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1

Diisopropylphosphofluoridate (DIPF) inactivates chymotrypsin by covalently modifying serine-195. This occurs because:

Serine-195 is in an environment which gives it a higher than normal reactivity with respect to DIPF

2

The mechanism of chymotrypsin can be viewed as a two step process—acylation of the enzyme active site followed by a deacylation reaction. The observation of "burst" kinetics in rapid kinetic studies of the hydrolysis of N-acetyl-L-phenylalanine p-nitrophenyl ester by chymotrypsin is due to:


the rate of the acylation reaction being faster than the deacylation reaction.

3

TPCK inactivates chymotrypsin but not trypsin because:


it looks like the substrate for chymotrypsin (but not trypsin) and thus can bind in its active site and modify His-57.

4

Which of the following is NOT true of a competitive inhibitor?


It irreversibly inhibits the enzyme by chemically modifying a group at the active site.

5

The cleavage of a peptide bond by chymotrypsin is caused by a nucleophilic attack by an active-site residue. Which of the following is true?

Ser is a strong nucleophile because of the action of Asp and His in the active site.

6

Which type of inhibitor causes Vmax to decrease while KM stays the same?

Noncompetitive

7

Which statement is NOT true regarding the effect of pH on enzymes?
A.
Each enzyme exhibits its optimal activity over a narrow range of pH values.

B.
One reason why enzymes have a bell-shaped curve for their pH dependence is that enzymes
tend to denature at extreme pH.

C.
Pepsin, a digestive enzyme operating in the intestine, displays its optimal activity at pH ~2.

D.
None of the above.

None of the above

8

Which type of inhibitor can only bind after the enzyme substrate complex has formed?

Uncompetitive

9

Why is penicillin called a suicide inhibitor for the biosynthesis of bacterial cell walls?
A.
In the presence of the penicillin, the transpeptidase enzyme no longer makes a covalent intermediate.

B.
Penicillin forms a covalent intermediate which is enzymatically inactive.

C.
Penicillin selectively removes an essential metal ion cofactor from a metalloenzyme.

D.
All of the above.

B.
Penicillin froms a covalent intermediate which is enzymatically inactive

10

The binding of oxygen to myoglobin and hemoglobin has what effect on the heme iron?

It causes the iron to move into the plane of the porphyrin ring

11

Which of the following indicates that the binding of oxygen to hemoglobin is cooperative?
A.
binding plot of Y (fraction of sites occupied) against pO2 is sigmoidal rather than hyperbolic.

B.
Hemoglobin has four subunits, each of which can bind oxygen.

C.
The fact that hemoglobin consists of αβ dimers, designated α1β1 and α2β2, which form the tetrameric hemoglobin.

D.
All of the above.

A.
sigmoidal

12

xygen and 2,3-bisphosphoglycerate (2,3-BPG) cannot bind to hemoglobin at the same time because:

the structure of hemoglobin is changed when oxygen binds such that 2,3-BPG can no longer bind.

13

Rapidly metabolizing tissues generate large amounts of protons and carbon dioxide. The result is that:

the oxygen-binding curve of hemoglobin (Y vs. pO2) is shifted to higher pO2 levels.

14

In addition to transporting oxygen from the lungs to the tissues, hemoglobin is also involved in transporting carbon dioxide from the tissues to the lungs. How is this accomplished?

Carbon dioxide reacts with terminal amino groups on hemoglobin in a reversible manner.

15

Each chain of hemoglobin can be viewed as existing in one of two states—the R state and the T state. What is the relationship of these states to oxygen binding?

Oxygen binds to the T state, converting it into the R state.

16

The difference between the "concerted" and "sequential" models of oxygen binding to hemoglobin is:

whether the transition between T and R states is "all-or-nothing" or has intermediate states(mixtures of R and T states in the same molecule).

17

In fetal hemoglobin the two β subunits are replaced with two γ subunits, resulting in fetal hemoglobin having a higher affinity for oxygen than the mother's normal adult hemoglobin. This is due to:

decreased binding of 2,3-BPG.

18

Hemoglobin S, the abnormal form of hemoglobin responsible for sickle cell anemia, is the result of a mutation in the gene for the β subunit. This mutation results in the change of:

negatively charged amino acid R-group to a hydrophobic amino acid R-group.
Glu->Val

19

The molecular consequences of the hemoglobin S mutation are that:
A.
the hemoglobin S forms aggregates and fibrous precipitates when oxygen is bound.

B.
the hemoglobin S forms aggregates and fibrous precipitates when oxygen is released.

C.
the hemoglobin S has a lower solubility and tends to precipitate in the lungs.

B.