1. The peptide unit is rigid and planar because: A. The R group interferes with the rotation.

1. 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. 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. 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. 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. 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.

6. . 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.

7. . 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.

8. 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.

9. . 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.

10. 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.