Question

Glucose are stored as glycogen in muscle cells, from which it can again be mobilized during exercise.

a) Write down the partial reactions as well as the net yield for the conversion of glucose-6-phosphate to glycogen and for the conversion of glycogen to glucose-6-phosphate.

b) Calculate the energy required for the consumption of the total process of converting glucose-6-phosphate to glycogen and back to glucose-6-phosphate expressed as energy-rich phosphate bonds (ATP equivalents). ΔG0’ for the reaction: ATP + H2O → ADP + Pi = –30.5 kJ∙mol-1.

c) Calculate ΔG’ for the reaction at 25 °C and pH 7, when [ATP] = 5 mM, [ADP] = 0.5 mM and [Pi] = 1.0 mM. Phosphocreatin constitutes a small extra storage of energy within muscle cells, and is formed from phosphorylation of creatin. The reaction is catalyzed by creatine kinase and ΔG0’ for the reaction is +2.3 kJ∙mol-1 (progressing towared the formation of phosphocreatin).

d) Calculate how much phosphocreatin formed relative to creatin at equilibrium in the creatin kinase catalyzed reaction (assuming the same temperature, pH and concentrations of ATP and ADP as indicated in the previous question).

Answer 1

a) glucose-6-phosphate to glycogen:

Glucose-6-phosphate is converted into glucose-1-phosphate by phosphoglucomutase. This is followed by formation of UDP-glucose. UTP is reacted with glucose-1-phosphate and UDP-glucose is formed. This reaction is done by UDP-glucose pyrophosphorylase. Next, the activated form of glucose, UDP-glucose is added to the glycogenin by glycogen synthase.

Glycogen to glucose-6-phosphate:

Glycogen phosphorylase catalyses the formation of glucose-1-phosphate from glycogen. Glucose-1-phosphate is converted into glucose-6-phosphate by phosphoglucomutase.

b) Only one UTP is required in the conversation of glucose-6-phosphate to glycogen.

c) ∆G = ∆G° - RTlnK

= -30.5 - (0.0083*298*ln 0.5*1/5)

= -30.5 +5.695 = -24.805 kJ/mol/K

d) The reaction of the formation of phosphocreatine is:

Creatine + ATP -> Phosphocreatine + ADP

∆G° = -30.5 + 2.3 = -28.2 kJ/mol/K

So, -28.2 = -RT ln ( phosphocreatine*0.5/creatine*5)

or,