Biochemistry : Carbohydrate Anabolism

Study concepts, example questions & explanations for Biochemistry

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Example Question #123 : Anabolic Pathways And Synthesis

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?

Possible Answers:

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.

NADH and FADH2, both of which are used to generate ATP in the cell via oxidative phosphorylation

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

Correct answer:

NADPH, which is used in reductive biosynthesis reactions, and Ribose-5-Phosphate, which is used as a major precursor to generate nucleotides

Explanation:

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 #123 : Anabolic Pathways And Synthesis

Which of the following carbohydrates cannot be continuously linearized with  glycosidic bonds? 

Possible Answers:

Glycogen

Galactose

Lactose

Sucrose

Correct answer:

Sucrose

Explanation:

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 #124 : Anabolic Pathways And Synthesis

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?

Possible Answers:

Gluconeogenesis

Glycogenesis

Glycolysis

Pentose phosphate pathway

Correct answer:

Gluconeogenesis

Explanation:

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.

Example Question #1 : Carbohydrate Anabolism

One of the key enzymes in the pentose phosphate pathway is glucose-6-phosphate dehydrogenase (G6PDH). This enzyme is responsible for oxidizing glucose-6-phosphate into the next intermediate in the pathway, with co-occuring production of NADPH. Which of the following is most likely to be true about the regulation of this enzyme?

Possible Answers:

G6PDH is activated by 

G6PDH is inhibited by 

None of these

G6PDH is inhibited by 

G6PDH is activated by 

Correct answer:

G6PDH is activated by 

Explanation:

From the question stem, we are told that glucose-6-phosphate dehydrogenase oxidized glucose into another compound, and also produces a molecule of NADPH in the process. In order to determine the way in which this enzyme is likely to be regulated, it's important to consider feedback mechanics.

Since this enzyme is producing NADPH when it is turned on, we would expect this product to negatively regulate the enzyme via feedback inhibition. Moreover, since we know that  is a reactant, we can correctly assume that having a high concentration of this will likely drive the reaction forward by turning the enzyme on. Thus,  would be expected to allosterically activate this enzyme. Furthermore, the question stem tells us nothing about the unphosphorylated forms of these cofactors, therefore we have no way of knowing how many NADH or  affects this enzyme, if they do at all.

Example Question #127 : Anabolic Pathways And Synthesis

One important chemical transformation that occurs in the pentose phosphate pathway is the conversion of glucose-6-phosphate (G6P) to ribulose-5-phosphate (R5P), which is shown below.

 

The conversion shown above is an example of which of the following type of reaction?

Possible Answers:

Oxidation of glucose-6-phosphate

Phosphorylation of glucose-6-phosphate

Isomerization of glucose-6-phosphate

Reduction of glucose-6-phosphate

Carboxylation of glucose-6-phosphate

Correct answer:

Oxidation of glucose-6-phosphate

Explanation:

From the question stem, we are shown the reaction in which glucose-6-phosphate is transformed into ribulose-5-phosphate. We are then asked to determine which type of reaction is occurring in this process.

We can also notice from the reaction that  is a reactant, and  is a product. Therefore, the  is being reduced to form . In order for this reduction reaction to happen, there needs to be a simultaneous oxidation reaction occurring, since the electrons need to come from somewhere. In this case, the electrons are coming from glucose-6-phosphate. Therefore, as  is reduced to , glucose-6-phosphate is oxidized to ribulose-5-phosphate. Thus, this is an oxidation reaction.

Also, it's important to note that this is not a carboxylation reaction. In fact, it is actually a decarboxylation reaction, since one of the carbon atoms on glucose is converted into carbon dioxide.

Moreover, this is also not a phosphorylation reaction, as the reactant and products have an equal number of phosphate groups.

And lastly, this is not an isomerization reaction because glucose-6-phosphate and ribulose-5-phosphate have different molecular formulas, thus they cannot ever be structural isomers.

 

Example Question #128 : Anabolic Pathways And Synthesis

In gluconeogenesis, where is oxaloacetate sequestered, and how is it able to reach the cytoplasm?

Possible Answers:

In the intermembrane space of the mitochondria; a carrier protein binds to oxaloacetate and traverses the mitochondrial membrane into the cytoplasm.

In the intermembrane space of the mitochondria; malate dehydrogenase reduces oxaloacetate to malate, which goes into the cytoplasm, and is converted back into oxaloacetate by malate dehydrogenase.

In the mitochondrial matrix; malate dehydrogenase reduces oxaloacetate to malate, which goes into the cytoplasm, and is converted back into oxaloacetate by malate dehydrogenase.

In the mitochondrial matrix; a carrier protein binds to oxaloacetate and traverses the mitochondrial membrane into the cytoplasm.

Correct answer:

In the mitochondrial matrix; malate dehydrogenase reduces oxaloacetate to malate, which goes into the cytoplasm, and is converted back into oxaloacetate by malate dehydrogenase.

Explanation:

Oxaloacetate is a metabolite of the citric acid cycle, which takes place in the mitochondrial matrix. Oxaloacetate cannot diffuse across the mitochondrial matrix, but malate can. So oxaloacetate is reduced to malate by malate dehydrogenase, and can now enter into the cytoplasm. Since malate dehydrogenase can catalyze the reverse reaction as well as the forward reaction, it can be used again to reform oxaloacetate. Once in the cytoplasm, oxaloacetate is converted into phosphoenolpyruvate (PEP) and continues gluconeogenesis.

Example Question #129 : Anabolic Pathways And Synthesis

Glycogen is a polysaccharide of which of the following molecules?

Possible Answers:

Fructose

Cellulose

Glucose

Ribose

Lactose

Correct answer:

Glucose

Explanation:

Glucose is converted into glycogen during the process called glycogenesis. Its structure consists of many linear alpha(14) glycosidic bonds, and also many branched alpha(16) glycosidic bonds. This heavily-branched structure means that there are many free ends, which are the substrates for glycogen phosphorylase. A debranching enzyme is needed to lyse the alpha(16) glycosidic bonds. Cellulose is a polymer glucose linked together via of beta(14) glycosidic bonds. Humans lack enzymes to catalyze the lysis of these bonds in cellulose.

Example Question #130 : Anabolic Pathways And Synthesis

Which of the following is false about the carbon fixation reaction?

Possible Answers:

Ribulose biphosphate carboxylase is among the most commonly found proteins in the biosphere

For each  molecule converted into carbohydrate, 2 ATP and 3 NADH are consumed

   combines with ribulose 1,5-biphosphate during the reaction

The enzyme which catalyzes the reaction is ribulose biphosphate carboxylase

Glyceraldehyde 3-phosphate is one of the products of the carbon fixation cycle (also known as the Calvin cycle)

Correct answer:

For each  molecule converted into carbohydrate, 2 ATP and 3 NADH are consumed

Explanation:

For each    molecule converted into carbohydrate, 3 ATPs and 2 NADPHs are consumed. The Calvin cycle is initiated when  and ribulose 1,5 biphosphate combine. The enzyme ribulose biphospate carboxylase catalyzes the reaction in the stroma of chloroplasts, and is considered one of the most the world's most abundant proteins. Glyceraldehyde 3-phosphate, which is a by-product of the cycle, goes on to be a building block of sugar, fatty acid, and amino acid synthesis.

Example Question #1 : Carbohydrate Anabolism

What is the major distinction between NADH and NADPH in biochemistry?

Possible Answers:

NADH and NADPH serve the same function in all reactions

NADH is used in reactions to create ATP, whereas NADPH is used in reactions to produce ADP

NADH is oxidized in catabolic reactions to produce ATP, wheres NADPH serves as a reducing agent in anabolic reactions

NADH is used primarily by eukaryotes, whereas NADPH is used primarily by prokaryotes

NADPH is oxidized in catabolic reactions to produce ATP, wheres NADH serves as a reducing agent in anabolic reactions

Correct answer:

NADH is oxidized in catabolic reactions to produce ATP, wheres NADPH serves as a reducing agent in anabolic reactions

Explanation:

The major distinction between NADH and NADPH is that NADH is generally used in catabolic reactions meant to produce ATP.  NADPH, on the other hand, is used primarily in anabolic reactions meant to build macromolecules from their smaller parts.

Example Question #132 : Anabolic Pathways And Synthesis

What is the role of phosphoenolpyruvate carboxykinase in carbohydrate metabolism?

Possible Answers:

Carbon dioxide is a byproduct of the reaction that it catalyzes

All of these

Guanosine triphosphate (GTP) is converted to guanosine diphosphate (GDP) by the enzyme

The reaction catalyzed by this enzyme is one of the first steps in gluconeogenesis

The enzyme converts oxaloacetate to phosphophenolpyruvate

Correct answer:

All of these

Explanation:

Gluconeogenesis is the production of glucose from other sources than carbohydrates, such as  from pyruvate, amino acids, lactate and glycerol. Phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenolpyruvate and carbon dioxide. It also produces GDP from GTP. It is regulated by hormones, such as glucagon and cortisol.

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