Question

Multiple Choice (2 pts each) 6: Calculate the pH of a buffer containing 0.010 M lactic acid (pK,-3.86) and 0.050 M sodium lactate 1: The aqueous environment of cells: A. Is generally irrelevant to biochemical structures and processes B. Serves as a source of energy for biochemical A. 3.16 B. 4.56 processes С. 3.82 D. 3.9 C. Provides an appropriate context for polar molecules and H-bonding that are critical for life D. Allows biochemical molecules to move freely in solution but is otherwise uninvolved in biochemistry 7: The amino acid structure shown here is appropriate at: No pH: this form is not biologically relevant A. 2: The metabolic processes that support life include: NH HN В. рH - 2 A. Reductive catabolism that builds up biochemical molecules using energy B. Oxidative catabolism that breaks down molecules to release energy C. Oxidative anabolism that builds up biochemical molecules using energy D. Reductive anabolism that breaks down molecules using energy NH C. pH 10 D. pH 6 OH нN E. pH 13 8: An amino acid with an ionizable side chain would 3: Proteins function in cells to: have how many plateaus in its titration curve? Form cellular structures A. A. four B. Catalyze chemical reactions C. Form regulatory complexes D. All of the above B. one C. three D. two Only A & B E. 9: The atoms in a peptide bond are held in a relatively rigid structure due to: A. Fixed Ramachandran angles for the atoms bonded to the a-carbon 4: Enzymes increase the rate of a chemical reaction by: A. Stabilizing the substrate B. Reducing the free energy level of the product C. Reducing the energy difference between the substrate and transition state levels B. Hydrogen bonding with peptide bonds farther along in the protein sequence C. Keto-enol tautomers that form with the side chain groups Increasing the amount of substrate converted to product D. D. Resonance structures that give the peptide bond partial double bond character 5: Buffering in biological systems is critical because: 10: Secondary structures form in proteins primarily because of: A. Many enzymes function via acid-base chemistry dependent on the protonation of chemical groups B. Solutions that are too acidic will promote deprotonation of amino acids C. Buffer molecules promote protein-protein interactions via H-bonding A. Hydrogen bonding between peptide bond atoms in the backbone B. Electrostatic interactions between acidic and basic side chains C. Hydrogen bonding between side chains on nonpolar amino acids D. Extensive van der Waals forces between the many protein atoms D. All of the above

Answer 1

1) Answer C is correct, sinca water is not only in cells for chemical species to move freely, but to allow many of them which are polar to dissolve and react and/or interact.

2) Catabolism involves oxidative reactions that break down molecules, while anabolism involves reductions to build up molecules. The right answer is therefore B.

3) A, B and C are correct, since they all describe activities in which proteins are involved.

4) Enzymes act as catalysts in cells, so their effect is the typical catalyst effect of reducing the energy difference between the substrate and transition state levels (C).

5) A is the right question, since enzymes have very specific pH intervals at which they can function, hence the need of efficient buffering in the cells.

6) We can use the Henderson-Hasselbach equation:

$pH=pKa+log\left ( \frac{[lactate]}{[lactic]} \right )=3.86+log(\frac{0.050M}{0.010M})=4.56$

7) The amino-acid is not fully protonated. Carboxylate (-COOH) groups are protonated, as is the imino group (C=NH), but the amino groups are not, so this is near neutrality; that is: pH = 6.

8) Each ionizable groups adds a plateau to the two plateaus that are present in all the aminoacid titration curves, so the answer is C: three.

9) The fact that keeps the peptide bond practically planar is the extended pi electron system from the resonance structures, that give the peptide bond a double bond character. D is the right answer.

10) The reason secondary structures occur is because of the hydrogen bonds between the atoms of the peptide bond. Side chains may help or impede the formation of a certain secondary structure, but the basis are the hydrogen bonds mentioned, so A is the right answer.