26. Hemoglobin has a quaternary structure and is made up of

A. six polypeptide chains, two α-chains and four β-chains
B. two polypeptide chains, one α-chains and one β-chains
C. four polypeptide chains, two α-chains and two β-chains
D. five polypeptide chains, two α-chains and three β-chains

27. When p^O₂= K_d of myoglobin, the fractional saturation (Y_O2 ) is about

A. 0.1
B. 0.5
C. 0.9
D. 1.7

28. Peptides in the fully extended chain conformation

A. have Y = F = 180°
B. do not occur in nature
C. also have a cis geometry in their peptide bonds
D. are equivalent to the (3-sheet structure

29. The oxygen in hemoglobin and myoglobin is bound to

A. the iron atom in the heme group
B. the nitrogen atoms on the heme
C. histidine residues in the protein
D. lysine residues in the protein

30. Which of the three subunits of the G proteins binds GDP and GTP?

A. Alpha
B. Beta
C. Gamma
D. Delta