1 The peptide unit is rigid and planar

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 p. H are predominantly dipolar ions (zwitterions).

• 5. The mean molecular weight of a protein composed of 2, 000 amino acids is: • A. 220, 000 k. Da. • B. 200, 000 daltons. • C. 2, 200 k. Da. • D. 220 k. Da. • 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. B. C. D. E. Identical amino acid sequence. Identical N-terminal amino acid. Identical C-terminal amino acid. Identical amino acid sequence and composition. 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.