Tutorial 2: Carbohydrate metabolism

Where does glycolysis occur in the cell?

• Mitochondria

• Nucleus

• Cytosol

• Ribosome

Answer explanation

Glycolysis occurs in the cytosol of the cell, where glucose is broken down into pyruvate, producing ATP and NADH. This process is essential for cellular respiration and energy production.

Which enzyme catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate?

• Phosphofructokinase

• Hexokinase

• Phosphoglyceromutase

• Phosphoglycerokinase

Answer explanation

The enzyme phosphoglyceromutase catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate by facilitating the transfer of a phosphate group within the molecule.

What is the rate-limiting step in glycolysis?

• Conversion of glucose to glucose-6-phosphate

• Phosphorylation of fructose-6-phosphate

• Conversion of 2-phosphoglycerate to phosphoenolpyruvate

• Oxidation of glyceraldehyde-3-phosphate

Answer explanation

The rate-limiting step in glycolysis is the phosphorylation of fructose-6-phosphate, catalyzed by phosphofructokinase. This step is crucial for regulating the pathway and controlling the flow of metabolites.

Which molecule is the final product of glycolysis under aerobic conditions?

• Lactate

• Acetyl-CoA

• Oxaloacetate

• Pyruvate

Answer explanation

The final product of glycolysis under aerobic conditions is pyruvate. It is produced from glucose through a series of enzymatic reactions, and can then enter the citric acid cycle for further energy production.

In the fed state, which hormone promotes glycolysis by increasing fructose 2,6-bisphosphate levels?

• Glucagon

• Insulin

• Adrenaline

• Cortisol

Answer explanation

Insulin promotes glycolysis in the fed state by increasing levels of fructose 2,6-bisphosphate, which activates key enzymes in the glycolytic pathway. In contrast, glucagon, adrenaline, and cortisol generally promote gluconeogenesis.

Which enzyme converts pyruvate to lactate?

• Pyruvate dehydrogenase

• Pyruvate carboxylase

• Lactate dehydrogenase

• Aldolase

Answer explanation

Lactate dehydrogenase is the enzyme that catalyzes the conversion of pyruvate to lactate, especially under anaerobic conditions, allowing for the regeneration of NAD+ needed for glycolysis.

Which enzyme catalyzes the first step of the TCA cycle?

• Isocitrate dehydrogenase

• Citrate synthase

• α-Ketoglutarate dehydrogenase

• Malate dehydrogenase

Answer explanation

Citrate synthase catalyzes the first step of the TCA cycle by combining acetyl-CoA and oxaloacetate to form citrate. This is the initial reaction that sets the cycle in motion.

What is the net yield of NADH from one cycle of the TCA cycle?

• 1 NADH

• 2 NADH

• 3 NADH

• 4 NADH

Answer explanation

In one cycle of the TCA cycle, 3 NADH molecules are produced: one from isocitrate to alpha-ketoglutarate, one from alpha-ketoglutarate to succinyl-CoA, and one from malate to oxaloacetate. Thus, the net yield is 3 NADH.

What happens to fructose in the liver?

• It is directly converted to glucose.

• It is phosphorylated to fructose-1-phosphate.

• It is oxidized to lactate.

• It bypasses glycolysis.

Answer explanation

In the liver, fructose is primarily phosphorylated to fructose-1-phosphate by the enzyme fructokinase. This process allows fructose to enter metabolic pathways, bypassing the regulatory step of glycolysis.

Which of the following enzyme deficiency causes fructose intolerance?

• Lactase deficiency

• Aldolase B deficiency

• Pyruvate kinase deficiency

• Pyruvate kinase deficiency

Answer explanation

Fructose intolerance is caused by a deficiency in Aldolase B, an enzyme crucial for fructose metabolism. Lactase deficiency affects lactose digestion, while pyruvate kinase deficiency relates to glycolysis, not fructose.