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Return to Biochemistry: The Molecular Basis of Life 7e Student Resources
Chapter 6 Review Quiz
Enzymes
Quiz Content
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Most enzymes function within a narrow range of pH. Many have a closely defined optimum pH. Enzymes which function in the stomach usually have lower pH optima than those which are found in the intestine. Enzymes could be expected to be sensitive to minor pH changes because:
At very high pH, enzymes are destroyed
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There are no physiological buffers which can withstand enzyme activity, so special proteins are needed in order to dilute the effects of strong acids and bases
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Acids have no effect on enzymes
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There are no organic acids or bases
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Changes in proton concentrations result in changes in charges of acidic or basic groups, resulting in changes in the enzyme structure
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Some very powerful digestive enzymes are produced as inactive zymogens in the pancreas and transported to the intestines, where they are activated by removal of a protein "keeper." These enzymes then act during digestion, and also act on other zymogens, activating them. Occasionally, an infection or injury to the pancreas causes these enzymes to become chemically activated within the pancreas. Under these conditions:
Digestion in the intestine would proceed faster than normal
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Fats would be absorbed more rapidly than normal
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The liver would require less energy supply than normal
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The patient would probably notice no differences in his or her well being
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The pancreas would become inflamed and severe pain would result
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Most growing cells require folic acid. Bacteria synthesize their folic acid from a number of materials, including para-aminobenzoic acid. We slow bacterial growth by treating infections in humans with sulfa drugs. (The sulfa drugs are very similar to para-aminobenzoic acid and act as competitive inhibitors, keeping the bacteria from synthesizing folic acid.) If the sulfa drugs kill bacteria, why don't they harm the human, too?
There are too many bacteria, and some consume the sulfa drugs and die, leaving the remaining bacteria to overwhelm the host
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Humans don't make folic acid (it=s a vitamin for us) anyway, so the sulfa drugs usually don=t harm humans
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Humans don't metabolize folic acid
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Humans don't metabolize sulfa drugs
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Humans can take extra folic acid to compensate for the sulfa drugs
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Enzymes have a distinct advantage over non-biological catalysts because they:
Are made of protein
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Are very efficient, specific, and sensitive to control
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Are all allosteric
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Can only be described by Lineweaver-Burk plots
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Can not be denatured by heating
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In a Lineweaver-Burk plot of enzyme activity, the highest values of [S] and Km are found:
Near the origin
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At the top of the vertical axis
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At the right end of the horizontal axis
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At the left end of the horizontal axis
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At the horizontal intercept of the curve
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Most synthetic reactions in which carbon dioxide is used require the vitamin biotin. The biotin becomes covalently bound to a lysine molecule which is part of an enzyme, and the biotin can then attach to and carry the carbon dioxide molecule. Biotin is best referred to as a/an:
Coenzyme
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Enzyme inhibitor
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Competitive inhibitor
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Organic acid
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Enzyme
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The fact that allosteric enzymes are remarkably sensitive to control makes them ideal candidates for:
The reverse of regulatory steps in a pathway
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The rate-limiting steps in a pathway
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The final steps in a pathway
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All the steps in a pathway
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Alternative pathways
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If you wanted to explain the concept of Km to someone who didn't know as much about biochemistry as you, you might say that:
Km is not really a useful concept for biochemists
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Km is a measure of how badly the enzyme reacts in the presence of other enzymes
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Km is the substrate concentration which allows the reaction to proceed at half the maximum velocity
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Km is only useful when considering noncompetitive inhibitors; competitive inhibitors don't have Km values
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Km is the reciprocal of the intercept on the vertical axis of a Lineweaver-Burk plot
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Methanol is a very poisonous substance, accounting for a number of deaths and many cases of blindness yearly. While the methanol itself is sometimes not toxic, it may be metabolized by the liver to more toxic molecules such as formic acid or formaldehyde. Before more efficacious treatments were developed, a person who had ingested methanol was often given a large amount of ethanol orally. The purpose of this treatment was:
To allow the ethanol to act as a competitive inhibitor of the metabolism of methanol
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To allow the ethanol to act as a noncompetitive inhibitor of the metabolism of methanol
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To make the patient forget about the pain so that the operation could proceed
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To deaden the tissues to pain
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To dissolve the methanol and move it through the circulation faster
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Most coenzymes may be included in classes of foodstuffs known as:
Minerals
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Carbohydrates
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Fats
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Vitamins
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Metallic ions
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The major effect of a catalyst on a reaction is to:
Keep the substrates apart
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Lower the equilibrium constant
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Raise the equilibrium constant
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Raise the activation energy
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Lower the activation energy
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It is more useful for biochemists to know the specific activity of an enzyme than the activity of the enzyme because:
High levels of activity indicate very busy enzymes
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Specific activity tells us exactly which reaction is being catalyzed and results in more positive identification of enzymes
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Biochemistry is a very specialized field
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Enzyme activation is brought about only under specialized conditions
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Specific activity is a measure of the purity of the enzyme preparation
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An enzyme in liver which removes glucose from blood has a high Km; another which does the same thing in brain tissue has a small Km. The usefulness of these two different values is that:
The enzyme in brain tissue can take glucose from the blood stream even when levels are low, and the enzyme in liver will not take glucose from blood unless levels are high
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The enzyme in brain tissue can take glucose from the blood stream only when levels are high, and the enzyme in liver will not take glucose from blood unless levels are low
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The enzyme in brain tissue can take glucose from the blood stream only when levels are high, and the enzyme in liver will not take glucose from blood at any time
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This phenomenon is of no apparent value to the organism
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It is actually dangerous for the organism; ideally, both Km=s should be high
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A molecule of an enzyme which has a high turnover number:
Can easily be denatured
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Can easily be replaced with another enzyme
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Needs a constant supply of cofactors
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Converts substrate to product very rapidly
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Can be easily controlled
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Most amino acids found in living things are L-form amino acids. A few amino acids found in antibiotics are D-form. The enzyme which converts D-alanine to L-alanine is best classified as an:
Transferase
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Hydrolase
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Lyase
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Isomerase
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Ligase
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In addition to activation and inhibition, enzymes may be controlled by
Polymerization, hydrolysis, and esterification
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Hydrolysis, esterification, and compartmentation
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Esterification, compartmentation, and induction
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Compartmentation, induction, and covalent modification
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Induction, covalent modification, and polymerization
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Consider the reaction: ethyl acetate + H
2
OC> ethanol + acetic acid The enzyme which catalyzes this reaction is classified as a/an:
Oxidoreductase
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Transferase
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Hydrolase
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Lyase
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Isomerase
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One way to tell whether an enzymatic reaction is carried out by an oxidoreductase is to:
Examine the substrate and products for loss or gain of hydrogen atoms
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Examine the substrate to determine whether oxygen is present
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Watch for flames to develop in the reaction bath
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Note the increasing temperature of the reaction solution
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Note any changes of color in the solution
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Parkinson's disease is caused by a deficiency of dopamine in the substantia nigra of the brain. Dopamine is produced in the brain from L-DOPA (L-dihydroxyphenylalanine) by removal of a molecule of carbon dioxide. The L-DOPA is produced by oxidation of tyrosine, which is in turn produced from phenylalanine by another oxidoreductase. The enzyme which converts L-DOPA to dopamine is best classified as a/an:
Oxidoreductase
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Transferase
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Hydrolase
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Lyase
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Isomerase
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As a solution containing an enzymatic reaction is heated, the temperature rises and the reaction velocity rises gradually for some period, but typically falls off very rapidly after reaching the optimum temperature because:
The reaction runs out of substrate and substrate must be continually added in order to attain optimal activity
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The enzyme functions more rapidly as the solution is heated because the molecules move faster, but after a certain temperature, the enzyme becomes denatured
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Enzymes can only function for a certain time period without resting
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Substrates wear out after a certain time period
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Heating darkens the solution, and the molecules need light in order to react
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During replication, a number of DNA fragments are produced. Energy is required for an enzyme to join these fragments together to form an intact strand of DNA. The enzyme is classified as a/an:
Transferase
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Hydrolase
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Lyase
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Isomerase
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Ligase
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Melanin, the dark pigment in our hair and skin, is produced by a series of enzymatic reactions which start with phenylalanine. The phenylalanine is converted to tyrosine, then to L-DOPA, and finally though a series of steps to melanin and other substances. If the conversion of phenylalanine to tyrosine were blocked, the individual affected would probably:
Enjoy enhanced mental abilities
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Be able to function in environments having low oxygen concentration
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Have light-colored hair
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Produce pleasant body odors
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Have dark-colored hair
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