All Biochemistry Resources
Example Questions
Example Question #1 : Regulating Carbohydrate Synthesis
When would you expect glycogen synthase to be activated?
When protein phosphatase I is activated
When glucagon is in high concentration
When glycogen synthase is phosphorylated
When protein kinase A is activated
When there is a low concentration of glucose in the blood
When protein phosphatase I is activated
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 #2 : Carbohydrate Synthesis
Which of the following is an oxidoreductase?
Aspartate amino-transferase
Glucose 6-phosphatase
Hexokinase
Trypsin
Lactate dehydrogenase
Lactate dehydrogenase
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 #2 : Regulating Carbohydrate Synthesis
Given that the pyruvate dehydrogenase complex is product-inhibited, which of the following molecules would act as an inhibitor to it?
Pyruvate
Phosphoenolpyruvate
Acetyl-CoA
Acetyl-CoA
The pyruvate dehydrogenase complex catalyzes the following reaction:
Since it is product inhibited, acetyl-CoA will inhibit the complex.
Example Question #1 : Regulating Carbohydrate Synthesis
In gluconeogenesis, how can the reaction carried out by pyruvate kinase be reversed?
PEP carboxykinase converts pyruvate to phosphoenolpyruvate (PEP)
A phosphatase can be used to dephosphorylate pyruvate into phosphoenolpyruvate (PEP)
Pyruvate kinase carries out a reversible reaction, and no other enzymes are needed
Pyruvate carboxylate converts pyruvate to oxaloacetate, which is converted by PEP carboxykinase into phosphoenolpyruvate (PEP)
Pyruvate carboxylate converts pyruvate to oxaloacetate, which is converted by PEP carboxykinase into phosphoenolpyruvate (PEP)
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 #4 : Carbohydrate Synthesis
What is the name of the enzyme that is found in the liver that converts glucose into glucose-6-phosphate?
Fructokinase
Glycogen synthase
Glucokinase
Glycogenin
Phospholipase
Glucokinase
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 #1 : Regulating Carbohydrate Synthesis
What is the name of the process involved in the conversion of glucose to glycogen?
Glycolysis
Glycogenesis
Gluconeogenesis
Glucogenolysis
Krebs cycle
Glycogenesis
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?
Citric acid cycle
Gluconeogenesis
Krebs cycle
Glycolysis
Electron transport chain
Gluconeogenesis
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.
Example Question #1 : Carbohydrate Anabolism
The pentose phosphate pathway is an important metabolic pathway within cells that allows them to synthesize two essential products. What are these two products, and what do they do?
NADH and FADH2, both of which are used to generate ATP in the cell via oxidative phosphorylation
Glutathione, which helps to maintain a reducing environment inside of cells, and 2,3-bisphosphoglycerate, which helps to reduce hemoglobin's affinity for O2.
Acetoacetate and beta-hydroxybutyrate, both of which are ketone bodies that serve as a fuel source for cells in the body when blood levels of glucose are low.
Fructose-2,6-bisphosphate, which plays a major regulatory role in glycolysis and gluconeogenesis, and glycerol-3-phosphate, which plays a role in the synthesis of triglycerides and phospholipids.
NADPH, which is used in reductive biosynthesis reactions, and Ribose-5-Phosphate, which is used as a major precursor to generate nucleotides
NADPH, which is used in reductive biosynthesis reactions, and Ribose-5-Phosphate, which is used as a major precursor to generate nucleotides
The pentose phosphate pathway (PPP) is a metabolic pathway in cells that is used to generate NADPH and/or ribose-5-phosphate for use in the cell, depending on the cell's needs. NADPH is used primarily to provide reducing power for several biosynthetic reactions, but it also serves as a means to keep glutathione predominately in its reduced form in the cell. This, in turn, helps maintain a reducing environment within cells. Furthermore, ribose-5-phosphate is used as a major precursor for the synthesis of nucleotides.
NADH and FADH2 are not produced by the PPP, but rather are produced by the oxidation of glucose via the aerobic respiration pathway. These two molecules are carriers of high-energy electrons, which are used to generate ATP via the electron transport chain.
Glutathione, as mentioned previously, is not produced by the PPP; however, it does use the NADPH produced by the PPP to maintain its reduced form within the cell, which, in turn, maintains a predominately reducing environment within the cell. 2,3-bisphosphoglycerate is an intermediate of glycolysis, not the PPP. One major function of 2,3-BPG is to bind hemoglobin and reduce its affinity for O2. This allows red blood cells to have an easier time releasing O2 to tissues that are in need of it.
Fructose-2,6-bisphosphate is not a product of the PPP. Rather, it is produced from a side reaction of the glycolytic intermediate fructose-6-phosphate. Fructose-2,6-bisphosphate serves as an allosteric regulator of the enzyme fructose-1,6-bisphosphatase, which is an important regulatory enzyme for glycolysis and gluconeogenesis. Hormones such as insulin and glucagon can stimulate cells to alter their concentration of fructose-2,6-bisphosphate, which in turn regulates the activity of glycolysis and gluconeogenesis. Glycerol-3-phosphate is also not produced from the PPP. Rather, it can be produced from the phosphorylation of glycerol or from the reduction of dihydroxyacetone phosphate, an intermediate of glycolysis. It is used as the backbone for the formation of triglycerides and phospholipids.
Acetoacetate and beta-hydroxybutyrate are both ketone bodies produced not by the PPP, but from the condensation of two molecules of acetyl-CoA plus additional modifications. Generally, when the body is in a fasting state and needs to reserve blood glucose levels, ketone bodies can be produced to act as an alternative energy source, thus allowing glucose to be mostly spared.
Example Question #121 : Anabolic Pathways And Synthesis
Which of the following carbohydrates cannot be continuously linearized with glycosidic bonds?
Lactose
Sucrose
Galactose
Glycogen
Sucrose
In order to linearize using a linkage, there needs to be an unbound carbon on the 1 position. However, sucrose is a
linkage and doesn't have a carbon available to linearize in the 1 position. It isn't a reducing sugar and therefore cannot be linearized. All of the other sugars have their anomeric carbon located at the 1 position and all of them are reducing sugars that can be linearized.
Example Question #1 : Carbohydrate Anabolism
The enzyme phosphoglucomutase is an enzyme responsible for the interconversion of glucose-6-phosphate and glucose-1-phosphate. In a person who is fasting, which of the following metabolic pathways is the most likely destination for glucose-6-phosphate?
Glycolysis
Gluconeogenesis
Glycogenesis
Pentose phosphate pathway
Gluconeogenesis
From the question stem, we're told that the enzyme phosphoglucomutase is responsible for interconverting two intermediate forms of glucose, both glucose-1-phosphate and glucose-6-phosphate. We're then asked to determine the most likely metabolic pathway that glucose-6-phosphate would be used for in a fasting individual.
First, it's important to remember that in an individual that is fasting, energy resources become more scarce. Therefore, the body tries to conserve as much energy as it can in this state. Furthermore, since the brain relies mostly on glucose for its metabolism, the body tries to keep a relatively stable level of glucose in the blood. As a result, many tissues in the body switch from using glucose to instead using other energy sources, such as fatty acids or ketone bodies. In order to help ensure that blood levels of glucose remain stable, the liver increases its rate of gluconeogenesis, which generates glucose from non-sugar substrates, such as pyruvic acid, certain amino acids, and glycerol. Therefore, we would expect glucose-6-phosphate to be funneled mostly into the gluconeogenesis pathway.
Even though glucose-6-phosphate can also be diverted to other pathways, such as glycolysis, glycerogenesis, or the pentose phosphate pathway, all of these pathways result in a net consumption of glucose. In a fasting state, this is the opposite of what we would want, since blood glucose levels need to be mostly stabilized in order to ensure that nervous tissue has an adequate supply.
All Biochemistry Resources
![Learning Tools by Varsity Tutors](https://vt-vtwa-app-assets.varsitytutors.com/assets/problems/og_image_practice_problems-9cd7cd1b01009043c4576617bc620d0d5f9d58294f59b6d6556fd8365f7440cf.jpg)