29 Questions
What is the effect of high glucose concentrations on glucokinase activity?
It is stimulated
What happens to glucose-6-phosphate concentrations in the cytoplasm when PFK-1 activity is inhibited?
They increase
What is the role of citrate in the cytoplasm?
It inhibits glycolysis
What is the fate of pyruvate in the mitochondria?
It is converted to acetyl-CoA
What is the effect of insulin on glucokinase activity?
It stimulates glucokinase
What is the result of citrate in the cytoplasm being cleaved by citrate lyase?
Initiation of lipogenesis and cholesterol biosynthesis
What is the role of the 1,6-glycosidase activity of the branching enzyme?
To liberate free glucose by catalyzing hydrolysis of the α1 → 6-glycoside bond
What is the purpose of phosphoglucomutase in glycogenolysis?
To convert glucose-1-phosphate into glucose-6-phosphate
What is the role of glucose-6-phosphatase in the liver?
To catalyze the hydrolysis of glucose-6-phosphate, yielding glucose
How are glycogen phosphorylase and glycogen synthase regulated?
By allosteric mechanisms and covalent modification by reversible phosphorylation and dephosphorylation
What is the net result of glycogenolysis in the liver?
Increased blood glucose concentration
What is the effect of phosphorylation on glycogen phosphorylase and glycogen synthase?
It increases the activity of glycogen phosphorylase and decreases the activity of glycogen synthase
What is the role of glucagon in glycogen metabolism in the liver?
It activates glycogenolysis and inhibits glycogenesis
What is the second messenger involved in the activation of glycogen phosphorylase?
cAMP
What is the effect of insulin on phosphodiesterase in the liver?
It activates its activity
Which of the following hormones activates glycogenolysis in muscle?
Epinephrine
What is the enzyme that phosphorylates glycogen phosphorylase?
Phosphorylase kinase
What is the characteristic of glucokinase in terms of its affinity to glucose?
Low affinity (high Km)
What is the effect of glucokinase activity on ATP-K+ channels in pancreatic β-islet cells?
Closes the ATP-K+ channel
What is the effect of increased insulin concentration in the blood on adipose and skeletal muscle tissue?
Increases glucose uptake
What is the characteristic of hexokinase in terms of its affinity to glucose?
High affinity (low Km)
What is the concentration of glucose in mmol/L that corresponds to 180 mg/dL?
10 mmol/L
Where is glucokinase found in the pancreas?
β-islet cells
What happens to glucose concentration in the portal blood coming from the small intestine during fasting state?
It decreases
What is the effect of glucagon on glycogen synthetase in the liver during fasting state?
It inhibits glycogen synthetase
What is the rate-limiting step in glycogenolysis catalyzed by?
Glycogen phosphorylase
What is the product of phosphorylytic cleavage of glycogen by glycogen phosphorylase?
Glucose-1-phosphate
What is the role of debranching enzyme in glycogenolysis?
It transfers a trisaccharide unit from one branch to the other
What is the fate of glucose-6-phosphate in the liver during glycogenolysis?
It is hydrolyzed by glucose-6-phosphatase
Study Notes
Glucokinase Enzyme
- Shows high affinity to glucose (low Km value)
- Found in pancreatic β-islet cells and liver
- Activated by high glucose concentrations in the portal blood (high Km for glucose)
Glucokinase in Pancreas
- Increased glucose phosphorylation leads to increased glycolysis, ATP formation, and insulin secretion
- Insulin stimulates glucose uptake in adipose and skeletal muscle tissue
Glucokinase in Liver
- Hexokinase has high affinity (low Km) for glucose, and is saturated under normal conditions
- Glucokinase has high Km, is activated after blood glucose concentrations exceed 180 mg/dL, and has high Vmax
- Insulin stimulates glycolysis, leading to pyruvate formation and citrate accumulation
Regulation of Glycolysis
- Citrate inhibits PFK-1 activity, slowing down glycolysis
- Citrate is converted to oxaloacetate and Acetyl-CoA, initiating lipogenesis and cholesterol biosynthesis
Fasting State
- Decreased insulin secretion, decreased glucose uptake in skeletal muscle and adipose tissue
- Increased glucagon secretion, inhibition of glycogen synthetase, and activation of glycogen phosphorylase
Glycogenolysis
- Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis
- Glucose-1-phosphate is converted to glucose-6-phosphate, which is hydrolyzed by glucose-6-phosphatase
- Glucose is released into the bloodstream for use by the brain and erythrocytes
Debranching Enzyme
- Removes terminal glucosyl residues from glycogen, exposing the 1 → 6 branch point
- Hydrolyzes the α1 → 6-glycoside bond to liberate free glucose
Regulation of Glycogenolysis and Glycogenesis
- Glycogen phosphorylase and glycogen synthase are regulated in opposite directions by allosteric mechanisms and covalent modification
- Phosphorylation of glycogen phosphorylase increases its activity, while phosphorylation of glycogen synthase reduces its activity
Covalent Modification of Allosteric Enzymes
- Insulin promotes dephosphorylation, while glucagon promotes phosphorylation of enzymes
- Phosphorylation activates glycogenolysis, gluconeogenesis, and lipolysis, while inhibiting glycogenesis and lipogenesis
Glycogen Metabolism in Liver and Muscle
- Glucagon (in liver), epinephrine, and norepinephrine (in muscle and liver) stimulate glycogenolysis
- cAMP activates protein kinase A, which phosphorylates and activates glycogen phosphorylase
This quiz assesses your understanding of glucokinase enzyme properties, including its affinity to glucose, vmax, and response to allosteric inhibitors and insulin.
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