Biochem I Chapter 7

A

A newly-identified protein shows a sigmoidally-shaped curve in a graph of fractional saturation versus ligand concentration. Which of the following statements about that protein is TRUE? A Option A: The protein undergoes conformational changes in quaternary structure when the ligand binds. B Option B: When the ligand binds to one subunit, the affinity of the other subunits for the same ligand remains the same. C Option C: The protein does NOT bind the ligand cooperatively. D Option D: The ligand binds irreversibly at a specific site on the protein, causing a global change in shape.

protons

CO2 binds at the N-terminal amino groups of blood proteins to form carbamates and release ______________ .

B

During vigorous exercise, the pH of blood passing through skeletal muscle decreases. How does this decrease affect the behaviour of hemoglobin? A Option A: It increases O2 binding to hemoglobin, because it decreases the binding of BPG. B Option B: It decreases O2 binding to hemoglobin, because it increases the binding of BPG. C Option C: It increases O2 binding to hemoglobin, because it increases the binding of BPG. D Option D: It decreases O2 binding to hemoglobin, because it decreases the binding of BPG.

actin

Each ___________ monomer can bind a myosin head.

Max Perutz

Elucidation of the structure of hemoglobin was pioneered by the researcher ______________ .

D

Fetal hemoglobin has a higher affinity for oxygen than maternal hemoglobin. Which of the following statements correctly outlines the mechanism behind this observation? A Option A: In fetal hemoglobin, the residue His143 is mutated to Ser143, and so the protein binds BPG with greater affinity. B Option B: In fetal hemoglobin, the residue Ser143 is mutated to His143, and so the protein binds BPG with greater affinity. C Option C: In fetal hemoglobin, the residue Ser143 is mutated to His143, and so the protein binds BPG with lower affinity. D Option D: In fetal hemoglobin, the residue His143 is mutated to Ser143, and so the protein binds BPG with lower affinity.

B

How are IgG molecules able to recognize a wide variety of different antigens? A Option A: The Fc portion has hypervariable sequences that form the antigen binding site. B Option B: Both light and heavy chains have hypervariable sequences that form the antigen binding site. C Option C: Each heavy chain has hypervariable sequences that form the antigen binding site, while the light chains do not vary. D Option D: Each light chain has hypervariable sequences that form the antigen binding site, while the heavy chains do not vary.

B

How does BPG decrease the affinity of hemoglobin for oxygen? A Option A: BPG reacts with oxygen to produce bicarbonate. B Option B: BPG binding to hemoglobin stabilizes the deoxy conformation. C Option C: BPG binding to hemoglobin stabilizes the oxy conformation. D Option D: BPG blocks the oxygen binding sites directly.

C

How does the sliding filament model explain the shortening of the sarcomere during muscle contraction? A Option A: Interdigitating thin filaments slide past each other. B Option B: Interdigitating thick and thin filaments compress by forming coiled coils. C Option C: Interdigitating thick and thin filaments slide past each other. D Option D: Interdigitating thick filaments slide past each other.

A

If a Lys residue that interacts with 2,3-bisphosphoglycerate (BPG) in the central cavity of hemoglobin is changed to a Ser residue, how would this affect hemoglobin behaviour? A Option A : The T state would be less stable. B Option B : Oxygen binding would be less sensitive to pH. C Option C : Oxygen binding would be more sensitive to pH. D Option D : The T state would be more stable.

high altitude

Increased numbers of erythrocytes and an increase in the amount of hemoglobin in erythrocytes are both part of the process of ______________ adaptation.

hypervariable

Most of the amino acid variation among antibodies occurs in three short segments called ______________ sequences.

structure-function

Mutant hemoglobins provided an opportunity to study protein _________________ relationships.

A

Noncooperative binding is characterized by a Hill coefficient of what value? A Option A: n = 1 B Option B: 0 < n < 1 C Option C: n = 0 D Option D: n > 1

hemoglobin

Oxygen binding has a dramatic effect upon the structure of ________________ .

D

Protein X binds reversibly to ligand Y such that X + Y ⇄ XY, and the molar concentrations of X, Y and XY are known. Which of the following represents the dissociation constant (K) for this reaction? A Option A: K = [XY]/[Y] B Option B: K could not be determined with the information provided. C Option C: K = [X] + [Y]/[X + Y] D Option D: K = [X][Y]/[XY] E Option E: K = [XY]/[X][Y]

cooperative

Sigmoidal binding curves are diagnostic of _____________ interactions between binding sites.

malaria

Some mutant forms of hemoglobin, such as hemoglobin S, provide resistance to the disease _______________ .

1st

The affinity of hemoglobin for the 4th oxygen is about 100 times greater than for the ______________ oxygen.

B

The binding of one O2 to a molecule of hemoglobin results in: A Option A: The release of any other O2 that may have bound earlier. B Option B: An increased affinity for O2 in the remaining subunits (which have not yet bound O2). C Option C: Dissociation of the hemoglobin subunits. D Option D: A decrease in hemoglobin's ability to bind a second O2. E Option E: The movement of hemoglobin to an organism's muscle tissue.

false

True or False?: CO is toxic because it competes with O2 for the oxygen binding site on heme proteins. It has a higher binding affinity to the Fe(II) of heme in Hb and Mb than O2, but it's binding could be even higher if not for the His F8 residue which causes it to bind Fe(II) in a bent manner (where CO would prefer to bind the Fe(II) in a linear manner).

false

True or False?: Myoglobin's oxygen dissociation curve shows cooperativity.

A

The enzyme 2,3-BPG mutase produces 1,3-BPG in red blood cells. Which of the following amino acids would you predict to find in the active site of 2,3-BPG mutase? A Option A: Arg B Option B: Leu C Option C: Glu D Option D: Asp E Option E: Phe

antiparallel

The immunoglobulin fold is a sandwich composed of three- and four-stranded _____________________ β-sheets.

ATP

The myosin head contains an enzyme activity for the hydrolysis of ____________ .

D

Treadmilling refers to A Option A: actin and myosin filaments sliding along each other. B Option B: synthesis and degradation of actin monomers. C Option C: the interactions between actin and tropomyosin. D Option D: actin monomers moving through a microfilament from the (+) end to the (-) end. E Option E: myosin heads walking along actin microfilaments.

A

What are the two conformations of hemoglobin? A Option A: The T state (the conformation of deoxyhemoglobin) and the R state (the conformation of oxyhemoglobin). B Option B: The R state (the conformation of deoxyhemoglobin) and the T state (the conformation of oxyhemoglobin). C Option C: The T state (the conformation of myoglobin) and the R state (the conformation of deoxyhemoglobin). D Option D: The T state (the conformation of dideoxyhemoglobin) and the R state (the conformation of deoxyhemoglobin).

B

What distinguishes monoclonal antibodies from the antibodies produced in a typical immune response. A Option A: Monoclonal antibodies have somewhat lower affinity for their antigens. B Option B: Monoclonal antibodies come from a clonal line of immortalized lymphocytes, and so they form an identical population. C Option C: Monoclonal antibodies have much higher affinity for their antigens. D Option D: Monoclonal antibodies come from memory B cells.

C

What is the fractional saturation of myoglobin at pO2 = 7.2 torr, if P50 = 2.8 torr? A Option A: 0.28 B Option B: 1.00 C Option C: 0.72 D Option D: 0.50

A

What is the primary role of nonmuscle actin in eukaryotic cells? A Option A: They form microfilaments that are part of the cytoskeleton. B Option B: They are stored in organelles, for future use in muscle function. C Option C: They form a protective barrier on the surface of the cell. D Option D: They are stored to provide a source of energy.

C

Where are disulfide bonds found in immunoglobulins? A Option A: They link the heavy chains to each other. B Option B: They are found between the beta sheets in many of the domains. C Option C: They are found in all of the places listed above. D Option D: They link the light chains to the heavy chains.

C

Which of the following is NOT one of the rules that defines the symmetry model of allosterism? A Option A: The ligand can bind to a subunit in either conformation. Only the conformational change alters the affinity for ligand. B Option B: An allosteric protein is an oligomer of symmetrically related subunits. C Option C: Ligand binding induces a conformational change in the subunit to which it binds. D Option D: Each subunit can exist in two conformational states, R and T, which are in equilibrium.

C

Which of the following occurs in hemoglobin when blood pH is lowered? A Option A: The distal histidine becomes charged at lower pH, resulting in a lower affinity of the heme. B Option B: Hemoglobin binds BPG with reduced affinity because histidine side chains in the central cavity of hemoglobin are charged at lower pH. C Option C: Histidine side chains at the subunit interface are charged at lower pH, forming salt bridges that stabilize the T state. D Option D: The proximal histidine becomes charged at lower pH, which weakens the binding of heme in its pocket.

C

Which of the following statements about 2,3 bisphosphoglycerate (BPG) is TRUE? A Option A: BPG binds more tightly to fetal hemoglobin than to adult hemoglobin. B Option B: BPG binds at a site which contains multiple negatively-charged groups. C Option C: The affinity of BPG binding to Hb would be reduced if the N-terminal groups of the four subunits were modified to make them uncharged. D Option D: BPG lowers myoglobin's affinity for oxygen.

A

Which of the following statements about 2,3-bisphosphoglycerate (BPG) binding is FALSE? A Option A: BPG aids oxygen delivery to tissues by increasing the affinity of myoglobin for oxygen. B Option B: BPG binds less tightly to fetal hemoglobin than to adult hemoglobin, thereby aiding oxygen transfer to a fetus. C Option C: BPG requires a binding site containing multiple positively charged groups. D Option D: BPG binds to hemoglobin at one site and lowers hemoglobin's affinity for oxygen at another site.

C

Which of the following statements about sickle cell anemia is FALSE? A Option A: Sickle cell anemia is a genetic disease. B Option B: The mutation in sickle cell anemia replaces a hydrophilic surface residue with a non-polar residue. C Option C: Sickle cell anemia is a consequence of a conservative mutation in the β-globin gene. D Option D: In sickle cell anemia, hemoglobin molecules aggregate to form long fibers that distort the shape of the red blood cell.

B

Which of the following statements about the T and R states of hemoglobin is FALSE? A Option A: The T state has a lower affinity for oxygen than the R state. B Option B: The T state is less stable than the R state at lower pH. C Option C: In the R state, the Fe2+ ion lies in the plane of the heme. D Option D: The R state has a smaller central cavity than the T state.

C

Which of the following statements about the structure of myoglobin is FALSE? A Option A: Myoglobin contains a heme prosthetic group that is slotted into a hydrophobic pocket between -helix E and -helix F. B Option B: A heme prosthetic group is tightly bound to myoglobin via a coordination bond. C Option C: Myoglobin contains all three types of secondary structure. D Option D: The tertiary structure of myogobin is a compact, roughly spherical shape.

B

Which of the following statements about the structure of myoglobin is TRUE? A Option A: Myoglobin contains a heme prosthetic group that is slotted into a hydrophilic pocket between -helix E and -helix F. B Option B: A heme prosthetic group is tightly bound to myoglobin via a coordination bond. C Option C: The heme group is loosely associated with myoglobin via hydrophobic interactions. D Option D: The heme prosthetic group is an integral part of the protein's secondary structure.

A

Which of the following statements correctly describes the interaction between an allosteric protein and an allosteric effector? A Option A: The effector binds reversibly at a specific site on one subunit of the protein, causing a global change in conformation. B Option B: The effector binds covalently at a specific site on the protein, causing a global change in shape. C Option C: The effector activates the protein by causing it to switch from its T (low affinity) to R (high affinity) form. D Option D: The effector binds non-specifically to one subunit and through induced fit initiates cooperativity between the subunits.

A

Which of the following statements is FALSE with respect to hemoglobin's transition from the T state to the R state? A Option A: Helix F changes its secondary structure in response to oxygen binding. B Option B: BPG is not required to stabilize the R state. C Option C: The Fe2+ ion is pulled into the plane of the heme prosthetic group when oxygen binds to hemoglobin in the T state. D Option D: Contacts between side chains in the different subunits of hemoglobin change upon binding of oxygen to one subunit.

B

Why is BPG essential for the delivery of O2 to the tissues? A Option A : BPG enables hemoglobin to adopt the R state conformation. B Option B : BPG enables stabilization of the T state conformation.

right

Would the oxygen dissociation curve for normal hemoglobin move to the right or left if there was a high concentration of BPG?

true

true or false?: A contracted muscle can be as much as one-third shorter than its fully extended length.

false

true or false?: Fetal hemoglobin has a lower affinity for oxygen than adult, because of changes at the BPG binding site.

true

true or false?: Hemoglobin is a tetramer of subunits that are similar, but not identical to myoglobin.

true

true or false?: Hydroxyurea is a treatment for sickle cell anemia that increases the percentage of erythrocytes that contain fetal hemoglobin.

false

true or false?: IgG molecules are always symmetrical.

true

true or false?: In myoglobin, the protein prevents the oxidation of Fe(II) to Fe(III) by oxygen.

false

true or false?: In sickle cell hemoglobin, a Glu replaces a Val, resulting in a structural change in hemoglobin.

false

true or false?: Oxygen is highly soluble in aqueous solution.

false

true or false?: The predominant secondary structure of muscle proteins is the β-sheet.

true

true or false?: The sequential model of allosterism states that ligand binding induces a conformational change in the subunit to which it binds.

false

true or false?: α1-α2 and β1-β2 contacts in hemoglobin are more extensive than the α-β contacts.