Question

When blood glucagon rises, which of the following hepatic enzyme activities falls? A. Adenyl cyclase B. Protein kinase C. 6 -Phosphofructo- 2 -kinase D. Fructose 1,6 -bisphosphatase E. Hexokinase

Short answer

Answer: C. 6-Phosphofructo-2-kinase

Step-by-step solution

Understanding the role of glucagon

Glucagon is a hormone released by the pancreas in response to low blood glucose levels. Its primary role is to increase blood sugar levels by stimulating the breakdown of glycogen in the liver and promoting gluconeogenesis (the production of glucose from non-carbohydrate sources). This process occurs primarily in the liver cells called hepatocytes.

Analyzing the hepatic enzymes

Now, let's analyze the hepatic enzymes mentioned in the question and their function in metabolism: A. Adenyl cyclase - This enzyme synthesizes cyclic AMP (cAMP) from ATP. cAMP is a secondary messenger that activates protein kinase A (PKA). PKA then goes on to phosphorylate other enzymes, regulating their activity. B. Protein kinase - This is a family of enzymes that regulate other enzymes and proteins through phosphorylation. In this context, the question most likely refers to protein kinase A (PKA), which is activated by glucagon through the cAMP signaling pathway. C. 6-Phosphofructo-2-kinase - This enzyme is involved in glycolysis, the process of breaking down glucose to generate ATP. It catalyzes the conversion of fructose-6-phosphate to fructose-2,6-bisphosphate, an activator of phosphofructokinase-1 (PFK-1), which is a key regulatory enzyme in glycolysis. D. Fructose 1,6-bisphosphatase - This enzyme is involved in gluconeogenesis, the process of generating glucose from non-carbohydrate sources. It catalyzes the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate, essentially reversing one of the key steps in glycolysis. E. Hexokinase - This enzyme is involved in glycolysis, catalyzing the conversion of glucose to glucose-6-phosphate, which is the first step in this pathway.

Identifying the enzyme whose activity falls

Glucagon promotes gluconeogenesis and inhibits glycolysis, as its primary function is to increase blood sugar levels. Thus, the enzymes involved in glycolysis (6-Phosphofructo-2-kinase and hexokinase) would be expected to have decreased activity when blood glucagon levels rise. Glucagon also activates adenyl cyclase and protein kinase A (PKA) through the cAMP pathway. Therefore, the hepatic enzyme whose activity falls when blood glucagon rises is: C. 6-Phosphofructo-2-kinase