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Wednesday, January 09, 2008

regulated by calcium or calmodulin, except Hexokinase

042. All of the following enzymes are regulated by calcium or calmodulin, except:

1. Adenylate cyclase

2. Glycogen synthase

3. Guanylyl cyclase

4. Hexokinase

Answer

4. Hexokinase

Reference:

Harper 26th Edition Page 458

Lippincott 3rd Edition Page 130

Ganong 22nd Edition page 43

Chaterjee 6th Edition Page 271

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Discussion

Enzymes and proteins regulated by Calcium and Calmodulin are (Harper)

1. Adenylyl cyclase

2. Ca2+-depended protein kinases

3. Ca2+- Mg2+ ATPase

4. Ca2+-phospholipid-dependent protein kinase

5. Cyclic nucleotide phosphodiesterase

6. Some cytoskeletal proteins

7. Some ion channels (eg L-type Calcium channels)

8. Nitric Oxide synthase

9. Phosphorylase kinase

10. Phosphoprotein phosphatase 2B

11. Some receptors (eg NMDA-type glutamate receptors)

Explanation

1. Several of the isoforms of the adenyl cyclase are stimulated by Calmodulin (Ganong)

2. Active Glycogen synthase (GS-a) is converted to inactive Glycogen synthase (GS-b) by phosphorylation, which is modulated by cAMP dependent protein kinase (Chaterjee)

3. Guanylyl cyclase is activated by Atriopeptins and Nitric Oxide, nitroprusside, nitroglycerin, sodium nitrate and sodium azide.(Harper). Nitric Oxide Synthase is activated by Calcium or Calmodulin. Hence we can assume that Calcium or Calmodulin regulate Guanylyl cyclase

4. Hexokinase is inhibited by Glucocorticoids and GH and Glucose-6-Phosphate. Insulin does not have effect on hexokinase

Comments

Cyclic AMP was the first intracellular signal identified in mammalian cells

Tips

Guanylate cyclase (EC 4.6.1.2, also known as guanylyl cyclase or GC) is a lyase, an enzyme that catalyzes the conversion of guanosine triphosphate (GTP) to 3',5'-cyclic guanosine monophosphate (cGMP) and pyrophosphate. There are soluble and membrane-bound forms of guanylyl cyclases. Soluble GC (sGC) is activated by nitric oxide, most notably involved in vasodilation. Membrane-bound forms are receptors: GC-A & GC-B for natriuretic factors such as atrial natriuretic factor (ANF), and GC-C for guanylin.

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