Answer to Question #101772 in Chemistry for Jeevan Jyoti behera

Question

Q. (a) Explain in brief the regulation of glycogen metabolism.
(b) how many moles of ATP and NADH are required to convert 6 moles of carbon dioxide to fructose -6-phosphate? Give all the reactions involved.

Answer 1

(a) Glycogen synthesis and the breakdown and reciprocally controlled by cAMP cascade acting through protein kinase A. Activation of the signalling pathways regulating glycogen metabolism are controlled by blood hormones (i.e., epinephrine, insulin, glucagon). In particular, epinephrine and glucagon activate adenylate cyclase that produces cAMP. This secondary messenger activates protein kinase A that phosphorylates and activates phosphorylase kinase. This enzyme activates glycogene phosphorylase b (active form phosphorylase a) that mediates glycogen breakdown. In addition, protein kinase A directly inactivates glycogen synthase. In contrast, insulin activates the specific insulin-sensitive protein kinase that activates protein phosphatase dephosphorylating glycogen synthase. The process triggers glycogen synthesis suppressing glycogen breakdown.

(b) CO₂ fixation occurs during the Calvin cycle in chloroplasts.

The first step of the cycle is mediated by the enzyme RuBisCO providing CO₂ fixation:

3 Ribulose-1,5-bisphosphate + 3 CO₂ = 6 3-Phosphoglycerate

The second step is accompanied by the reduction of 3-Phosphoglycerate:

6 Phosphoglycerate + 6 ATP = 6 1,3-Bisphosphoglycerate + 6 ADP

6 1,3-Bisphosphoglycerate + 6 NADPH = 6 Glyceraldehyde 3-phosphate + NADP⁺

The third step is associted with the regeneration of Ribulose-1,5-bisphosphate:

5 Glyceraldehyde-3-phosphate + 3 ATP = 3 Ribulose-1,5-biphosphate + 3 ADP

In parralel, one molecule of Glyceraldehyde-3-phosphate is involved in the formation of fructose-6-phospate:

Glyceraldehyde-3-phosphate + dihydroxyacetone phosphate = fructose-6-phosphate

As dihydroxyacetone phosphate is obtained from Glyceraldehyde-3-phosphate, three more cycles are required to produce fructose.

The net equation of the reaction is as following:

6CO₂ + 18ATP + 12NADPH + 12 H₂O = C₆H₁₂O₆ + 18 ADP + 18 P_i + 12 NADP⁺ + 6H⁺

Answer: 18ATP and 12NADPH