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1. The peptide unit is rigid and planar because:A. The R group interferes with the rotation.B. The carbon-nitrogen bond has partial double-bond character.C. The bond between the carbon atom and the peptide nitrogen atom is not free to rotate.D. The secondary structure affects the strength of the peptide
unit.E. There is a large degree of freedom of rotation on either side of the peptide unit.
• 2. helices:• A. Constitute a common type of secondary
structure, characterized by an almost planar, fully extended configuration of the polypeptide chain.
• B. Are stabilized by hydrogen bonds between adjacent chains.
• C. Are always left-handed.• D. Exhibit a tightly coiled polypeptide
main chain from which the R groups extend outward.
• E. Are a major structural component of collagen.
• 3. Gel-filtration chromatography is a method for purification of proteins which is based on:
• A. The charge of the proteins.• B. The specific binding of the proteins to
a certain matrix.• C. The capacity of the proteins to
traverse a semipermeable membrane.• D. The combined effect of solubility and
charge.• E. The size of the proteins, which
determines their migration through a column filled with special carbohydrate beads.
• 4. Proteins are made of a set of 20 amino acids that differ in the size, shape, charge and chemical nature of the R group. Which of the following statements is not true?
• A. The aliphatic side chains of leucine and isoleucine are highly hydrophobic
• B. Amino acids cysteine and methionine include a sulfur atom in their side chains.
• C. Both cysteine and methionine can form disulfide bridges (-S-S).
• D. Phenylalanine, tyrosine and tryptophan have aromatic side chains.
• E. Amino acids at neutral pH are predominantly dipolar ions (zwitterions).
• 5. The mean molecular weight of a protein composed of 2,000 amino acids is:
• A. 220,000 kDa.• B. 200,000 daltons.• C. 2,200 kDa.• D. 220 kDa.• E. 2,000 atomic mass units.
• . The term zwitterion denotes:
• A. The un-ionized form of an amino acid.• B. A commonly used synonym for an amino
acid.• C. The presence of single + and - charges on
the same molecule (i.e., a dipolar ion).• D. The fully dissociated from of an amino acid.• E. The fully protonated form of an amino acid.
• . An enzymatically-catalyzed reaction is faster than the same reaction performed in the absence of enzyme because:
• A. The enzyme allows the reaction to occur under standard conditions.
• B. The enzyme is usually a bigger molecule than the reactants.
• C. The enzyme lowers the free energy of activation of the reaction.
• D. The enzyme shifts reaction equilibrium towards the products.
• E. The enzyme increases the free energy of activation of the reaction.
• The tripeptides Trp-Ala-Ser and Ser-Ala-Trp exhibit:
• A. Identical amino acid sequence.
• B. Identical N-terminal amino acid.
• C. Identical C-terminal amino acid.
• D. Identical amino acid sequence and composition.
• E. Identical amino acid composition.
• . Myoglobin does not bind oxygen in a cooperative fashion because:
• A. The affinity of myoglobin for oxygen is very low.
• B. Myoglobin is a monomeric protein.
• C. The heme group is covalently bound to the polypeptide chain.
• D. There is no 2,3-BPG in muscles.
• E. Myoglobin does not bind 2,3-BPG.
• Phosphorylation:
• A. Is an efficient mechanism for regulation of enzymatic activity.
• B. Takes place both in intracellular and extracellular (secreted) proteins.
• C. Is an irreversible process.• D. Takes place only on tyrosine residues.• E. Is catalyzed by phosphatases.