Biochemistry : Glycolysis Enzymes

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Glycolysis

Which of these enzymes catalyzes the first reaction in glycolysis?

Possible Answers:

Pyruvate kinase

Triose phosphate isomerase

Aldolase

Hexokinase

Correct answer:

Hexokinase

Explanation:

The first step in glycolysis is the conversion of glucose to glucose-6-phosphate through the consumption on one ATP molecule. Glucose is reacted upon by the enzyme hexokinase to carry out this step. Kinases are a group of enzymes that add phosphate groups by removing them from an ATP. All of these other enzymes catalyze subsequent reactions in glycolysis.

Example Question #2 : Glycolysis

Dihydroxyacetone is converted to glyceraldehyde-3-phosphate by what category of enzyme?

Possible Answers:

Isomerase

Enolase

Dehydrogenase

Kinase

Correct answer:

Isomerase

Explanation:

Dihydroxyacetone phosphate (DHAP) is converted to glyceradehyde-3-phosphate (G3P) by the enzyme triose phosphate isomerase. As the name suggests, this enzyme catalyzes the isomerization of a three-carbon sugar into another three-carbon sugar. Since the molecular formulas of DHAP and G3P are the same, we know that they are isomers of each other.

The balance between DHAP and G3P is extremely important in regulating overall cell metabolism. DHAP is a precursor to triglycerides, and is used in their synthesis, while G3P is an intermediate in glycolysis, an ATP-producing process. In order to favor the conversion of DHAP into G3P, and not the opposite, the cell must keep G3P levels low (Le Chatelier's Principle). Consider the following equilibrium: . This should make sense: if there is lots of ATP around in the cell, there is no need for glycolysis to proceed. Thus the equilibrium will be pushed to the left, increasing the concentration of DHAP in the cell. In humans, DHAP is converted into triglycerides, which get stored as fat. One way to shift this equilibrium to the right is to "create" an ATP need. This can be done by exercising. Exercise utilizes ATP and will thus pull the equilibrium to the right, removing DHAP (which was destined to be converted into fat) and facilitates its conversion into G3P to proceed with cellular respiration. 

Example Question #1 : Glycolysis Enzymes

The enzyme pyruvate kinase is responsible for catalyzing the conversion of phosphoenolpyruvate into __________.

Possible Answers:

dihydroxyacetone

glucose

glucose-6-phosphate

pyruvate

Correct answer:

pyruvate

Explanation:

The tenth and final reaction of glycolysis involves the conversion of phosphoenolpyruvate (PEP) into pyruvate. This step is catalyzed by the enzyme pyruvate kinase. This kinase is going to remove a phosphate group from PEP and put it on ADP to yield ATP. Pyruvate, a three-carbon molecule, is the end product of glycolysis. It can be sent to the pyruvate dehydrogenase complex to be turned into acetyl-CoA, which enters the Krebs cycle. Alternatively, it can be reduced into lactate and/or ethanol (depending on the organism) to regenerate  for glycolysis via anaerobic respiration.

Example Question #1 : Glycolysis

The enzyme enolase catalyzes the ninth reaction of glycolysis. What is the product of this reaction?

Possible Answers:

Phosphoenolpyruvate (PEP)

glyceraldehyde-3-phosphate (G3P)

Dihydroxyacetone phosphate

Pyruvate

Correct answer:

Phosphoenolpyruvate (PEP)

Explanation:

The ninth reaction involves the conversion of 2-phosphoglycerate into phosphoenolpyruvate. The enzyme enolase, which produces a double bond by removing the hydroxyl group on 2-phosphoglycerate catalyzes this reaction. Note that the resulting molecule is an enol (double bond -ene, and alcohol - ol).

Example Question #4 : Glycolysis

Which of the following best describes the action of an isomerase?

Possible Answers:

Reduces high energy molecules such as 

Changes the molecular formula of a molecule

Rearranges a molecule without changing the molecular formula

Creates a double bond in the molecule

Correct answer:

Rearranges a molecule without changing the molecular formula

Explanation:

Isomerases catalyze the isomerization, or rearrangement of atoms within a molecule, of its substrate. Isomerases are seen in glycolysis inn the second step where glucose-6-phosphate is converted into fructose-6-phosphate by phosphoglucose isomerase. Glucose-6-phosphate is rearranged into fructose-6-phosphate such that the molecular formula is unchanged. Another isomerase is triose phosphate isomerase. It catalyzes the isomerization of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate. 

Example Question #5 : Glycolysis

Which of the ten glucose reactions uses the enzyme GAPDH?

Possible Answers:

Third

Seventh

First

Sixth

Correct answer:

Sixth

Explanation:

Glyceraldehyde phosphate dehydrogenase (GAPDH) is used in the sixth reaction, where G3P is converted to 1,3-bisphosphoglycerate (1,3-BPG). A hydrogen is removed from G3P and added to , yielding NADH. Also, G3P has one phosphate group, while 1,3-BPG has two. The energy released as G3P is oxidized (causing subsequent reduction of ) is highly exergonic. This energy, sometimes referred to as the energy of oxidation, drives the addition of inorganic phosphate onto G3P, yielding the doubly-phosphorylated 1,3-BPG. 

Example Question #6 : Glycolysis

Which enzyme in glycolysis is responsible for the conversion of glucose to glucose 6-phosphate?

Possible Answers:

Hexokinase

Phosphoglucose isomerase

Phosphofructokinase

Triose phosphate isomerase

Aldolase

Correct answer:

Hexokinase

Explanation:

Hexokinase is the first enzyme in the glycolytic pathway and it is responsible for the phosphorylation of glucose to glucose 6-phosphate. The other enzymes catalyze subsequent reactions in glycolysis.

Example Question #7 : Glycolysis

Which of the following enzymes catalyzes the rate-limiting step of glycolysis?

Possible Answers:

Phosphofructokinase

Pyruvate kinase

Lactate dehydrogenase

Hexokinase

Correct answer:

Phosphofructokinase

Explanation:

The rate-limiting step of glycolysis is the conversion of glucose-6-phosphate to fructose-6-phosphate. This reaction is catalyzed by the enzyme phosphofructokinase. Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate, pyruvate kinase converts Phosphoenolpyruvate to pyruvate, and lactate dehydrogenase converts pyruvate into lactose.

Example Question #11 : Carbohydrate Metabolism

Which of the following enzymes carries out a redox reaction in glycolysis?

Possible Answers:

Glyceraldehyde-3-phosphate dehydrogenase

Phosphoglucose isomerase

Aldolase

Pyruvate kinase

Correct answer:

Glyceraldehyde-3-phosphate dehydrogenase

Explanation:

Glyceraldehyde 3-phosphate dehydrogenase is the only enzyme in glycolysis that carries out a redox reaction. Glyceraldehyde 3-phosphate is oxidized to 1,3-bisphosphoglycerate while  is reduced to

Example Question #9 : Glycolysis

Which of the following choices is responsible for the decarboxylation in the pyruvate dehydrogenase complex?

Possible Answers:

Thiamine pyrophosphate (TPP)

CoA

FAD

Lipoamide

Correct answer:

Thiamine pyrophosphate (TPP)

Explanation:

The pyruvate dehydrogenase complex essentially carries out a two part reaction: a decarboxylation and an oxidation. All these choices play important roles in the pyruvate dehydrogenase complex. Thiamine pyrophosphate (TPP) is the only choice, however, that is responsible for the decarboxylation step. Lipoamide acts as transporter, transferring the substrate to a distant active site. FAD then reoxidizes lipoamide for the next substrate. CoA is important in producing the substrate.

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