Questions:

Answer each of the following questions.

1. Which enzyme-catalyzed reactions are shared between the gluconeogenesis and glycolysis pathways? (Note: just provide the name of the enzyme that catalyzes each of the shared reactions).

2. Why are some enzymes needed only in the gluconeogenesis pathway and not in the glycolysis pathway?

3. Which one of the following reactions only occurs during gluconeogenesis (i.e., not during glycolysis)?

1. 1,3-bisphosphglycerate into 3-phosphoglycerate
2. Glucose 6-phosphate into fructose 6-phosphate
3. Oxaloacetate into Phosphoenolpyruvate


Phosphoenolpyruvate into Pyruvate

4. An increase in g
   lucagon levels can

1. Promote the degradation of glycogen (i.e., glycogenolysis)
2. Occurs during the fasting state (starvation)
3. Promotes the synthesis of glycogen
4. A and B

5. A high school girl who is self-conscious about her appearance has been fasting for several days to fit into a dress she intentionally bought a size too small for a school dance. Which of her organs/tissues is producing the glucose that is being synthesized through gluconeogenesis?

1. Red blood cells
2. Muscles
3. Liver
4. Brain

6. Why do only some tissues (e.g., liver) contain the enzyme glucose-6-phosphatase?

7. What are the main products of the pentose phosphate pathway, and how does the cell use them?

8. How many molecules of ATP are produced during the metabolism of one (1) molecule of glucose through the pentose phosphate pathway?

9. Which two (2) molecules are produced during the pentose phosphate pathway that are also produced during glycolysis?

10. Describe two (2) specific things that make NADH and NADPH different from each other

Answers:

1:

Catalyzed reaction that are shared by glycolysis and gluconeoganasis are –

• Reaction by aldolase.
• Reaction by triose phosphate isomerase.
• Reaction by glyceraldehydes 3 phosphate dehydrogenase.
• Reaction by phosphoglycerate kinase
• Reaction by phosphoglycerate mutase
• Reaction by enolase.
• Reaction by pyruvate kinase (Voet et al.2013).

2:

The glycolysis is a metabolic process in which glucose is converted to pyruvate and the reverse process in which generation of glucose occurred from non-carbohydrate carbon substances. However, both the process use same steps and enzymes except 3 steps – firstly, in glycolysis glucose 6 phosphate converts in to fructose 6 phosphate, whereas in gluconeogenesis the reverse is done by using glucose 6 phosphatase.  Secondly, the conversion of fructose 1,6  bisphosphate to fructose 6 phosphate by the enzyme phosphofructokinase is occur during glycolisis, but the process is reversed by using fructose 1,6 bisphospahtase in gluconeogenesis. Lastly the conversion of phosphoinolpyruvate is reversed by the enzyme pyruvate carboxylase and phosphoenol pyruvate carboxy kinase in gluconeogenesis. Thus, four enzymes glucose 6 phosphatase, fructose 1,6 bisphospahtase, pyruvate carboxylase and phosphoenol pyruvate carboxy kinase are used in gluconeoganesis but not in glycolysis (Marshall et al. 2014). In glycolysis the conversion of glucose to glucose 6 phosphate is important whereas in gluconeogenesis the reverse is important in order to generate free glucose, thus, glucose 6 phosphatase is used that helps to hydrolyze glucose 6 phosphate and produce phosphate group and free glucose. In glycolysis conversion of fructose 6 phosphate to fructose 1,6 bisphosphate is done by using phosphofructokinase, but the reaction is one directional, thus another enzyme fructose 1,6 bisphosphatase is used to reveresd the reaction.in gluconeogenesis. In gluconeogenesis the conversion of pyruvate to phosphoinolpyruvate is important in order to reverse the process of glycolysis and produce glucose from non-carbohydrate carbon resource. Thus, the enzyme pyruvate carboxylase is used that helps in carboxylation of pyruvate and produce oxaloacetae. The oxaloacetate then converts into phosphoinolpyruvate by the enzyme phosphoenol pyruvate carboxy kinase (Voet et al. 2013).

3:

The reaction that includes conversion of oxaloacetate into phosphoinolpyruvate is occurred in the gluconeogenesis pathway but not in glycolysis (option c) (Voet et al. 2013).

4:

An increase in glucagon levels can promote the degradation of glycogen i.e., glycogenolysis and it occurs during the fasting state or starvation (option d) (Voet et al. 2013).

5:

As the girl is fasting from several days thus the glucose level will decrease and the liver will produce the glucose through gluconeogenesis in order to maintain the glucose level in the body and produce energy for carry out the function of the body (Option c) (Vasudevan, Sreekumari and Vaidyanathan 2013).  

6:

The enzyme glucose 6 phosphatase is present only in some tissues such as liver and kidney. A little amount of the enzyme is also found in the skeletal muscle fibres and the beta pancreatic islets cells of kidney. The enzyme is highly available in liver because it is required for the process of gluconeogenesis and glycogenolysis. The enzyme helps to convert the glucose 6 phosphate into free glucose, which is then reached to the blood and provide energy to continue the activity of the body (King 2014).

7:

 The main products of pentose phosphate pathway include NADPH and ribose 5 phosphate. The cell use NADPH in order to carry out anabolic reaction such as lipid synthesis, nucleic acid synthesis and reduction of glutathione. The ribose 5 phosphate is used in nucleotide biosynthesis and histidine biosysnthesis, both the process starts with the conversion of ribose 5 phosphate into phosphoribosyl pyrophosphate (Stincone et al. 2015).

8:

In the pentose phosphate pathway the complete oxidation of one molecule of glucose provides 12 molecules of NADPH₂, thus produces 36  molecules of ATP (Wood 2012).

9:

Fructose 6 phosphate and glyceraldehydes 3 phosphate are the two molecules produced during both the pentose phosphate pathway and glycolysis (Wood 2012).  

10:

The NADH molecule is made of two neucleotides that are joined through their phosphate groups and the neucleotide is consist of 5 carbon deoxyribose sugar which is attached to the adenine base and a phosphate group. On the other hand the NADPH molecule is also composed of two neucleotides, however it contains one extra phosphate group that is attached to the ribose sugar of adenine. The presence of extra molecule of phosphate helps the NADPH molecule in order to combine with different sets of enzymes (Voet et al. 2013).

The NADH and NADPH molecule functions differently in the biological process. For example, the NADH molecule is an oxidizing agent and involved in the catabolic reactions in which energy is generated, where as the NADPH molecule is a reducing agent and involved in the anabolic reaction that consumed energy (Voet et al. 2013).

References:

King, M.W., 2014. Integrative medical biochemistry: Examination and board review. McGraw-Hill Education Medical.

Marshall, W.J., Lapsley, M., Day, A. and Ayling, R., 2014. Clinical Biochemistry E-Book: Metabolic and Clinical Aspects. Elsevier Health Sciences.

Stincone, A., Prigione, A., Cramer, T., Wamelink, M.M., Campbell, K., Cheung, E., Olin?Sandoval, V., Grüning, N.M., Krüger, A., Tauqeer Alam, M. and Keller, M.A., 2015. The return of metabolism: biochemistry and physiology of the pentose phosphate pathway. Biological Reviews, 90(3), pp.927-963.

Vasudevan, D.M., Sreekumari, S. and Vaidyanathan, K., 2013. Textbook of biochemistry for medical students. JP Medical Ltd.

Voet, D., Voet, J.G. and Pratt, C.W., 2013. Fundamentals of biochemistry: life at the molecular level (No. 577.1 VOE). Hoboken: Wiley.

Wood, T., 2012. The pentose phosphate pathway. Elsevier.