1 Exam 1 Biol 213-503 Molecular Cell Biology Fall 2009 Name____________________________ In drawing structures, neatness counts. The space provided for each question should be sufficient for your answers. Note that there are 102 points on this exam but the maximum score you can receive is 100. Use your time wisely. Questions 1-17 are worth 3 points each. Circle your answer(s). 1. Which type of chemical bond is characterized by the transfer of electrons from one atom to the other? (a) Hydrogen bond (b) Ionic bond (c) Covalent bond (d) van der Waals attraction 2. Which of the following statements about amino acids is TRUE? (a) Twenty-two amino acids are commonly found in proteins. (b) Most of the amino acids used in protein biosynthesis have charged side chains. (c) Amino acids are often linked together to form branched polymers. (d) D- and L-amino acids are found in most proteins. (e) All amino acids contain an NH2 and a COOH group. 3. DNA differs from RNA in (a) the number of different bases used. (b) the number of phosphates between the sugars in the sugar-phosphate backbone. (c) the kind of sugar found in the sugar-phosphate backbone. (d) the kind of purines used. (e) the structural polarity of the polynucleotide chain. 4. Energetically favorable reactions (a) decrease the entropy of a system. (b) increase the free energy of a system. (c) have a positive ΔG . (d) decrease the free energy of a system. (e) create order in a system. 5. Which of the following CANNOT be a reason that cells use enzymes rather than heat to increase the rate of biochemical reactions? (a) The temperature increase required to speed up a reaction by an appreciable extent is often huge. (b) Reactions cannot be coupled without enzymes. (c) An enzyme catalyzes just one or a very small number of different reactions; heat would affect all the reactions in a cell. (d) Enzymes change the equilibrium of biochemical reactions. (e) Enzymes can accelerate reactions to a much greater extent than can heat. 6. Any reaction A ⇔ B is at equilibrium when (a) [A] = [B]. (b) ΔG ° = 0. (c) ΔG = 0. (d) ΔG = ΔG °.2 7. You are studying a biochemical pathway that requires ATP as an energy source. To your dismay, the reactions soon stop, partly because the ATP is rapidly used up and partly because an excess of ADP builds up and inhibits the enzymes involved. You are about to give up when the following table from a biochemistry textbook catches your eye. Hydrolysis reaction ΔG° creatine + ATP enzyme A creatine phosphate + ADP +3 kcal/mole ATP + H20 enzyme B ADP + phosphate –7.3 kcal/mole pyrophosphate + H20 enzyme D 2 phosphate –7 kcal/mole glucose 6-phosphate + H20 enzyme E glucose + phosphate –3.3 kcal/mole Which of the following reagents are most likely to revitalize your reaction? (a) A vast excess of ATP (b) Glucose 6-phosphate and enzyme E (c) Creatine phosphate and enzyme A (d) Pyrophosphate (e) Pyrophosphate and enzyme D 8. Which of the following statements about enzymes is correct? (a) Catalysis of an energetically unfavorable reaction by an enzyme will enable that reaction to occur. (b) An enzyme can direct a molecule along a particular reaction pathway. (c) An enzyme can catalyze many chemically different reactions. (d) An enzyme can bind to many structurally unrelated substrates. (e) Enzymes are permanently altered after catalyzing a reaction. 9. The following reactions take place in a cell located next to a blood vessel. X → Y ΔG ° = –10 kcal/mole Y + O2 → Z + CO2 ΔG ° = +0.5 kcal/mole Normally, the blood vessel brings in oxygen and takes away carbon dioxide, but years of overindulgence have taken their toll, and it has become completely clogged with cholesterol, cutting off the blood supply. Which of the following molecules would be expected to accumulate in large amounts? (a) X (b) Y (c) Z (d) Y and Z (e) X and Z 10. Enzymes help catalyze chemical reactions by (a) lowering the activation energy. (b) increasing the free energy. (c) increasing the activation energy. (d) lowering the free energy.3 11. Which of the following statements is FALSE? (a) The three dimensional structure of a protein dictates its function by determining its binding specificity for other molecules. (b) Many proteins have more than one binding site. (c) Binding between protein and ligand generally involves noncovalent bonds. (d) Proteins are designed to bind their ligands as tightly as possible. (e) Changes in the amino acid sequence of a protein can decrease binding to a ligand, even if the altered amino acid does not lie in the binding site for the ligand. 12. Although all protein structures are unique, there are common structural building blocks that are referred to as regular secondary structures. Some have α helices, some have β sheets, and still others have a combination of both. What makes it possible for proteins to have these common structural elements? (a) hydrogen bonding between the amino acid side chains most commonly found in proteins. (b) noncovalent interactions between amino acid side chains and the polypeptide backbone. (c) ionic interactions between charged amino acid side chains. (d) hydrogen bonding between atoms of the polypeptide backbone. (e) hydrophobic interactions between the many nonpolar amino acids. 13. Disulfide (S–S) bonds (a) are formed by the cross-linking of methionine residues. (b) are formed mainly in proteins that are retained within the cytosol. (c) stabilize but do not change a protein’s final conformation. (d) can be broken by oxidation. (e) rarely form in extracellular proteins. 14. Protein structures have several different levels of organization. The primary structure of a protein is its amino acid sequence. The secondary and tertiary structures are more complicated. Consider the definitions below and select the one that best fits the term “protein domain.” (a) a small cluster of α helices and β sheets (b) the tertiary structure of a substrate-binding pocket (c) a complex of more than one polypeptide chain (d) a protein segment that folds independently 15. The biosynthetic pathway for the two amino acids E and H is shown schematically below. You are able to show that E inhibits enzyme V, and H inhibits enzyme X. Enzyme T is most likely to be subject to feedback inhibition by __________________ alone.