MCAT bio class 1

Compounds A and B react very slowly to form Compound C. Addition of a very small quantity of Enzyme X doubles the reaction rate. Addition of twice as much Enzyme X would most likely: A. Increase the reaction rate by the same amount as the first addition of enzyme. B. Have no effect, since reaction rate is independent of enzyme concentration. C. Have no effect since the enzyme will be saturated with substrate D. Increase the reaction rate by increasing the equilibrium constant.

A

What best explains the reason for the inability of the human intestinal tract to digest cellulose? A. β(1,4) linkages B. (1,6) linkages at branch points C. 5'-3' phosphodiester bonds D. φ-ψ peptidyl linkages

A

Which of the following statements regarding enzymes is NOT true? A. Enzymes increase the kinetic barrier. B. Enzymes speed up reactions by lowering the energy of activation. C. Enzymes are not used up during reactions. D. Enzymes do not affect the thermodynamics of a reaction.

A

Carbon dioxide is a small, hydrophobic molecule that diffuses through cell membranes. Its movement can be described as: A. thermodynamically favorable because it decreases entropy. B. thermodynamically favorable because it increases entropy. C. thermodynamically unfavorable because it increases entropy. D. thermodynamically unfavorable because it decreases entropy.

B

During the absorption of dietary fat, the molecule is broken down before being reassembled following absorption. What is dietary fat broken down into before absorption? A. Three fatty acids and one monoglyceride B. Two fatty acids and one monoglyceride C. Triacylglycerol D. Triglycerides

B

Epigenetics is the heritable increases or decreases in gene expression due to environmental (among other) causes. One of the mechanisms involved this differential expression is DNA methylation. What is the most likely mechanism for this process? A. Addition of a methyl group to ribose, decreasing the interaction with histones B. Addition of a methyl group to cytosine, resulting in methyl-cytosine base-pairing with guanine C. Addition of a methyl group to diphosphate, increasing the interaction with histones D. Addition of a methyl group to cytosine, resulting in methyl-cytosine mismatch base-pairing with thymine

B

Phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) phosphorylates phosphatidylinositol (4,5)-bisphosphate (or PIP2) to generate the second messenger phosphatidylinositol (3,4,5)-trisphosphate (PIP3). The morpholine-containing chemical LY294002 is cell permeable and is a competitive inhibitor of ATP binding PI3K. Each of the following is true EXCEPT: A. LY294002 can simply diffuse into the cell across the plasma membrane, and increases the Km for the PI3K phosphorylation reaction. B. LY294002 likely undergoes simple diffusion to enter the cell and will decrease the Vmax but not change the Km for the PI3K phosphorylation reaction. C. LY294002 and adenosine triphosphate bind the same pocket of PI3K and must therefore have some structural similarities. D. LY294002 can be outcompeted for binding to PI3K by increasing the concentration of ATP in the reaction.

B

Which of the following is NOT a function of cholesterol? A. Precursor for steroid hormones B. Energy storage C. Increase fluidity of plasma membranes D. Precursor for bile

B

Which of the following will change the ∆G° of an enzyme catalyzed reaction? A. Decreasing product concentration B. Increasing temperature C. ∆G is a constant and cannot be readily changed D. Increasing enzyme concentration

B

Which of the following would be true of the Lineweaver-Burk plot for a non-competitive inhibitor? A. The y-intercept of the graph would be bigger and the x-intercept would be smaller. B. The y-intercept of the graph would be bigger, and the x-intercept would not change. C. The y-intercept of the graph would not change and the x-intercept would be bigger. D. The y-intercept of the graph would be smaller and the x-intercept would not change.

B

A biochemist assesses the changes in reaction rate of an enzyme-catalyzed reaction in a solution containing saturating quantities of a known inhibitor. When substrate is added, the reaction rate increases, but fails to reach the published Vmax for this enzyme concentration, even at concentrations of substrate well above that of the inhibitor. Interestingly, the researcher notes that the concentration of substrate required to reach half of the observed maximal rate matches the published value. What is true of this enzyme and its inhibitor? A. The inhibitor binds to active site of the enzyme. B. Inhibitor concentration has no impact on reaction rate. C. Addition of inhibitor does not impact enzyme affinity for the substrate. D. The Vmax is independent of enzyme concentration.

C

In which of the following situations would there be the greatest yield of products at equilibrium following the addition of an enzyme? A. A reaction favoring reactants and large activation energy. B. A reaction with more stable reactants and small activation energy. C. A reaction with less stable reactants and large activation energy. D. A reaction favoring reactants and small activation energy.

C

Methoxy arachidonyl fluorophosphates (MAFPs) covalently link to the active sites of serine proteases. What best characterizes the activity of a serine protease following MAFP binding? A. Competition between the substrate and the MAFP which can be overcome with high substrate concentrations B. Increased protease activity C. Negligible protease activity D. Activation of the protease activity of the serine protease to cleave the MAFP

C

Which of the following is NOT a monosaccharide? A. Glucose B. Fructose C. Lactose D. Ribose

C

Which of the following is an example of feedback inhibition? A. Decreased activity of hexokinase due to a decreased concentration of its substrate glucose B. Decreased activity of hexokinase due to elevated levels of a competitive inhibitor, 2-deoxyglucose C. Decreased activity of PFK due to elevated levels of phosphoenolpyruvate, a glycolytic intermediate D. Increased activity of PFK due to high levels of AMP

C

Reaction coupling allows for: A. Formation of multimeric enzymatic complexes to decrease the overall energy of activation required. B. Addition of the equilibrium constants in a series of sequential steps to give a more spontaneous equilibrium constant. C. Increased reaction rates due to coupling of a fast reaction to a slow reaction. D. Generation of products that would not normally be formed spontaneously.

D

Which of the following is NOT a known mechanism of direct enzymatic regulation? A. Interaction of the enzyme with downstream products B. Peptide hydrolysis C. Removal of a phosphate with the use of a phosphatase D. Varied activity of transcription factors affecting enzymatic expression

D

Which of the following would most affect the Km of an enzyme? A. A mutation in a highly variable region of the protein coding sequence B. Addition of a noncompetitive inhibitor C. Increasing substrate concentration D. A mutation in the active site

D

What bonds/interactions are responsible for maintaining quaternary structure of a protein? A. The same bonds/interactions responsible for the tertiary structure B. Polar covalent bonds between alpha carbons C. The same bonds/interactions responsible for the primary structure D. The same bonds/interactions responsible for the secondary structure

a