283 **Question 3**When you are awake and your blood sugar levels are very low, which of the following enzymes would you expect to be activated in your liver cells? (Select all that apply!)- [ ] Hexokinase- [ ] Glycogen synthase- [ ] Protein Phosphatase-1 ("Many proteins" phosphatase)- [ ] Glycogen phosphorylase- [ ] Glucose-6-phosphate dehydrogenase (catalyzes first reaction of PPP)- [ ] Phosphofructokinase-1 (PFK-1)- [ ] Glucose-6-phosphatase

Fructose 2,6-bisphosphate mediate the regulation of glycolysis and gluconeogenesis in the liver. It acts as an allosteric effector of the enzymes PFK-1 and FBPase-1. The liver has a special role in maintaining constant blood glucose levels. When the blood glucose level decreases, the hormone glucagon signals the liver to produce more glucose. Fuctose2,6-bisphosphate activates PFK-1 and stimulates glycolysis in the liver and at the same time, inhibits FBPase-1 activity and slowdowns gluconeogenesis when the blood glucose level is high. The Fuctose2,6-bisphosphate is formed by phosphorylation of fructose 6-phosphate and catalyzed by phosphofructokinase-2 (PFK-2) and broken by fructose 2,6-bisphosphatase (FBPase-2). The balance of these two enzyme activities determines the cellular level of Fuctose2,6-bisphosphate, which is regulated by insulin and glucagon. The glucagon stimulates the formation of cAMP in the liver, which further activates cAMP-dependent protein kinase. It transfers the phosphoryl group to bifunctional protein PFK-2/FBPase-2. Phosphorylation of protein enhances FBPase-2 activity and inhibits PFK-2 activity. GLucagon in this way lowers the Fuctose2,6-bisphosphate level and stimulates gluconeogenesis and inhibits glycolysis. The enzymes PFK-1, hexokinase, protein phosphatases are the enzymes that stimulate glycolysis in the liver. They are not active when the blood sugar level is low. The glycogen synthase is the enzyme that promotes glycogenesis, the conversion of glucose into glycogen. When the blood sugar level is low, it does not promote glycogenesis. Glucose-6-phosphate dehydrogenase is responsible for the first step in the pentose phosphate pathway, a series of chemical reactions that convert glucose to other sugar(ribose-5-phosphate). It is not active when blood sugar levels are low. Ans: Two enzymes that play important role in activating liver cells during low blood sugar levels are, Glycogen phosphorylase- It catalyzes the release of glucose monomers from the glycogen polymer stored in the liver. Glucose-6-phosphatase-in liver, it catalyzes the terminal step of glycogenolysis and gluconeogenesis.