MCAT Question of the Day 101-200 Daily MCAT Tutor Axilogy.com

Part Two:(Questions 101-200)

The control steps are shown in boxes. PHOS, glycogen ph osphorylase; GLUT, glucose transporter; HK, hexokinase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase. The dotted lines represent the allosteric control of activity of PFK -1 and PFK-2. Question 101: The analysis of the changes in heart metabolite concentrations found under various glycolytic conditions indicates that flux control is exerted at a few control steps. One intermediate, fructose-6-bisphosphate, is implicated in synchronizing the rates of the first and last glycolytic reactions by a positive feed-forward mechanism. Based on your knowledge of glycolysis, which of the following substrates and/or enzymes are likely to function in feedback regulation for the glycolytic pathway?

A. Glucose-6-phosphate

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B. Fructose-6-phosphate C. Pyruvate kinase D. Both A & B. Question 102: The pathway above depicts an interaction between citrate (an intermediate of the TCA Cycle) and phosphofructokinase-1 (PFK-1). Based on your understanding of intermediary metabolism, which of the following statements are true? A. Citrate inhibits glycolysis in the mitochondria, acting as a “clue” that plenty of substrates are available for the production of cellular energy. This allows flux to be attenuated so that glucose isn’ t

being broken down needlessly. B. Citrate inhibits glycolysis in the cytosol, acting as a “clue” that plenty of substrates are available for the production of cellular energy. This allows flux to be attenuated so that glucose isn’t being broken

down needlessly. C. Citrate inhibits glycolysis in the mitochondria, acting as a “clue” that not enough substrates are

available for the production of cellular energy. This allows flux to be b e enhanced so that glucose is being broken down and sent to the TCA Cycle to be further processed. D. Citrate inhibits glycolysis in the cytosol, acting as a “clue” that not enough substrates are available

for the production of cellular energy. This allows flux to be enhanced so that glucose is being broken down and sent to the TCA Cycle Cy cle to be further processed. Question 103: Which of the following statements about glycolysis is correct?

A. Pyruvate is converted to lactate under hypoxic conditions. B. Pyruvate is converted to acetyl-CoA under hypoxic conditions. C. Pyruvate is converted to acetyl-CoA when there is plenty of oxygen available to the cell. D. Both A & C. Question 104: Which of the following is true about sugars?

A. The alpha and beta forms of D-glucose D -glucose contain a different number of chiral carbons. B. Alpha D -glucose, but not beta D -glucose, is a reducing sugar. C. There are 16 possible open-chain stereoisomers in the family of ketohexoses. D. There are 8 possible open-chain stereoisomers in the family of ketohexoses. Question 105: Which of the following is true about lactose?

A. It is a disaccharide with the two sugar units joined by an acetal a cetal linkage. B. It is a monosaccharide. C. It diffuses freely across phospholipid bilayer membranes. D. The biological synthesis of lactose produces a mixture containing equal amounts of alpha and beta glycosidic linkages. Question 106: Which of the following statements about D-glucose is correct?

A. Carbon 1 is chiral in the open-chain form of the sugar but is achiral in the cyclic form. B. It is a ketose. C. It is a pentose. D. The chair-form is the favored conformation for the intramolecular hemiacetal. Question 107: Which of the following statements about glycosidic linkages in starch is true?

A. They arise from the reaction of a hemiacetal with an alcohol to form an acetal.  B. They can exist in only a single stereochemical form. C. They arise from the reaction of a hemiketal with an alcohol to from f rom a ketal.











C. The dominant form of glucose in solution results from a nucleophilic attack by the C4 OH group on the C1 aldehyde. D. The dominant form of glucose in solution results from a nucleophilic attack by the C6 OH group on the C1 aldehyde. Question 109: Which of the following is true about monosaccharides?

A. Ketoses and aldoses that contain the same number of carbon atoms have the same number of chiral centers. B. Both the alpha and beta forms f orms of glucose contain rings composed of five carbon atoms and one oxygen atom. C. The alpha form of glucose contains a six-membered ring, while the beta form is an open-chain aldehyde. D. Both the alpha and beta forms fo rms of glucose contain rings composed of six carbon atoms. Question 10: Which of the following statements about sugars is true ?

A. Sucrose can be correctly described as a monosaccharide, while lactose is a disaccharide. B. The formation of a glycosidic linkage between two glucose monomers can be described as the formation of an acetal. C. The dominant form of glucose in solution results from a nucleophilic attack by the C4 OH group on the C1 aldehyde. D. The dominant form of glucose in solution results from a nucleophilic attack by the C6 OH group on the C1 aldehyde. Question 11: Which of the following statements about sugars is true?

A. A disaccharide, formed by the linkage of two aldohexoses, is not a reducing sugar. B. There are 8 different possible stereoisomers possible for the open-chain forms of al dopentoses. C. All naturally occurring disaccharides contain alpha, rather than beta, glycosidic bonds. D. Starch is a polyglucose that contains a large number of 1,6-glycosidic 1,6 -glycosidic bonds and a much smaller number of 1,4-glycosidic bonds. Question 12: Which of the following statements about sugars is true?

A. There are four different stereoisomers in the family of aldotrioses. B. Dihydroxyacetone, a ketotriose, contains one chiral carbon. C. The open-chain aldehyde and cyclic hemiacetal forms of D-glucose contain the same number of chiral carbons. D. There are 4 pairs of enantiomers in the family of ketohexoses.











Question 13: Which of the following statements are true regarding the data presented above?

A. During fluorescence recovery after photobleaching (FRAP), square area of

plasma membrane fluorescence is bleached and a curve is is fit to the recovering fluorescence intensity. B. From the curve equation, the effective diffusion coefficient and relative mobility of the protein can be calculated. C. This type of experiment would be appropriate for membrane phospholipids, but not membrane proteins. D. Both A & B. Question 14: Which of the following statements concerning the information presented in this graph are correct?

A. It is NOT possible to use techniques like this to investigate the 3-dimensional diffusion and binding of molecules inside of the cell.

B. This data could NOT be used to quantify the two dimensional lateral diffusion of a molecularly thin film containing fluorescently labeled probes C. This data can be used to quantify the two dimensional lateral diffusion of a molecularly thin film containing fluorescently labeled probes.

D. Both A & C. Question 15: Which of the following statements about biological membranes are false?

A. A given peripheral protein is likely to be found attached a ttached to either surface of the membrane. B. Decreasing the average length of the fatty acid tails of membrane lipids will increase the transition temperature of the membrane. C. According to the Fluid Mosaic Model, integral membrane proteins can move at a significant rate perpendicular to the membrane plane (i.e., transversely) but not within the membrane plane (i.e.,











C. Replacing some mono-unsaturated fatty acid tails with saturated fatty acids decreases the membrane transition temperature. D. Decreasing the average length of the saturated fatty acid tails increases the transition temperature. Question 17: Which of the following is true about phospholipid bilayers?

A. They form spontaneously, from dispersed phospholipids, because their formation results in a net increase in the entropy of water molecules. B. Their phosopholipid building blocks are amphipathic molecules that contain two domains: (1) non-polar derivatives of phosphoric acid and (2) polar fatty acid groups. C. They form spontaneously, from dispersed phospholipids, because their formation, although entropically unfavorable, is enthalpically favorable. D. They form spontaneously, from dispersed phospholipids, because their formation results in a net increase in the entropy of phospholipid molecules. Question 18: Which of the following amino acids is most likely to be found in membranespanning helix of an integral membrane protein?

A. arginine B. lysine. C. aspartate. D. isoleucine Question 119: Which of the following statements about biological membranes is true? A. Increasing the percentage of unsaturated fatty acids present in the “tails” of membrane

phospholipids will increase the transition temperature of the membrane. B. Decreasing the temperature will increase the fluidity of the membrane. C. An integral protein is more likely to “flip” from one side of the membrane to the other than to move

within the plane of the membrane. D. The formation of lipid bilayer membranes is an exergonic process that is largely driven by a favorable entropy change. Question 120: Which of the following statements are false regarding membrane transport?

A. Water is able to enter the cell through simple diffusion. B. Water is able to enter the cell through facilitated diffusion. C. Water able to enter the cell through both simple and facilitated fa cilitated diffusion. D. Water a polar molecule, so it is only able to transverse the nonpolar region of the bilayer via active transport.











[A] N-linked glycans are almost always attached to the nitrogen atom of an asparagine side chain that is present as a part of Asn-X-Ser/Thr consensus sequence, where X is any amino acid except proline (Pro). Site-directed mutagenesis and inhibitor binding studies indicate critical cysteine 189 resides at the external membrane surface, which serve as the si te of mercurial water transport inhibitors. [B] Polarity analysis of the four membrane-spanning hydrophobic domains. Each membranespanning helix is shown along with Kyte-Doolittle hydrophobicity indices at 40° intervals.

Question 21: Based on the figure and information provided in the passage, which of the following combinations of proteins regions in part A correspond to the four reg ions depicted in part B?

A. 1, 4, 6, 7 B. 1, 2, 6, 7 C. 1,3,4,7 D. 1, 3, 4, 6













C. T, S, C, Q, Y D. D, E, R, P Question 23: Based on information presented in part B of the above figur e, which of the following type(s) of membrane transport is this protein likely involved in?

A. Active transport. B. Facilitated diffusion. C. Simple diffusion. D. Both B & C. Question 24: Which of the following is a correct statement regarding the numbers occurring in 40° intervals on part B?

A. This information provides us with information regarding the type of amino acid. For example, the negative numbers correspond to amino acids that are polar in nature, which allow them to interact with the water molecule as it passes through the protein. B. This information provides us with knowledge regarding position of the amino acid. For example, the negative numbers correspond to amino acids that are polar in nature, which allow them to interact with the water molecule as it passes through the protein. C. This information provides us with knowledge regarding the type of amino acid. For example, the negative numbers correspond to amino acids that are hydrophobic in nature, which allow them to interact with the water molecule as it passes through the protein. D. Both B & C. Question 25: The paragraph under the figure refers to a “critical cysteine 189 residue”. Based on your knowledge of biochemistry, which of the following is the best-proposed idea as to why this amino acid residue is involved in the inh ibition of water transport?

A. Cysteine, in the presence of Hg 2+, will transform into a proline residue. Proline is a helix-breaker; and this will disrupt the protein’s secondary structure.

B. Hg2+ binds to H20 molecules and pulls them away from the transport protein, thus blocking water uniport into the cell. C. Hg2+ binds to the oxygen atom on cysteine’s side chain, which prevents the passage of uniport water through the porin. D. None of the above propositions are biochemically feasible. Question 26: Which of the following is true about aquaporins, such as the bovine AQP1 protein?

A. They transport small, polar solutes (e.g., glycerol and urea) at rates comparable to t he rate at which water itself is transported.











B. Transport from the interior of the intestinal lumen, through brush border cells, to the capillaries involves the operation of both a Na +/glucose antiport and a glucose uniport. Na+/glucose antiport and a glucose uniport. C. Transport from the interior of the intestinal lumen, through brush border cells, to the capillaries involves the operation of both a Na +/glucose symport and a glucose uniport. D. Inhibition of the Na +/K+ ATPase by adding a specific cardioglycoside inhibitor of the enzyme to a functional in vitro preparation of intestinal epithelial tissue would be expected to have little o r no effect on glucose transport by this preparation. Question 129: Which of the following statements about the movement of water across biological membranes is correct?

A. Water, as a non-polar molecule, diffuses rapidly across membranes without the need for a transport protein. B. Aquaporins catalyze water movement across membranes in a fashion that facilitates “proton jumping” from one water molecule to another.

C. Hydrogen bonding from conserved histidine and arginine residues in aquaporins form hydrogen hy drogen bonds with transiting water molecules that replace the hydrogen bonds formed between interacting water molecules in solution. D. Aquaporins are relatively non-specific, and catalyze movements of anions such as NO 3- and Cl- at rates comparable to rate for water movement. Question 130: Which of the following statements about biological ion transport systems is true?

A. The bacterial Na+/H+ antiport plays a major role in pH homeostasis. B. The bacterial Na+/H+ antiport catalyzes an electrogenic reaction. C. In respiring bacterial cells, the bacterial Na +/H+ antiport catalyzes the net import of Na + in exchange for the net export of H +. D. The Na+/H+ antiport plays the dominant role in establishing the Na + gradient maintained in the mammalian intestinal epithelium.











Question 132: Based on the data presented in part A of the above figure, which of the following helices in part B are most likely to be out of place in the proposed model.

A. 1 & 2 B. 2 & 5 C. 5 & 6 D. 1 & 3 Question 133: Which of the following is a correct statement regarding part A of the figure above?

A. Negative values on the y-axis indicate positively charged amino acid residues, whereas positive values indicate those that are negatively charged. B. Positive values on the y-axis indicate positively charged amino acid residues, whereas negative values indicate those that are positively charged. C. Negative values on the y-axis indicate nonpolar amino acid residues, whereas positive values indicate those that are polar. D. Negative values on the y-axis indicate polar amino acid residues, whereas positive values indicate those that are nonpolar. Question 134: Which of the following statements is true?

A. Beta sheets are stabilized by hydrogen bonds between every fourth f ourth amino acid. B. Peptide oxygen atoms serve as the major hydrogen bond donor responsible for stabilizing alpha helices. C. Peptide oxygen atoms serve as the major hydrogen bond acceptor responsible for stabilizing alpha helices. D. The hydrogen bonds found in parallel and in anti-parallel a nti-parallel beta-sheets are of equal strength. Question 135: Which of the following statements about structures of proteins is true?

A. Most structures determined by x-ray x- ray crystallography have sufficient resolution so that hydrogens are readily visualized in the structures. B. NMR spectroscopy is routinely used us ed to determine tertiary structures for proteins with masses as large as 250 kDa. C. The native structures of proteins are extremely rigid so that no portions of the protein exhibit any significant flexibility. D. In several cases it has been shown that the structures of proteins in solution, as determined by NMR spectroscopy, are very similar to those present in protein crystals, as determined by x-ray diffraction. Question 136: Which of the following statements is true?











C. Iodoacetamide is widely used to alkylate thiols after reductive cleavage. D. Both methionine and cysteine can be involved in disulfide bond formation in proteins. Question 139: Which of the following statements about peptide bonds is true?

A. The carbonyl oxygen of the peptide bond is usually carries a negative charge at pH values between 4.0 and 10.0. B. Peptide bonds are highly resistant to acid-catalyzed hydrolysis. C. It would be incorrect to characterize a peptide bond in between betw een proline and the carboxylic group of another amino acid as an amide.  D. The C-N peptide bond and its four substituents all lie in a single plane. Question 140: Which of the following is true about amino acids?

A. The side chain of tyrosine is always uncharged at all pH values between 2 and 13. B. The phenyl side chain of tryptophan is the major contributor to the abso rbance of proteins at 280 nm. C. Organic chemistry nomenclature would classify the side chain functional group of asparagine as an ester. D. The hydrolysis of oligopeptides into their constituent amino acids is an exergonic reaction.











C. Initial reaction velocities (v o) are used in the analysis of enzyme reactions, as soon as enzyme and substrate are mixed. During this time, the concentration of product is very small, so the rate of the reverse reaction from product to substrate can be ignored. D. All of the above. Question 143: Which of the following statements about Michaelis-Menton kinetics are true?

A. A numerically small (low) K m reflects a high affinity of the enzyme for substrate in most cases. B. A numerically large (high) K m reflects a high affinity of the enzyme for fo r substrate in most cases. C. A numerically large (high) K m reflects a low affinity of enzyme for substrate in most cases. D. Both A & C. Question 144: Which of the following statements about the Michaelis-Menten model is true?

A. The model cannot be used, under any circumstances, for enzyme-catalyzed, reversible reactions. B. The equation derived using a Michaelis-Menten model that includes several enzyme-bound intermediates will be very different in form from the equation derived from a model that includes only a single enzyme-bound intermediate. C. The model assumes that the ES complex is present at approximately constant concentration during most of the time-course of the reaction. D. The model can be used for allosteric enzymes with sigmoidal rate vs. [S] profiles. Question 145: Which of the following statements about the Michaelis-Menten model is true?

A. The model cannot be used, under any circumstances, for enzyme-catalyzed, reversible reactions. B. The model assumes that the ES complex is present at approximately constant concentration during most of the time-course of the reaction. C. The equation derived using a Michaelis-Menten model that includes several enzyme-bound intermediates will be very different in form from the equation derived from a model that includes only a single enzyme-bound intermediate. D. The model can be used for allosteric enzymes with sigmoidal rate vs. [S] profiles. Question 146: Which of the following statements about transition state analogs is true?

A. They have very short half-lives  B. They are highly effective catalysts  C. They are identical to transition states in every detail. D. Using them as antigens is often a useful way of producing catalytic antibodies. Question 147: Which of the following statements is true? A. If one knows the value of ΔG ‡ for a reaction, one can calculate the equilibrium constant for the

reaction.













D. ΔG0 for a reaction is a measure of

the free energy required to reach the transition state from the

starting reactants. Question 150: Which of the following is true about catalysis

A. General base catalysis to make water a better nucleophile is a common feature of catalysis by ribonuclease, serine proteases and lysozyme. B. The transition states are identical for catalyzed and uncatalyzed reactions. C. ∆G‡ is negative for an exergonic reaction but positive for a n endergonic reaction. D. The transition state is the lowest point on a ∆G vs. Reaction Coordinate diagram.











B. His MM isozyme concentration decreased after cardioconversion. C. His MB isozyme concentration increased on day two after cardioconversion. D. His MB isozyme concentration increased on day two before b efore cardioconversion. Question 154: Which of the following statements about competitive inhibition is true?

A. The presence of a competitive inhibitor increases K M and decreases V max. B. The presence of a competitive inhibitor decreases K M without affecting V max. C. Competitive inhibitors bind equally well to free enzyme and the enzyme/substrate complex. D. Lineweaver-Burk plots for kinetics measured in the absence of an inhibitor and in the presence of different concentrations of inhibitor produce a series of straight lines with a common 1/v 0 axis intercept. Question 155: Which of the following statements about allosteric regulation is true?

A. Allosteric modulators always decrease the activity of an allosteric enzyme. B. Allosteric modulators always increase the activity of an allosteric enzyme. C. Allosteric modulators always affect the substrate concentration at which half- maximal velocities are observed for an allosteric enzyme without affecting its Vmax. D. Binding of an allosteric modulator to an enzyme e nzyme often causes a conformational change at the catalytic site of the enzyme. Question 156: Which of the following statements about Lineweaver-Burk plots for enzymes that display Michaelis-Menten kinetics is correct?

A. The information available from Lineweaver-Burk plots obtained at several different concentrations of a competitive inhibitor is NOT sufficient to allow calculation of the equilibrium binding constant for the enzyme/inhibitor e nzyme/inhibitor complex. B. The addition of a competitive inhibitor alters the intercept on the 1/v 0 axis without affecting the intercept on the 1/[S] axis.











B. ATP serves as a positive allosteric effector for the enzyme. C. CTP and ATP bind to the same types of regulatory subunits. D. Double-reciprocal plots of 1/v0 vs. 1/[aspartate] yield straight lines.











B. The rates of the ATP- requiring reactions would decrease, due to ATP’s decreased ability to be hydrolyzed to ADP. C. The rates of the ATP- requiring reactions would increase, due to decreased stability in the enzyme’s active site. D. The rates of the ATP- requiring reactions would increase, due to ATP’s decreased ability to be hydrolyzed to ADP. Question 163: Compared with resting state metabolism, contracting skeletal muscle displays:

A. an increased conversion of pyruvate to lactate (anaerobic metabolism) B. decreased oxidation of pyruvate to carbon dioxide (CO 2) and water (H2O). C. a decreased NADH/NAD+ ratio. D. a decreased concentration of adenosine monophosphate (AMP).  Question 164: The reaction catalyzed by phosphofructokinase-1:

A. is activated by high concentrations of adenosine triphosphate (ATP) and citrate.  B. uses fructose-1-phosphate (F 1P) as its substrate.  C. is the rate-limiting reaction of glycolysis. D. is near equilibrium in hepatocytes (liver cells). Question 165: Which one of the following statements concerning the glycolytic pathway is correct?

A. The conversion of glucose to lactate is an aerobic process.  B. Hexokinase functional in liver carbohydrate metabolism only in the absorptive period following consumption of a carbohydrate-containing meal.  C. Fructose 2,6-bisphosphate is a potent inhibitor of phosphofructokinase (PFK-1). D. The regulated reactions of the glycolytic pathway are also the irreversible reactions. 











Question 169: Under physiological conditions, which of the following glycolytic enzymes are subject to strict regulation?

A. glucokinase (or hexokinase), phosphofructokinase and pyruvate kinase. B. glucokinase (or hexokinase), phosphofructokinase and dihydroxyacetone-phosphate. C. glucokinase (or hexokinase), phosphofructokinase and glyceraldehyde-3-phosphate. D. glucokinase (or hexokinase), phosphofructokinase and triose phosphate isomerase.













B. They are often cleaved by organic solvents as well as high concentrations of urea.  C. They occur most commonly in the cis configuration. D. All of the above. Question 174: Which of the following statements about protein tertiary structure is true?

A. Salt bridges (i.e., ion/ion pairs) make the largest energetic contribution to stabilizing the native structures of most soluble proteins. B. Proteins fold by sampling all possible conformations in a completely random fashion. C. In general, hydrophobic interactions make the largest energetic contribution to stabilizing the native structures of most soluble proteins.  D. Cells do not possess any mechanisms for refolding improperly folded proteins. Question 175: Which of the following statements about protein beta-sheets is true?

A. The hydrogen bonds in parallel beta-sheets and in anti-parallel an ti-parallel beta-sheets are of equal strength. B. The hydrogen bonds in anti-parallel beta-sheets are stronger than those in parallel beta-sheets. C. Interactions between adjacent side-chain R-groups contribute significantly to stabilizing beta-





















A. The three-dimensional arrangements arrangements of the three components of the catalytic triad are very similar in many serine proteases, even though the primary structure positions of the triad are not well conserved. B. The histidine component of the catalytic triad is the amino acid that forms the acyl-enzyme intermediate. C. Inhibition resulting from treatment with organic phosphates can be reversed by either dialysis or gel filtration chromatography. D. The conversion of the acyl-enzyme intermediate into a tetrahedral intermediate involves a change from sp3 hybrid orbitals on a C to sp 2 hybrid orbitals. Question 184: Which of the following statements about chymotrypsin is true?

A. The substrate-binding pocket contains a negatively charged residue that is an important determinant for substrate specificity. B. The enzyme’s active site contains an “oxyanion hole” that participates directly in substrate

binding. C. The enzyme contains an essential aspartate residue that functions as a general acid/general base











samples. D. The isoelectric point of a protein does not affect the way it will elute during ion-exchange chromatography. Question 189: Which of the following is true about gel filtration chromatography?

A. Blue dextran, a polysaccharide with a molecule mass of ca. 1 million Da with a covalently-attached blue chromophore, is widely used as a calibration standard for the smallest elution volume possible for a protein. B. Blue dextran, a polysaccharide with a molecule mass of ca. 1 million Da with a covalently attached blue chromophore, is widely used as a calibration standard for the largest elution volume possible for a protein. C. Octyl-Sepharose is a widely used stationary phase for gel filtration chromatography. D. Diethylaminoethyl-cellulose (DEAE-cellulose) is a widely used stationary phase for gel filtration chromatography.











A. The relationship shown above is true for all weak acids. B. The relationship shown above is true for all strong acids. C. The relationship shown above is not true for all weak acids. D. Both B & C. Question 192: What can be said about the endpoint of the titration curve above?

A. It occurs at around pH 7, and all the acetic acid has lost its protons to OH -, to form H2O and acetate. B. It occurs at around pH 7, and all the acetic acid gained protons to form H 2O and acetate from OH -. C. Throughout the titration of acetic acid the two equilibria coexist, each always conforming to its equilibrium constant. D. Both A & C. Question 193: Which of the following statements about amino acids is A. Methionine and cysteine both contain thiol groups in their side chains.

NOT

true?

B. No amino acids absorb light to a significant extent in the visible region of the spectrum. C. Proline is the only common amino acid with a cyclic side chain.













D. Glutamine and asparagine are the only two common amino acids with amide groups in their side chains. Question 199: Which of these groups of amino acids contains only hydrophilic, uncharged amino acids?

A. glutamate, threonine, aspartate

MCAT Question of the Day 101-200 Daily MCAT Tutor Axilogy.com

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