Biochemistry : Regulating Carbohydrate Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Regulating Carbohydrate Synthesis

When would you expect glycogen synthase to be activated? 

Possible Answers:

When protein kinase A is activated

When there is a low concentration of glucose in the blood

When glycogen synthase is phosphorylated

When protein phosphatase I is activated

When glucagon is in high concentration 

Correct answer:

When protein phosphatase I is activated

Explanation:

Glycogen synthase is turned on when unphosphorylated. The enzyme responsible for this is protein phosphatase I. Protein kinase A inactivates glycogen synthase. Low glucose concentration causes a release in glucagon, which activates glycogen phosphorylase and deactivates glycogen synthase. 

Example Question #1251 : Biochemistry

Which of the following is an oxidoreductase?

Possible Answers:

Aspartate amino-transferase

Trypsin

Lactate dehydrogenase

Glucose 6-phosphatase

Hexokinase

Correct answer:

Lactate dehydrogenase

Explanation:

An oxidoreductase catalyzes the transfer of electrons from one molecule to the other, usually using ; i.e., it is an enzyme that catalyzes a redox reaction. Trypsin cleaves peptide bonds. Hexokinase phosphorylates hexose sugars. Glucose 6-phosphatase hydrolyzes glucose 6-phosphate into a phosphate group and glucose. Aspartate amino-transferase catalyzes the transfer of an amino group between aspartate and glutamate. Lactate dehydrogenase interconverts pyruvate to lactate, and at the same time  and .

Example Question #1252 : Biochemistry

Given that the pyruvate dehydrogenase complex is product-inhibited, which of the following molecules would act as an inhibitor to it? 

Possible Answers:

Acetyl-CoA 

Pyruvate

Phosphoenolpyruvate 

Correct answer:

Acetyl-CoA 

Explanation:

The pyruvate dehydrogenase complex catalyzes the following reaction:

Since it is product inhibited, acetyl-CoA will inhibit the complex. 

Example Question #1253 : Biochemistry

In gluconeogenesis, how can the reaction carried out by pyruvate kinase be reversed?

Possible Answers:

Pyruvate kinase carries out a reversible reaction, and no other enzymes are needed

PEP carboxykinase converts pyruvate to phosphoenolpyruvate (PEP)

A phosphatase can be used to dephosphorylate pyruvate into phosphoenolpyruvate (PEP)

Pyruvate carboxylate converts pyruvate to oxaloacetate, which is converted by PEP carboxykinase into phosphoenolpyruvate (PEP)

Correct answer:

Pyruvate carboxylate converts pyruvate to oxaloacetate, which is converted by PEP carboxykinase into phosphoenolpyruvate (PEP)

Explanation:

There are 3 enzymes in glycolysis that carry out irreversible reactions: phosphofructokinase-1 (PFK-1), hexokinase and pyruvate kinase. While phosphatases are used to reverse the reactions for PFK-1 and hexokinase, they are not used in reversing the pyruvate kinase reaction. 2 enzymes are needed to convert pyruvate back into phosphoenolpyruvate (PEP). First, pyruvate carboxylase converts pyruvate into oxaloacetate, and then PEP carboxykinase converts this into PEP.

Example Question #1254 : Biochemistry

What is the name of the enzyme that is found in the liver that converts glucose into glucose-6-phosphate?

Possible Answers:

Phospholipase

Glycogenin

Glucokinase

Glycogen synthase

Fructokinase

Correct answer:

Glucokinase

Explanation:

Fructokinase catalyzes the reaction of fructose converting into fructose-1-phosphate. Glycogenin acts as a primer for glycogen synthesis, by polymerizing the first few molecules of glucose. Glycogen synthase converts glucose to glycogen. Phospholipase hydrolyzes phospholipids.

Example Question #1255 : Biochemistry

What is the name of the process involved in the conversion of glucose to glycogen?

Possible Answers:

Glycogenesis

Glycolysis

Krebs cycle

Gluconeogenesis

Glucogenolysis

Correct answer:

Glycogenesis

Explanation:

The process of glycogenesis is when glucose is converted into glycogen. This occurs in the muscle and the liver after food is consumed. Gluconeogenesis is the process where glucose is produced from non-carbohydrate precursors. Glycogenolysis is the breakdown of glycogen. Glycolysis is the breakdown of glucose into two molecules of pyruvate. The Krebs cycle does not generate glycogen or glucose, rather it produces high energy electrons to be carried to the electron transport chain for ATP production.

Example Question #2 : Regulating Carbohydrate Synthesis

Which reaction occurs during a fasting state when glycogen stores are depleted?

Possible Answers:

Krebs cycle

Citric acid cycle

Electron transport chain

Gluconeogenesis

Glycolysis

Correct answer:

Gluconeogenesis

Explanation:

Glyconeogenesis occurs in the hepatic pathway when no glucose is available. The electron transport chain occurs during cellular respiration. Glycolysis is the break down of glucose, not the production of glycogen. The Krebs cycle and citric acid cycle are the same thing, and both are used to produce NADH and ATP.

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