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.