Biochemistry
6th Edition
ISBN: 9781305577206
Author: Reginald H. Garrett, Charles M. Grisham
Publisher: Cengage Learning
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Textbook Question
Chapter 27, Problem 20P
Figure 27.3 illustrates the response of R (ATP-regenerating) and U (ATP-utilizing) enzymes to energy charge.
a. Would hexokinase be an R enzyme or a U enzyme? Would glutamine: PRPP amidotransferase, the second enzyme in purine biosynthesis, be an R enzyme or a U enzyme?
b. If energy charge = 0.5: Is the activity of hexokinase high or low? Is ribose-5-P pyrophosphokinase activity high or low?
c. If energy charge = 0.95: Is the activity of hexokinase high or low? Is ribose-5-P pyrophosphokinase activity high or low?
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The following Michaelis-Menten plot shows the response of phosphofructokinase-1 (PFK-1), an enzyme in the glycolytic
pathway, to the presence of ATP and AMP. Based on the data below, which of the following statements apply? Select all
that apply.
No inhibitors (low (ATP))
1 mM ATP
+ 0.1 mM AMP
1 mM ATP
1.0
2.0
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Fig. : Regulation of PFK activity
AMP is an allosteric modifier that enhances the binding of the substrate fructose 6-phosphate
а.
O b. AMP is an allosteric modifier that reduces the binding of the substrate fructose 6-phosphate
ATP is an allosteric modifier that reduces the binding of the substrate fructose 6-phosphate
O c.
O d. ATP is an allosteric modifier that enhances the binding of the substrate fructose 6-phosphate
Phosphofructokinase activity
When ATP and xylose are added to hexokinase, the enzyme produces a small amount of xylose-5-phosphate along with a large amount of ADP. How does this observation support the induced fit model?
The enzyme aspartate transcarbamoylase catalyzes an early step in pyrimidine biosynthesis. The two
states of the multi-subunit enzyme are shown below. Note that the binding of the regulatory molecule
CTP (cytosine triphosphate) causes the enzyme complex to be inactive. Is this situation an example of
positive or negative regulation? Explain why the use of CTP as the regulatory molecule is logical givén
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INACTIVE ENZYME: T STATE
catalytic
subunits
regulatory
subunits
OFF
6 CTP
ON
ACTIVE ENZYME: R STATE
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