Which of the following enzyme activity decrease in fasting?
**Core Concept**
Glucagon-like peptide-1 (GLP-1) secretion and insulin gene expression are regulated by the nutritional state of the body. When fasting, the body's metabolic needs shift to favor gluconeogenesis and ketogenesis, and the pancreas adapts by altering the activity of various enzymes involved in glucose metabolism.
**Why the Correct Answer is Right**
During fasting, the activity of the enzyme glucokinase decreases. Glucokinase is a key enzyme in glucose phosphorylation, which is the first step in glucose utilization by the pancreas. When blood glucose levels are low, glucokinase activity decreases, reducing glucose uptake and utilization by the pancreas. This decrease in glucokinase activity helps to conserve glucose and promote gluconeogenesis, which is essential for maintaining blood glucose levels during fasting. The decrease in glucokinase activity also helps to increase the activity of gluconeogenic enzymes, such as glucose-6-phosphatase, which is necessary for gluconeogenesis to occur.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose-6-phosphatase activity actually increases during fasting, as it is a key enzyme in gluconeogenesis.
* **Option B:** Glycogen synthase activity decreases during fasting, as the body's energy needs are met by gluconeogenesis and ketogenesis rather than glycogen breakdown.
* **Option D:** Phosphofructokinase activity is not directly related to fasting, as it is a key enzyme in glycolysis, which is suppressed during fasting.
**Clinical Pearl / High-Yield Fact**
During fasting, the body's metabolic state shifts from anabolic to catabolic, and the pancreas adapts by altering the activity of various enzymes involved in glucose metabolism. This shift is critical for maintaining blood glucose levels and ensuring that the body's energy needs are met.
**Correct Answer:** B. Glycogen synthase.