Question

Question 1 Liver cells and the skeletal muscle cells regulate glycogen in different ways. List their key differences. Not complete Answer: Marked out of 1.00 Check P Flag question How is Protein phosphatase 1 regulated by phosphorylation? What enzymes are involved, and what is the effect of phosphorylation on PP1? Question 2 Not complete Answer: Marked out of 1.00 Check Flag question Question 3 What reaction replenishes oxaloacetate? Name the enzyme and it's substrates. Not complete Answer: Marked out of 1.00 Check P Flag question

Answer 1

Answer 1: Glycogen is a branched polymer of glucose. It serves as an energy storage molecule in animals and present in the cytosol in the form of granules. The liver and skeletal muscles are the primary sites of glycogen storage in the body and store most of the glycogen, heart, brain, and kidney also store some amount of glycogen. In the liver, glycogen metabolism is regulated as per body glucose demand and maintain the blood glucose level to serve as a fuel for other body organs. In contrast to the liver, glycogen stored in skeletal muscle is only for their internal use to meet its own energy demand. Muscle cells lack the glucose-6-phosphatase enzyme which is required for the generation of glucose and then can be exported from the cell, therefore, glycogen stored in the muscle cells is only used by them. The glycogen metabolism is mainly regulated by hormones that regulate glycogen enzymes (glycogen synthase and glycogen phosphorylase) by their phosphorylated status. For example, glycogen synthase (glycogen synthesis) is active when the enzyme is dephosphorylated whereas glycogen phosphorylase (glycogen degradation) is active when the enzyme is phosphorylated. Due to the difference in their function in the liver ( maintain blood glucose level for other organs) and skeletal muscle cells ( meet their own energy demand) it has a difference in regulation. Apart from hormonal regulation skeletal muscle cells have non-hormonal allosteric regulation through ATP, calcium ions, and AMP. Glucose Inhibits the liver glycogen phosphorylase but not muscle enzyme. Muscle contraction increases the intracellular calcium levels and activates the muscle glycogen phosphorylase to release more glucose molecules as energy is required to perform the function. Increase AMP levels indicate the lack of energy and activate the phosphorylase enzyme.

Answer2. Protein phosphatase 1 (serine/threonine phosphatase) belongs to the phosphatase class of enzyme and plays an important role in glycogen metabolism, muscle contraction and many more. The enzyme has catalytic as well as regulatory subunit and regulates blood glucose levels in the liver and glycogen metabolism. The enzyme catalyzes the dephosphorylation of phosphorylases and inhibits glycogen breakdown. The activity of protein phosphatase itself is regulated by its phosphorylation status. The enzyme is phosphorylated by cyclin-dependent kinases (CDK1 and 2) and its activity inhibited in its phosphorylated form.

Answer 3. Oxaloacetate is an important intermediate in many pathways of animals such as the citric acid cycle, gluconeogenesis, amino acid synthesis, etc. The citric acid cycle requires a minimum level of oxaloacetate to occur. oxaloacetate is formed by carboxylation of pyruvate, a reaction catalyzed by the enzyme pyruvate carboxylase (a biotin-dependent-enzyme).

Pyruvate carboxylase Pyruvate+ CO2+H2O+ATP Oxaloacetate +ADP + Pi+ 2H+