catlitic stratigies

Chapter 15 Questions

Glycogen Metabolism

1) Book questions 2,6, 7 and 8

2) Is the energy required to synthesize glycogen from glucose 6-phosphate the same as the energy required to degrade glycogen to glucose 6-phosphate?

No. More energy is required to synthesize glycogen from glucose 6-P. During glycogen synthesis, one high energy phosphoanhydride linkage is hydrolyzed since PPi formed by the action of UDP glucose pyrophosphorylase is quickly converted to 2 Pi. In contrast no cleavage of a high energy phophoanhydride bond is needed to degrade glycogen to glucose 6 phosphate because the glucose residue is removed by a phosphorolysis reaction.

3) The phosphorolytic cleavage of glycogen is key for glycogen metabolism. Why?

Hydrolysis of glycogen will generate glucose which is free to leave the cell and will require the additional input of energy to phosphorylate to glucose 6-P. Phosphorolytic cleavage of glycogen produces glucose 1-P which can be converted to glucose 6 P and be utilized by several different pathways. This reaction does not require ATP and is therefore more efficient because it decreases the ATP investment.

4) T/F

glycogen is stored in muscle and liver T
glycogen is a major source of stored energy in brain F
glycogen reserves can exceed the equivalent of 10,000 kcal in the average human F
glycogen fills the nucleus of the cells that store glycogen F
glycogen storage occurs in the form of dense granules in the cytosol of cells T

5) Which enzyme is not required for the production and release of LARGE amounts of glucose from the liver. (#2)

glucose 6-phosphatase
fructose 1,6-bisphosphatase
debranching enzyme
phosphoglucosmutase
glycogen phosphorylase

6) Why is it important to have different pathways for glycogenesis and glycogenolysis in liver and muscle cells?

The separate pathways for the synthesis and degradation of glycogen allow the synthesis of glycogen to proceed despite a high ratio of Pi to glucose 1 phosphate. The separate pathways allow the coordinated reciprocal control of glycogen synthesis and degradation by hormonal regulation.

7) How does the regulation of phosphorylase in the liver differ from the scheme for phosphorylase regulation in muscle shown in figure 23-8 in Stryer?

AMP doesn’t activate liver phosphorylase (the B to A conversion shown n fig 23-8), and glucose shifts the equilibrium between the activated phosphorylase a toward the inactivated form ( the D to C conversion).

8) For figure 23-8, determine which forms of the phosphorylase are active and the enzyme that catalyzes the conversion of the phosphorylated/dephosphorylated form of phosphorylase.

9) T/F

glycogen synthase is activated when it is dephosphorylated T
glycogen synthase is activated when it is phosphorylated F
glycogen synthase is activated when it is phosphorylated and in the presence of high levels of glucose 6-phosphate T
glycogen synthase is activated when it is phosphorylated and in the presence of high levels of AMP F

10) Which of the following statements about hormonal regulation of glycogen synthesis and degradation are correct?

insulin increases the capacity of the liver to synthesize glycogen T
insulin is secreated in response to low levels of blood glucose F
glucagon and epinephrine have opposing effects on glycogen metabolism F
glucagon stimulates the breakdown of glycogen, particularly in the liver T
the effects of all three of the regulating hormones are mediated by cAMP F

11) An investigator has a sample of purified muscle phosphorylase b that she knows is inactive. Suggest two test tube (invitro) methods that could be used to make the enzyme active. After the phosphorylase is activated, she incubates the enzyme with a sample of unbranched glycogen. However she finds no uncleaved glycosal units. What else is needed for the cleavage of glycosal residues by active phosphorylase?

She can activate the enzyme by adding AMP to the sample or by using active phosphorylase kinase and ATP to phosphorylate the enzyme. Inorganic phosphate is also required for the conversion of glycosyl residues in glycogen to glucose 1-phosphate.