1.46: Enzymes and Isoenzymes

1. Enzymes accelerate the reaction rate by:
   1. decreasing the amount of free energy of activation necessary for the reaction
   2. shifting the equilibrium position of a reaction
   3. causing thermodynamically incompatible reactions to occur
   4. increasing the rate of the reaction in one direction only
   5. c and d
2. Enzyme nomenclature is used to describe:
   1. the reaction taking place
   2. the physical conditions of the assay
   3. the conversion of products to substrates
   4. only two point reactions e. the buffer in the assay
3. A substance which when added to the enzyme attaches to a site removed from the active site so that the enzyme cannot bind its natural substrate is a (an):
   1. competitive inhibitor
   2. non-competitive inhibitor
   3. substrate analog
   4. enzyme cofactor
   5. coenzyme
4. To assure that zero order kinetics is maintained in an enzyme reaction, the substrate concentration should be:
   1. equal to the K_m
   2. less than the K_m
   3. at least 10 times greater than the K_m
   4. equal to 1/K_m
   5. equal to the enzyme concentration
5. For which order of reaction is the rate dependent only on the enzyme concentration?:
   1. zero order
   2. first order
   3. second order
   4. mixed order
   5. none of the above
6. The point at which an enzyme reaction is proceeding at the greatest rate is:
   1. the Michaelius constant (K_m)
   2. zero order kinetics
   3. first order kinetics
   4. point where the rate of the reaction is dependent on the substrate concentration
   5. [S] = K_m
7. Enzymes of metabolism:
   1. are present in all cells
   2. are plasma specific enzymes
   3. have a known function in serum
   4. have a known function in plasma
   5. are produced in large quantities after eating
8. Isoenzymes are:
   1. multiple molecular forms of an enzyme family that catalyze the same reaction
   2. different enzymes which exhibit the same enzymatic specificity
   3. multiple molecular forms of different enzymes which catalyze the same reaction
   4. different enzymes which exhibit the same electrophoretic mobility
   5. enzymes with the same tertiary structure which catalyze the same reaction
9. CK isoenzymes are diagnostically important because:
   1. erythrocyte and cardiac sources of elevation can be distinguished
   2. cardiac and hepatic sources of elevation can be distinguished.
   3. statistical analyses of the patterns of the 5 isoenzymes give a diagnostic classification of the various liver diseases
   4. the isoenzyme pattern indicates the specific tissue involved in a malignancy
   5. they are absolutely tissue specific
10. The distribution of isoenzymes:
    1. is the same throughout the body
    2. varies greatly during adult life
    3. is dependent on physiological needs
    4. varies by individual
    5. varies by organ
11. Serum from adults contain which isoenzyme of alkaline phosphatase?:
    1. heart, kidney, liver
    2. liver, kidney, bone
    3. kidney, red cells, liver
    4. red cells, liver, brain
    5. placenta, lungs, brain
12. In coupled enzyme systems, the enzyme in the last reaction:
    1. is employed as an activator
    2. must exhibit the same kinetic properties as the enzyme in the primary reaction
    3. is the one whose product is measured
    4. is of no significance
    5. forms the substrate for the enzyme being measured
13. The plot of 1/velocity vs. 1/substrate concentration is known as the:
    1. Arrhenius plot
    2. initial rate
    3. Michaelis-Menten plot
    4. maximal velocity
    5. Lineweaver-Burk plot
14. Situation: You are running an assay of enzyme “X” which utilizes the coenzyme NAD. Its activity is measured in terms of NADH produced. You have obtained the following data:
    
    Micromolar extinction coefficient (\(\epsilon\)) of NADH (at 340 nm) = 6.22 x 10^-3 L \(\cdot\) \(\mu\)mol^-1 \(\cdot\) c^-1; light path (b) = 1 cm total volume of assay = 3.0 mL sample volume = 0.5 mL absorbance change 7 minutes = 0.350
    
    Calculate the enzyme activity in IU/liter:
    1. 0.048
    2. 1.30
    3. 48
    4. 336
    5. 4.8
15. If zero order kinetics are followed, allowing the reaction to run twice the time will:
    1. cause the product to be denatured
    2. halve the amount of product formed
    3. have no effect on the amount of product formed
    4. double the amount of product formed
    5. double the absorbance change per minute
16. Skeletal muscle tissue contains which of the following CK isoenzymes?
    1. MM only
    2. MB only
    3. BB only
    4. MM + MB
    5. MM + BB
17. Creatine kinase (CK) isoforms are:
    1. degradative forms of individual CK isoenzymes
    2. polymerized forms of individual CK isoenzymes
    3. intracellular form of individual CK isoenzymes
    4. imunoglobulin-CK complex of individual isoenzymes
    5. none of the above.
18. If the sample volume is halved in an enzymatic reaction, the final calculated enzyme activity will:
    1. increase four-fold
    2. decrease four—fold
    3. increase two—fold
    4. decrease two-fold
    5. remain the same
19. According to IUB classification, creatine kinase is a/an:
    1. oxidoreductase
    2. transferase
    3. hydrolase
    4. lyase
    5. ligase

Use the following Key to answer Questions 20-27:

1. 1, 2, and 3 are correct
2. 1 and 3 are correct
3. 2 and 4 are correct
4. 4 only is correct
5. all are correct

20. An enzyme is:
    1. a protein
    2. highly specific
    3. not consumed in a reaction
    4. a catalyst
21. The type of curve below shows the effect of enzyme activity in relation to:
    1. substrate concentration
    2. temperature
    3. activator concentration
    4. pH

22. An advantage of kinetic methods of measuring enzyme activity is that:
    1. temperature is not critical
    2. pH is not critical
    3. there is elimination of the lag phase
    4. linearity is demonstrable
23. Which of the following statements concerning the measurement of serum enzymes is/are true?:
    1. enzymes are increased in serum following cell destruction and release of cellular constituents.
    2. enzymes are totally organ specific. A rise in an enzyme tells the physician exactly which organ is diseased.
    3. enzymes can rise to significant levels over background and help indicate the nature of the disease
    4. enzymes are depressed after stimulation of the exocrine glands
24. Acid phosphatase and alkaline phosphatase are:
    1. substrate specific
    2. isoenzymes
    3. the same enzyme acting at different pH levels
    4. separate enzymes
25. Different isoenzyme patterns are useful for determining:
    1. the age of the sample
    2. the severity of the disease process
    3. the condition under which the sample was stored
    4. which tissue was involved in the disease process
26. Isoenzymes can be measured by which of the following procedures?
    1. chromatography
    2. electrophoresis
    3. immunoassay
    4. heat stability
27. Which of the following compound(s) is (are) an enzyme cofactor?
    1. chloride
    2. pyridoxyl-5-phosphate
    3. magnesium
    4. thiamine-pyrophosphate
28. Enzyme measurements are usually performed in the:
    1. lag phase
    2. linear phase
    3. substrate depletion phase
    4. none of the above
    5. doesn’t matter which phase is used
29. An international unit of enzyme activity is defined as the number of:
    1. \(\mu\)moles product formed/second
    2. \(\mu\)moles product formed/minute
    3. mmoles substrate consumed/liter
    4. mmol substrate consumed/second
    5. none of the above
30. Nicotinamide adenine dinucleotide (NAD) is best considered a enzyme:
    1. cofactor
    2. activator
    3. substrate
    4. inhibitor
    5. holoenzyme
31. An organ’s isoenzyme composition never changes except as a result of disease.
    1. True
    2. False

Answer

1. a (p. 1047)
2. a (p. 1050-1051)
3. b (p. 1057)
4. c (p. 1054)
5. a (p. 1053-1054)
6. b (p. 1053-1054)
7. a (p. 1061)
8. a (p. 1065)
9. b (p. 1067-1070)
10. e (p. 1067)
11. b (p. 1068)
12. c (p. 1060)
13. e (p. 1055)
14. c (p. 1055)
15. d (p. 1054)
16. d (p. 1067)
17. a (p. 1066)
18. e (p. 1055)
19. b (p. 1051)
20. e (p. 1045, 1046, 1048)
21. c (p. 1056, 1059)
22. c (p. 1052-1053)
23. b (p. 1060-1061)
24. d (p. 1051)
25. d (p. 1068-1069)
26. e (p. 1071)
27. c (p. 1057)
28. b (p. 1054)
29. b (p. 1055)
30. c (p. 1044, 1057)
31. b (p. 1068)