Practice Quiz 8: Enzyme Kinetic Analysis
Questions 1-7:  ATCase kinetics.
Aspartate transcarbamoylase (ATCase) catalyzes the condensation of carbamoylphosphate (CP)and Aspartate (Asp) as co-substrates in the biosynthesis of N-carbamoylaspartate, which is one of the intermediary precursors in the multi-step biosynthesis of cytidine triphosphate (CTP) as indicated below.
  • The activity of this enzyme is affected by two regulatory molecules -- ATP and CTP.
  • Disulfide blocking agents, like HgNO3, also affect the enzymatic activity of ATCase.
  • For this exam, consider the rates of product formation catalyzed by ATCase when analyzed in the presence of varying Asp concentrations, [Asp], with a fixed carbamoylphosphate concentration, [CP], in excess over the total enzyme concentration, [E]tot.
  • With a fixed [CP] excess over [E]tot, the reaction becomes essentially 2nd order in terms of [Asp] and [E].
Open the Hill plot showing the experimental kinetic data obtained for ATCase catalysis under varying conditions as indicated.  Examine this plot to  answer the following questions.   Note:  The adjustable green ruler line corresponds to the Hill equation for this enzyme.  Use this line to help you answer these questions.
1)
Select the following number that most closely approximates the value of KM for ATCase in the absence of ATP, CTP, and HgNO3 based on this Hill plot.
  (a) (b) (c) (d) (e)
  1.00E-09 9.00E-02 1.00E-02 1.00E-04 0.1
2)
Compare the overall reaction rates of ATCase in the presence and absence of 0.4 mM CTP as shown on this Hill plot.  Which statement best summarizes the apparent effect CTP has on the kinetic activity of ATCase?
  (a) (b)
  CTP stimulates enzyme activity. CTP inhibits enzyme activity.
  (c) (d)
  CTP has no effect on enzyme activity. CTP both stimulates and inhibits enzyme activity.
3)
From the data shown on this Hill plot, which number below, if any, corresponds most closely to the minimum valence of ATCase- i.e., it's minimum number of active sites - that can be inferred from these data?
  (a) (b) (c) (d) (e) (f) (g)
  1 2 3 4 5 6 The minimum number of active sites cannot
accurately be determined from the Hill plot.
4)
Under what conditions does ATCase exhibit appear to exhibit noncooperative kinetics as determined from this Hill plot?
  (a) (b) (c) (d) (e)
  Asp + 0.4 mM CTP Asp + 0.2 mM ATP Asp alone Asp + 1 mM HgNO3 Under none of these conditions does
ATCase appear to exhibit noncooperative kinetics.
5) Under what conditions does ATCase exhibit its maximum degree of cooperativity in converting Asp to product as determined from this Hill plot?
  (a) (b) (c) (d)
  Asp + 0.4 mM CTP Asp + 0.2 mM ATP Asp alone Asp + 1 mM HgNO3
6) Under what conditions does ATCase exhibit the least activity as determined from this Hill plot?
  (a) (b) (c) (d)
  Asp + 0.4 mM CTP Asp + 0.2 mM ATP Asp alone Asp + 1 mM HgNO3
7) At what concentration of [Asp] - to the nearest integer - does ATCase achieve 75% of its maximal activity in presence of 0.4 mM CTP?
For 75% ATCase activity in the presence of 0.4 mM CTP, [Asp] =    mM.
Questions 8-11: Small Molecule Structure Analysis
Click here to view the structure of a hexapeptide pepstatin inhibitor of HIV protease, with its Ca-carbons numbered "(1)-(5)," and answer the following questions in reference to this structure.
8) Residue numbers 1 and 2 of this hexapeptide pepstatin inhibitor probably derive directly from which standard amino acid?
  A
 		C
 		D
 		E
 		F
 		G
 		H
 		I
 		K
 		L
  M
 		N
 		P
 		Q
 		R
 		S
 		T
 		V
 		W
 		Y
  Residue numbers 1 and 2 is not likely to derive directly from any of the standard amino acids.
9) Residue number 3 of this hexapeptide pepstatin inhibitor probably derives directly from which standard amino acid?
  A
 		C
 		D
 		E
 		F
 		G
 		H
 		I
 		K
 		L
  M
 		N
 		P
 		Q
 		R
 		S
 		T
 		V
 		W
 		Y
 
Residue numbers 3 is not likely to derive directly from any of the standard amino acids.
10) Residue number 4 of this hexapeptide pepstatin inhibitor probably derives directly from which standard amino acid?
  A
 		C
 		D
 		E
 		F
 		G
 		H
 		I
 		K
 		L
  M
 		N
 		P
 		Q
 		R
 		S
 		T
 		V
 		W
 		Y
  Residue numbers 4 is not likely to derive directly from any of the standard amino acids.
11) Residue number 5 of this hexapeptide pepstatin inhibitor probably derives directly from which standard amino acid?
  A
 		C
 		D
 		E
 		F
 		G
 		H
 		I
 		K
 		L
  M
 		N
 		P
 		Q
 		R
 		S
 		T
 		V
 		W
 		Y
  Residue numbers 5 is not likely to derive directly from any of the standard amino acids.
12-13 : Protein Structure Analysis
Click here to examine the active sites of three serine proteases -- chymotrypsin, elastase and trypsin.
Identify the corresponding active sites for these enzymes, designated  "I," "II," and "III," by answering the following questions.
12) Active site "I" of the three serine proteases is most likely that of which serine protease?
  (a) (b) (c)
  chymotrypsin elastase trypsin
13) Active site "II" of the three serine proteases is most likely that of which serine protease?
  (a) (b) (c)
  chymotrypsin elastase trypsin
14) The sidechain carboxyl group of Asp194 in all three of these proteases forms a salt bridge with the N-terminal group of amino acid residue number 16.  In two of these proteases, amino acid residue 16 is an isoleucine.  In the third protease, however, amino acid residue 16 is not an isoleucine. 
Select one of the choices below to identify the amino acid whose N-terminal group makes a salt bridge with the carboxyl group of Asp194 in this third protease.
  A
 		C
 		D
 		E
 		F
 		G
 		H
 		I
 		K
 		L
  M
 		N
 		P
 		Q
 		R
 		S
 		T
 		V
 		W
 		Y
After you have finished answering all questions and are ready to have your quiz graded, click the button.
© Duane W. Sears
Revised: October 07, 2010