Biochemistry Test III Questions --------------------------------

Question 1

Lactate dehydrogenase removes 2 electrons and one proton from the molecule lactate to form the molecule pyruvate. What type of enzyme is lactate deyhdrogenase?

Hydrolase

Isomerase

Ligase

Oxidoreducatase

Answer 1

oxidoreductase

Question 2

You are treating a patient who has a mutation in her hexokinase enzyme. Instead of a glutamate residue at position 256, she has threonine residue. Which of the following would be a likely effect on glucose binding to this altered enzyme?

Glucose won’t bind at all to the enzyme.

Glucose will bind, but with a lower affinity than normal.

Glucose will bind, but with a higher affinity than normal.

Glucose will act as an
irreversible inhibitor.

Answer 2

Glucose will bind, but with a lower affinity than normal.

Question 3

A patient has a defect in her chymotrypsin enzyme. In place of the catalytic triad histidine residue, she has phenylalanine residue. How will this difference affect the function of her chymotrypsin enzyme?

No change – Phe is very similar in structure to His.

Increase in Function – Phe is more hydrophobic and will be more effective at cleaving hydrophobic polypetides.

Decrease in Function – Phe is only a weak base and cannot readily accept a proton from the nearby Ser residue needed to activate the Ser.

Loss of Function – Phe is not a basic residue and cannot accept a proton from the nearby Ser residue needed to activate the Ser.

Answer 3

Loss of Function – Phe is not a basic residue and cannot accept a proton from the nearby Ser residue needed to activate the Ser.

Question 4

The liver enzyme alcohol dehydrogenase (ADH) converts ethanol to acetaldehyde. After a drinking binge, when a patient’s blood alcohol level is high, ADH operates near Vmax. However, after a few hours the patient’s blood alcohol has dropped to a concentration equal to ½ Km for ADH. The velocity for ADH at this substrate level will be
¼ Vmax

⅓ Vmax

½ Vmax

⅔ Vmax

Answer 4

⅓ Vmax

Question 5

For a given enzyme, a competitive inhibitor has a structure resembling the transition state of the reaction catalyzed by the enzyme. Because of this, the affinity of the enzyme for the inhibitor will be ___________ than that for the substrate.

Higher

Lower

About the same

The inhibitor won’t bind because of this

Answer 5

higher

Question 6

You and your study partner are having an argument. You say that HIV protease inhibitor drugs are competitive inhibitors. Your partner says these drugs inhibit HIV protease by covalent modification (suicide inhibition) of the active site. Which of these statements is right?

Competitive inhibition: binds to active site non-covalently, but with very high affinity.

Suicide inhibition: enzyme amino acid side chains chemically react with the inhibitor and form an unbreakable covalent bond.

Both are correct: the inhibitor binds in the active site (competitive inhibition) but also forms a covalent adjunct with the enzyme (suicide inhibition).

Answer 6

Competitive inhibition: binds to active site non-covalently, but with very high affinity.

Question 7

In an early step of cholesterol synthesis, the molecule HMG-CoA is converted to the molecule mevalonate by the enzyme HMG-CoA Reductase. Not surprisingly, cholesterol is an allosteric inhibitor of HMG-CoA Reductase. Which of the following is the most likely mechanism by which cholesterol inhibits HMG-CoA Reductase?

A. This can’t be determined without inhibition data.

B. Competitive inhibition

C. Feedback inhibition

D. Reversible covalent modification

E. Suicide inhibition

Answer 7

C. Feedback inhibition

Question 8

You are treating a cancer patient who has a defect in her Ras protein, causing it not to bind the SOS protein at all. What is the most likely effect of this defect on the function of her Ras protein?

Her Ras protein will be predominantly in the Ras-GDP form.

Her Ras protein will be predominantly in the Ras-GTP form.

Her cells will produce more Ras protein to compensate.

Her cells will produce more SOS protein to compensate.

Answer 8

Her Ras protein will be predominantly in the Ras-GTP form.

Question 9

You are treating a patient who is hypoventilating. How will this most likely affect bicarbonate transport by red blood cells (RBCs) in the lungs?

Bicarbonate transport out of the RBCs will decrease.

Bicarbonate transport into RBCs will decrease.

Glucose import into RBCs will be increased.

The patient will develop respiratory acidosis.

The patient will develop respiratory alkalosis.

Answer 9

Bicarbonate transport into RBCs will decrease.

Question 10

You are treating a patient who has ingested an inhibitor of the sodium pump. How would this affect glucose uptake by intestinal epithelial cells as depicted in this diagram?

Glucose import into the cell will decrease.

Glucose import into the cell will increase

Na+ entry into the cell will decrease.

K+ entry into the cell will decrease.

Glucose release into the blood will decrease.

Answer 10

Glucose import into the cell will decrease.

Question 11

Which of the following operates by paracrine signaling?

a. glucagon
b. insulin
c. epinephrine
d. cortisol
e. acetylcholine

Answer 11

acetylcholine

Question 12

What is meant by signal amplification in a signal cascade?
a. One hormone binds to many receptors to activate them.

b. One receptor when activated by one hormone has many different functions.
c. One hormone activates one receptor that results in the generation of a large pool of second messenger molecules that diffuse in the cell and activate many target proteins.
d. One hormone activates one receptor that results in the generation of one second messenger molecule that diffuses within the cell to activate many target proteins.
e. Cells amplify signals by propagating hormones to nearby cells.

Answer 12

d. One hormone activates one receptor that results in the generation of one second messenger molecule that diffuses within the cell to activate many target proteins.

Question 13

Signal cascades generated by G-protein coupled receptors are terminated when

a. the second messenger molecules are degraded.
b. the hormone dissociates from the receptor.
c. target molecules are dephosphorylated.
d. the G protein returns to the GDP-bound form.
e. All of the above.

Answer 13

a. the second messenger molecules are degraded.

Question 14

Which enzyme cleaves PIP2 (also called PI-4,5-bisP) to IP3 and DAG to start the IP3 signal cascade?
a. Phospholipase A1

b. Phospholipase A2
c. Phospholipase B
d. Phospholipase C
e. Phospholipase D

Answer 14

d. Phospholipase C

Question 15

The insulin receptor substrate (IRS) is

a. phosphorylated by the insulin receptor, which activates it for binding a variety of other protein targets.
b. binds insulin then activates the insulin receptor’s autophosphorylation activity.
c. an insulin molecule that binds the intracellular face of the receptor.
d. a kinase receptor.
e. is activated at all times, but is held in place on the receptor until insulin binds.

Answer 15

a. phosphorylated by the insulin receptor, which activates it for binding a variety of other protein targets.

Question 16

Cholera toxin modifies a G-protein so that the G-protein

a. cannot communicate with its signaling target.
b. terminates the signal cascade.
c. prevents the receptor from recycling.
d. cannont hydrolyze GTP.
e. becomes an inhibitory G-protein.

Answer 16

d. cannont hydrolyze GTP.

Question 17

A patient with manic depressive disorder is treated with lithium, which slows the production of inositol phosphates and phosphatidyl inositol derivatives in cells. Which of the following signaling adapter proteins is most likely affected by this drug?

A. Phospholipase C
B. Phospholipase D
C. Protein kinase A
D. Protein kinase C
E. Jak-Stat Receptor

Answer 17

A. Phospholipase C

Question 18

A 58 year-old man has been placed on a “Beta-blocker drug,” which is designed to control the activity of his β-adrenegic receptors in his cardiac muscle. β-adrenergic receptors are G-protein coupled receptors that result in Protein Kinase A activation. Which of the following cardiac muscle cell proteins would most likely exhibit reduced activity following the use of this drug?

A. Adenylate cyclase
B. IP3 receptor of ER
C. Gi protein
D. Protein kinase C
E. PIP2 kinase

Answer 18

A. Adenylate cyclase