include the central nervous system and the spinalcord through what is called the blood brain and blood cerebrospinalfluid barrier Others include the testes developing fetusesplacenta and varioustissue stem cellsThe liver intestinaltract and kidney lining cells work to excrete toxins into the bile blood urine and intestinal lum en to keep them out or remove them from the body Cancer cells often have PGP over or highly expressed In addition cancer cells have been shown to quickly pum p out hydrophobic dies relative to normal somatic cells using flow cytometry experim ents Multidrug resistance MDR in cancer cells A primarily targets polar molecules B involves a membrane protein that binds compounds in the cytoplasm of the cell C develops through over expression of several transport proteins each specific to a drug D often arises through expression of Pglycoprotein E all of the answers are correct Of the following mechanisms which would not contribute to enzyme catalysis a The formation ofa covalent intermediate between the enzyme and the substrate bThe active site having a greater af nity for binding the substrate relative to binding the transition state intermediate cThe binding of two substrates to bring them together and orient them properly to react dHydrogen bonding of basic amino acid R groups with a substrate The organism likely to have the highest percentage of unsaturated fatty acid chains in its membranes is A an Antarctic fish B a desert iguana C a human D a polar bear E a thermophilic bacterium The organism likely to have the highest percentage of unsaturated fatty acid chains in membranes is o Antarctic sh uid membranes allow greater uidity at lower temperatures 9 Desert iguana 9 Human 9 Polar bear 9 Thermophilic bacteria Peptide bonds A have free rotation except for glycines B often break from chance intermolecular collisions C have planar structures D have LP but not D angles Pglycoprotein A is a protein obtained by cancer cells through viral infection B normally functions in signal transduction and cell recognition C functions to keep toxins out of the central nervous system D is not expressed in intestinal endothelial cells E is the malaria quotreceptorquot on red blood cells Pglycoprotein inhibitors A also inhibit cytochrome p450 detoxi cation B are found in peanuts and beer C have been found to cause severe toxin buildup in the central nervous system and spinal fluid D strangely reduce anticancer drug levels in the blood E do not work in combination with anticancer drugs phi angle of rotation occurs around NCalpha The polarnonpolar nature of amino acid R groups side chains are difficult to measure using free amino acids A due to the insolubility of hydrophobicam ino acids in water B because the charged amino acidstend to associate and precipitate C since the Rgroups often switch from polarto nonpolar when the amino acid is incorporated into a protein Dasa result ofthe polaramino and carboxyl groupsfound on each amino acid The polarnonpolar nature ofamino acid R groups side chains are dif cult to measure using free amino acids You could get around this problem A by measuring their effect on the polarity of a protein B through measuring the solubility ofthe R groups alonein water C by measuring the solubility of the R groups alone in organic solvent D both B and C The production ofjust one molecule of each ofthe possible amino acid sequences of polypeptides 300 amino acids in length will require more atoms than exist in the universe True Proteins and nucleic acids both A are made up of amino acids B contain sugars C are hydrophobic D are large polymers E consist of four basic building blocks PSC833 A is a bleaching agent B is likely a carcinogen C activates Pglycoprotein D inhibits Pglycoprotein E is a second fluorescent dye psi angle of rotation occurs around CCalpha the relative Gs of the substrate S and product P for a reaction that will occur spontaneously The relative Gs of a reaction that will occur spontaneous are G of S gt than G of P A sequence of amino acids in a certain protein is found to be SerGlyProGly The sequence is most probably part of an A antiparallel B sheet B parallel B sheet C a helix D a sheet E B turn Sodium Na transport across a cell plasma membrane a by the NaK ATPase uses 30 of the ATP hydrolyzed in mammalian cells b often uses secondary active transport c has linear kinetics in response to a concentration gradient d has no role in glucose transport into intestinal epithelial cells Some patient39s cancers respond to chemotherapy but many do not due to resistance to the anticancer drug What is the range of reasons cancers become resistant to anticancer drugs discussed in class and how can they be classified The range of reasons some patients ca ncersfailto respond or stop responding to chemotherapy drugs can be drug specificor more generalThe amino acid changes making Bcr Abl resistant to Gleevecare an example ofa specific change in cancer cells M ore general changes include im paired drug delivery increased D NA repair blockage of signaling for program med cell death a poptosis activation ofthe cytochrome p450 detoxification system decreased cellular uptake through mem bra ne transporters of polar drugs and increased cellular export by ABC pum ps of non polar drugs The detoxification and increased export of non polar drugsare often up regulated together Changes leading to drug resistance are through a natural selection processfor cancer cells displaying these changes The tertiary structure of proteins A is stabilized but not dictated by disulfide bond formation B consists of ohelices and Bsheets C is stabilized by multiple Hbonds between non polar amino acid Rgroups with water D is always completely unrelated to that of other proteins Three enzymes puri ed from a snake venom were tested for their ability to lyse red blood cells The phospholipase produced phosphorylcholine and diacylglycerol Question 3 The results are explained by A red blood cells having no proteins or sialic acid in their membranes B the phospholipase removing the fatty acid tails C the phospholipase producing two very polar products D producing a large number essentially non polar membranelipids Transition state intermediate stabilization ais not part of the serine protease enzymatic mechanism bexplains why most transition state analogs are weak enzyme inhibitors cdecreases the activation energy of a reaction by increasing the concentration of the transition state intermediates daffects only the fo nNard rate of a reaction Transmembrane proteins a can rapidly ip from one side of the membrane to the other b can contain a single amphipathic alphahelix c are stabilized by disul de linkages in the cytosolic portions d form channels from multiple amphipathic alphahelices or a beta barrel Alpha helixes form more exible and versatile structures compared to beta barrels Triosephosphate isomerase catalyzes the above reaction and is competitively inhibited by phosphoglycolate by A competing with the substrate B mimickingthe product C binding the enzyme outside the active site D acting as a transition state analog ValyltRNA synthetase has two amino acid binding sites One binds valine preferentially but still binds threonine The second binds threonine but not valine by A having a very differently shaped binding site B adding a hydrophobic amino acid to the binding site C increasing the size of the binding site D including an additional hydrogen bond acceptor in the binding site What are the key attributes of enzymes Enzymes are generally proteins some are RNAs or ribozymes They function catalytically and their names usually end in quotasequot What makes them particularly special is that they are very specific for substrate and reaction catalyzed they can be regulated and they can couple two or more separate reactions What are the properties ofthe membrane lipid portions that are important in membrane formation Describe what makes a membrane lipid amphipathic Membrane lipids are amphipathic macromolecules because the have a polar portion and a non polar portion The polar head group and the non polar tails What bonds and interactions influence quaternary structure driven by hydrophobic forces and stabilized by the other three types of noncovalent bonds What do disulfide bonds play into protein folding does not help a protein fold properly Enzymes disul de isomerase are needed to help the disul de bonds break to allow folding to occur and then help the correct crosslinking bonds to form later but function to stabilize the correct protein structure Disul de bonds are not formed in the reducing environment of the cytoplasm However they can form in the relatively oxidizing environment outside cells and inside the endoplasmic reticulum lumen and in vesicles What does quaternary protein structure consist of multiprotein subunit complexes forming dimers trimers etc What does quaternary structure consist of consists of the association of separate proteins subunits into macromolecular complexes What does the primary protein structure consist of amino acid sequence What does the secondary protein structure consist of consists of dhelices Bstrand Bsheets and turns What does the tertiary protein structure consist of domains that are formed from secondary structures combining to form the structure of the whole proteins polypeptide What is found on the exterior of a globular protein hydrophilic amino acids they are water soluble roughly circular proteins What is the quotbusiness endquot of an enzyme what does it look like what is it made up of and very generally how does it work The quotbusiness endquot of an enzyme is the active site which is made up of amino acid R groups and sometimes peptide bonds This is the site when substrate is bound and the reaction is catalyzed through the four mechanisms described in the lectures and in the answer to question 1 of the Module 5 study guide questions answers What is typically found in the interior of a watersoluble globular protein hydrophobic amino acids When a lipid bilayer is torn it does not seal by forming a hemimicelle cap because A membrane lipids are conical B membrane lipids are amphipathic C membrane lipids are cylindrical D the cytoplasm would leak out of the cell E it violates the second law of thermodynamics Where are disulfide bonds most commonly found extracellular proteins like antibodies and intestinal enzymes Which of the following is correct with respect to the amino acid composition of proteins A Larger proteins have a more uniform distribution of amino acidsthan smaller proteins B Proteins contain at least one each ofthe 20 different standard amino acids C Proteinswith different functions usually differ significantly in theiram ino acid composition D Proteinswith the same molecularweight have the same amino acid com position EThe average molecularweight ofan amino acid in a protein increases with the size of the Which of the following molecules can pass most readily through a biological membrane by simple diffusion a Na b glucose6P c C02 d aspartate an amino acid Which ofthe following pairs of bonds within a peptide backbone show free rotation around both bonds A Cd C and N Cd B CO and N C C CO and N Cd D N C and Cd C E N Cd and N C Which of the following statements about oligomeric proteins is false aAsubunit may be similarto other proteins b All subunits must be identical c Many have regulatory roles d Some oligomeric proteinscan further associate into large fibers e Some subunits may have nonprotein prostheticgroups ex histones or nucleosom es come in piecesthat make up bunches of subunites Which of these parameters determine the rate kinetics and which determines the direction thermodynamics of a chemical reaction The Aern determines the direction thermodynamics S to P or P to S ofthe reaction and the rate kinetics of the reaction is inversely proportional to the A645 activation energy Which of these portions and properties is responsible for membranes being selfsealing and why The hydrophobicfatty acid tailsand the hydrophobicforcesare responsible for mem bra nes being self sealing because it isalways energetically more favorable for these tailsto be sequestered away inside the lipid bilayer away from water Why are glycine and proline often found within a B turn A 3 turn results in a tight 1 80 reversal in the direction of the polypeptide chain Glycine is the smallest and thus most flexible amino acid and proline can readily assume the cis configuration which facilitates a tight turn and comes in a circular configuration needing glycine on either side to facilitate the turn
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