Glucagon works via G-protein system. Which of the following best explains why gl

Question

Glucagon works via G-protein system. Which of the following best explains why glucagon stimulates glycogen breakdown in liver cells, but stimulates lipid break down in fat cells.

a. Fat cells contain Gs-protein while liver cells contain a Gi-protein

b. Liver cells don’t contain adenylyl cyclase

c.The two cell types contain a different set of proteins phosphorylated by protein kinase A

d. Glycogen and lipid breakdown are not governed by cyclic AMP levels.

phate from ATP to select protein amino acid residues. The Passaggettfrusstigaom93-100) biological effects of cyclic AMP are mediated by changes in protein phosphoryla General hormones which circulate in the blood can be divided into 3 categories: 1) Steroid hormones, synthesized in In 1957, Earl Sutherland found tha ver homogenates ncul the smooth endoplasmic reticulum of endocrine glands and bated with either epinephrine or glucagon would stimulate the secreted from the cell via exocytosis, bind to proteins for trans activity of glycogen phosphorylase. an enzyme which govern port to their target tissue. They then diffuse through the target the conversion of giycogen to glucose-1-phosphate. However, cell membrane and normally bind to a large receptor protein in f the membranes present in the homogenate were removed by the cytosol, which in turn camies them into the nucleus where longer be still be acti activated by epinephrine or glucagon, but could they exert their effect directly at the transcriptional level. 2) Peptide hormones bind to membrane bound receptors and act vated by the addition of cyclic AMP via a second messenger system. 3) Tyrosine derivatives are further divided into thyroid hormones and the catecholamines, diffuse epinephrinc and norepinephrine. Thyroid 93. Cortisol most likely binds to a receptor protein hrough the cell membrane and bind to receptors in the nucleus. bound to the ce membrane They also act directly at the transcription level. The cate- cholamine ac membrane through a second n the cytosol at the ce C. on the nuclear membrane messenger system. just outside the cel 94. Phosphodiesterase breaks down cyclic AMP to AMP. Caffeine, a drug abundant in coffee, suppresses the activ ity of phosphodiesterase. According to the information in receptor he passage, which of the following would most likely be found in a blood sample of someone who has recently drunk large amounts of coffe adenyly cyclase A. high ADH levels stimulation B. ow insulin levels C. high glucose levels GTP G protein ATP CAMP D. ow glucose levels GDP activation 95. A certain mutant tumor cell line has normal epinephrine receptors and normal adenylyl cyclase; however. fails to Protein Kinase A increase c in the presence of epinephrine. The mos likely explanation for this is: The G -prot ein in the cel s either missing or mal functio The Gi-protein in the ce s either missing or mal various proteins functioning Figure 1 Epinephrine diffuses directly into the cell to act on protein kinase A One second messenger system, shown in Figure 1, works as D. The mutation resuits in a change in the structure of follows: The activated membrane bound hormone receptor acti protein kinasc A nside the cell, called a G-protein. Once vates a protein activated, the G-protein exchanges G for GTP, which causes Depending upon the a portion of the G-protein to dissociate type of G-protein, the dissociated portion may stimulate or inhibit adenylyl cyclase, another membrane bound protein. Inhibitory G-proteins are called Giproteins; stimulating G proteins are called G proteins. After activating adenyly dissociated portion of the G-protei cyclase, the mu hydrolyze GTP to GpP in order to become inactive and recom bine with ts other portion. Adenylyl cyclase converts ATP to cyclic AMP Cyclic AMP activates protein kinase A. Kinases are a family of enzymes that catalyze the transfer of y-phos 210 Copyright 2007 Examkra ckers GO ON TO THE NEXT PAGE.

Explanation / Answer

93. B. in cytosol Reason : though Cortisol is a steroid so it will bind to the large receptor protein in the cytosol of a target tissue.

94. C. high glucose level (Reason : there will be more cyclic AMP in blood so. )

95. A (Reason : the mutant cell can't bring a change in Protein kinase A but it can inhibit the function of G-Protein either on or off the binary signaling switch.

last question's answer : C The two cell types contain a different set of proteins phosphorylated by protein kinase A (because it requires different type of GPCRs)

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