Chapter 6 Questions
Type of Enzyme Inhibitors
1. A competitive inhibitor has a structure that is so similar to the substrate that it can bond to the enzyme just like the substrate. 2. A noncompetitive inhibitor binds to a site on the enzyme that is not the active site. 3. Usually, an irreversible inhibitor forms a covalent bond with an amino acid side group within the active site, which prevents the substrate from entering the active site or prevents catalytic activity. 4. The competitive inhibitor competes with the substrate for the active site on the enzyme. 5. When the noncompetitive inhibitor is bonded to the enzyme, the shape of the enzyme is distorted. 6. Enzyme inhibitors disrupt normal interactions between an enzyme and its substrate.
Factors that affect enzymes
1. An enzyme is denatured when it loses its native conformation and its biological activity. 2. An enzyme is considered a catalyst because it speeds up chemical reactions without being used up. 3. An enzyme is considered specific because of its ability to recognize the shape of a particular molecule. 4. A cofactor, such as a vitamin, binds to an enzyme and plays a role in catalysis. 5. When properly aligned, the enzyme and substrate form an enzyme-substrate (ES)complex. 6. A substrate binds to an enzyme at the active site, where the reaction occurs. 7. In a catalyzed reaction a reactant is often called a substrate.
Remember that the slope of this line is calculated as Δy/Δx (the "rise" over the "run"), in μmol/mL • min. What is the approximate slope of the line between 5 and 20 minutes?
17
You have an enzymatic reaction proceeding at the optimum pH and optimum temperature. You add a competitive inhibitor to the reaction and notice that the reaction slows down. What can you do to speed the reaction up again?
Add more substrate; it will outcompete the inhibitor and increase the reaction rate.
The image (Figure 1) shows a hypothetical enzymatic pathway with four enzymes, labeled E1, E2, E3, and E4. The enzymes make products, labeled P0, P1, P2, P3, and P4. Which of the following statements is most likely to be true in the case of the feedback-regulated enzymatic pathway shown?
P4 binds E1 and deactivates it.
Which of the following is NOT a way in which an enzyme can speed up the reaction that it catalyzes?
The active site can provide heat from the environment that raises the energy content of the substrate.
A graphical version of the data (similar to the one you just made) is shown at left (Figure 2) . (Select Figure 2 in the drop-down menu above the table.) How would you describe the pattern you see in the graph?
The concentration of in the buffer increases slowly at first, then rises rapidly, then reaches a maximum and stays there over the time course.
Which of the following statements about ATP (adenosine triphosphate) is correct?
The cycling between ATP and ADP + Pi provides an energy coupling between catabolic and anabolic pathways.
Which of the following statements about the role of ATP in cell metabolism is true?
The energy from the hydrolysis of ATP may be directly coupled to endergonic processes by the transfer of the phosphate group to another molecule.
Which of the following statements about the combustion of glucose with oxygen to form water and carbon dioxide (C6H12O6 + 6 O2 → 6 CO2 + 6 H2O) is correct?
The entropy of the products is greater than the entropy of the reactants.
You have added an irreversible inhibitor to a sample of enzyme and substrate. At this point, the reaction has stopped completely. What can you do to regain the activity of the enzyme?
The enzyme is inactive at this point. New enzyme must be added to regain enzyme activity.
What is the free energy change (ΔG) of the hydrolysis of ATP to ADP?
The free-energy change (ΔG) of the hydrolysis of ATP to ADP and Pi may vary considerably with variations in pH, temperature, atmospheric pressure, and concentrations of reactants and products.
If your blood sugar level is low from skipping lunch, the following reaction will occur in your liver cells: Glucose 6-phosphate ----> (Glucose 6-phosphate) = Glucose + Pi
Your blood sugar level will increase as glucose is released from your liver cells into your bloodstream.
The process of cellular respiration, which converts simple sugars such as glucose into CO2 and water, is an example of _____.
a catabolic pathway
Which of these is exhibiting kinetic energy?
a space station orbiting Earth
What variable was measured by the researchers, and what are the units for this variable?
(inorganic phosphate) concentration, in micromoles per milliliter of sampled buffer
Consider that the rate of enzyme activity is related to the slope of the line, Δy/Δx (the "rise" over the "run"), in μmol/mL • min, with the steepest slope indicating the highest rate of enzyme activity. What part of the time course had the highest rate of enzyme activity?
between 5 and 20 minutes
In your body, what process converts the chemical energy found in glucose into the chemical energy found in ATP?
cellular respiration
The reaction ADP + P --> ATP is a(n) _____ reaction.
endergonic
This graph illustrates a(n) _____ reaction.
endergonic
Which of these reactions requires a net input of energy from its surroundings?
endergonic
"Conservation of energy" refers to the fact that _____.
energy cannot be created or destroyed but can be converted from one form to another
What is the correct label for "A"?
energy of activation (difference between final and initial energy)
A(n) _____ reaction occurs spontaneously.
exergonic
The energy for an endergonic reaction comes from a(n) _____ reaction.
exergonic
The following reaction A --> B + C + heat is a(n) _____ reaction.
exergonic
Select the INCORRECT association.
exergonic ... uphill
Use the data at left to make a line graph. To create the graph, first click on "add graph" and select the label "Concentration of Inorganic Phosphate Over Time." (For more help using the graphing tool, open the hint.)
graph with data
In this reaction _____. (lower energy)
heat has been released to the environment
Which of these are by-products of cellular respiration?
heat, carbon dioxide, and water
What type of reaction breaks the bonds that join the phosphate groups in an ATP molecule?
hydrolysis
Enzymes are described as catalysts, which means that they _____.
increase the rate of a reaction without being consumed by the reaction
Which of the following would increase the rate of the reverse reaction?
increasing the concentration of ammonia
An enzyme _____.
is an organic catalyst
As a result of its involvement in a reaction, an enzyme _____.
is unchanged
The graph presents three activation energy profiles for a chemical reaction (the hydrolysis of sucrose): an uncatalyzed reaction, and the same reaction catalyzed by two different enzymes. (uncatalyzed highest curve, A in the middle, B on the bottom) Rank these by reaction rate, as measured by the rate of product formation per unit time, from lowest reaction rate to highest reaction rate. To rank items as equivalent, overlap them.
lowest -> highest reaction rate uncatalyzed reaction, reaction catalyzed by enzyme A, reaction catalyzed by enzyme B
Chemical energy is a form of _____ energy.
potential
In general, enzymes are what kinds of molecules?
proteins
Enzymes work by _____.
reducing EA
In general, an enzyme has one active site at which catalysis can occur. When the substrates are bound to the active site, the enzyme will catalyze the reaction. As the concentration of substrate increases, the reaction rate increases, until the point where the active site is saturated with substrate. When the enzyme is saturated, the rate of the reaction will not increase with the concentration of substrates. Look at the graph of reaction rate versus substrate concentration for an enzyme. (Figure 1) In which region does the reaction rate remain constant?
region C
In which region is the enzyme saturated with substrate?
region C
What name is given to the reactants in an enzymatically catalyzed reaction?
substrate
In this reaction _____.
the products have less potential energy than the reactants
What is energy coupling?
the use of energy released from an exergonic reaction to drive an endergonic reaction
Does the rate of glucose 6-phosphatase activity change over time in isolated liver cells? Glucose 6-phosphatase, which is found in mammalian liver cells, is a key enzyme in control of blood glucose levels. The enzyme catalyzes the breakdown of glucose 6-phosphate into glucose and inorganic phosphate (). These products are transported out of liver cells into the blood, increasing blood glucose levels. In this exercise, you will graph data from a time-course experiment that measured concentration in the buffer outside isolated liver cells, thus indirectly measuring glucose 6-phosphatase activity inside the cells. Isolated rat liver cells were placed in a dish with buffer at physiological conditions (pH 7.4, 37°C). Glucose 6-phosphate (the substrate) was added to the dish, where it was taken up by the cells. Then a sample of buffer was removed every 5 minutes and the concentration of measured. The table (Figure 1) at left shows the data collected at each time interval. Part A - Making a graph with the data To see patterns in the data from a time-course experiment like this, it is helpful to graph the data. But first, you must determine which variable should go on each axis of the graph. What variable did the researchers intentionally vary in the experiment, and what are the units for this variable?
time of reaction, in minutes
Which part of the adenosine triphosphate molecule is released when it is hydrolyzed to provide energy for biological reactions?
γ-phosphate (the terminal phosphate)
Now that you have determined which variable goes on each axis, the graph can be constructed. An effective graph marks off the axes with just enough evenly spaced tick marks to accommodate the full set of data. Assuming that the x-axis tick marks will be separated by 5 (5, 10, 15, and so on), what is the largest value that should appear on the x-axis?
40
Assuming that the y-axis tick marks will be separated by 50 (50, 100, 150, and so on), what is the largest value that should appear on the y-axis?
400
Which of these is ATP?
A, Ribose, 3P
In cells, what is usually the immediate source of energy for an endergonic reaction?
ATP
Biological processes involve both catabolism, the breaking down of high-energy molecules into simple molecules, and anabolism, the building of complex molecules from simple ones. In general, catabolic processes generate energy, whereas anabolic processes require energy. The energy is usually stored in intermediate energy-carrying molecules such as adenosine triphosphate (ATP). Select the highest energy form of adenosine from the following images.
Adenosine triphosphate (ATP) is the high-energy form of adenosine because it contains the most phosphate groups (three). This molecule fuels many different endergonic (energy-requiring) enzymatic processes in biological organisms. ATP molecules diffuse or are transported to the place where the energy is needed and deliver chemical energy from the breaking of their phosphate bonds.
When the substrate (glucose 6-phosphate) is first added to the liver cells, it is transported into the cells and processed by the enzyme (glucose 6-phosphatase). Then the product is released from the cells into the buffer. Which of the following statements is the most reasonable biological explanation for the pattern you see on the graph?
As the enzyme activity slows down (when the slope reaches zero), there is no additional being released because the cells have used up the substrate.
What will happen to the rates of the forward and reverse reactions when a catalyst is added?
Both forward and reverse rates increase.
Consider a situation in which the enzyme is operating at optimum temperature and pH, and has been saturated with substrate. What is your best option for increasing the rate of the reaction?
Increase the enzyme concentration.
What is the fate of the phosphate group that is removed when ATP is converted to ADP?
It is acquired by a reactant in an endergonic reaction.
Select the correct statement about chemical energy, a term used by biologists to refer to potential energy available for release in a chemical reaction.
Light energy is converted to chemical energy during photosynthesis.
Which statement about the binding of enzymes and substrates is correct?
When substrate molecules bind to the active site of the enzyme, the enzyme undergoes a slight change in shape.
To learn how temperature, concentration, and catalysts affect the rate of a reaction. The rate of a reaction is defined as the ratio of the change in concentration of a product to the change in time: rate of reaction=change in concentration / change in time Three major factors will affect the rate of the reaction: temperature, concentration, and catalysts. In general, an increase in temperature, an increase in reactant concentration, or the presence of a catalyst will all increase the rate of the reaction. Ammonia, NH3, is used in numerous industrial processes, including the production of pharmaceuticals such as sulfonamide and antimalarials and vitamins such as the B vitamins. The equilibrium equation for the synthesis of ammonia (sometimes known as the Haber process) is N2(g)+3H2(g)⇌2NH3(g) The Haber process is typically carried out at a temperature of approximately 500∘C. What would happen to the rate of the forward reaction if the temperature were lowered to 100∘C?
The reaction rate would decrease .
What would happen to the rate of the forward reaction if the concentration of nitrogen were decreased?
The reaction rate would decrease .