Question 1

You have a patient exhibiting severe episodes of hypoglycemia. Measurement of circulating insulin and glucagon levels indicate that they are normal and responsive to cycles of feeding and fasting. Which of the following enzyme defects would most likely explain the hypoglycemia?

Accepted Answer

A hepatic glucokinase with a Km equal to that of muscle hexokinase would result in increased glucose uptake and phosphorylation in the liver, leading to excessive glycogen storage and hypoglycemia due to reduced glucose availability in the blood.

Question 2

You are examining the effects of an experimental compound on the metabolic activities of primary hepatocytes in culture. You discover that addition of your compound to these cells activates changes in metabolic fluxes highly similar to those induced by addition of glucagon. Determination of phosphate incorporation shows that several proteins become hyperphosphorylated following addition of the compound. Which of the following statements would most accurately reflect these changes?

Accepted Answer

The compound mimics glucagon's effects, which typically increase glycogen phosphorylase activity by promoting phosphorylation, leading to glycogen breakdown.  Within hepatocytes, glucagon activation of its receptor results in activation of the kinase PKA. Activation of PKA will then lead to the phosphorylation of numerous substrates. One of the substrates for PKA is glycogen synthase/phosphorylase kinase (phosphorylase kinase) which results in activation of this PKA substrate. Activation of phosphorylase kinase results in the phosphorylation of both glycogen synthase and glycogen phosphorylase. When glycogen phosphorylase is phosphorylated its level of activity is increased.

Question 3

You are treating an 11-year-old female type 1 diabetic patient who forgot to eat after she self-administered her afternoon insulin. Blood work demonstrates that her serum glucose level is 1 mM (18mg/dL).
Which of the following statements correctly defines the metabolic situation in this patient?

Accepted Answer

In a hypoglycemic state, insulin levels are low, and glucagon is high, leading to the phosphorylation of hepatic glycogen synthase, which prevents glucose incorporation into glycogen, thus promoting glycogen breakdown to release glucose.  As the level of blood glucose falls, the pancreas will secrete glucagon as a means to stimulate hepatic gluconeogenesis. Within hepatocytes, glucagon activation of its receptor results in activation of the kinase, PKA. Activation of PKA will then lead to the phosphorylation of numerous substrates. One of the substrates for PKA is glycogen synthase/phosphorylase kinase (phosphorylase kinase), which results in activation of this PKA substrate. Activation of phosphorylase kinase results in the phosphorylation of both glycogen synthase and glycogen phosphorylase. When glycogen synthase is phosphorylated, its level of activity is decreased resulting in reduced incorporation of glucose into glycogen.

Question 4

You have been studying a cell line derived from a muscle tumor and comparing its activities, with respect to the effects of high levels of fatty acids on glucose oxidation, to those of normal skeletal muscle cells from the same individual. You find that exposing the normal cells to high levels of free fatty acids results in impaired levels of glucose oxidation. These results are expected based upon the interactions of the glucose-fatty acid cycle. However, the tumor-derived cells do not exhibit reduced glycolysis in the presence of high levels of free fatty acids. Which of the following best explains these observations?

Accepted Answer

The tumor cells likely have a mutant form of PFK1 that is insensitive to allosteric inhibition by citrate, allowing glycolysis to continue even in the presence of high levels of fatty acids, which normally would inhibit glycolysis in normal cells. According to the glucose-fatty acid cycle, these 2 fuel sources reciprocally antagonize the utilization of each other. In other words, glucose oxidation can restrict fatty acid metabolism and fatty acid oxidation leads to reduced utilization of glucose. There are 2 major mechanisms by which fatty acid oxidation can restrict the flow of glucose through glycolysis. High levels of fat oxidation lead to generation of acetyl-CoA in excess of the cells' ability to fully oxidize it in the TCA cycle. Some of this acetyl-CoA is converted to citrate via the citrate synthase reaction of the TCA cycle, but the citrate cannot be further oxidized, so it is transported to the cytosol where it allosterically inhibits the PFK1 reaction. In addition, the acetyl-CoA allosterically activates pyruvate dehydrogenase complex (PDHc) kinases that, when activated, phosphorylate the PDHc. Phosporylated PDHc is less capable of oxidizing acetyl-CoA and leads to increased levels of pyruvate in the cytosol. The increased pyruvate restricts the ability of pyruvate kinase to oxidize phosphoenolpyruvate, resulting in reduced flow through glycolysis.

Question 5

You are studying the effects of an experimental compound on the process of glucose metabolism in laboratory animals. Addition of the compound to the animal chow is associated with decreased glucose output by the liver of these animals. Analysis of liver biopsy cells isolated 2-days following compound addition indicates that citrate transport into the cytosol is impaired. The activity of which of the following glycolytic enzymes is most likely to be affected by your experimental compound?

Accepted Answer

PFK1 represents the rate-limiting enzyme of glycolysis. This enzyme is subject to allosteric regulation, both positive and negative, by numerous effectors including citrate, which normally inhibits the enzyme allowing carbons to be diverted into glucose via gluconeogenesis

Question 6

Under conditions of anaerobic glycolysis, the NAD₊ required by glyceraldehyde-3-phosphate dehydrogenase is supplied by a reaction catalyzed by which of the following enzymes?

Accepted Answer

Under anaerobic conditions, lactate dehydrogenase catalyzes the conversion of pyruvate to lactate, regenerating NAD+ from NADH, which is necessary for glycolysis to continue.

Question 7

You are examining the regulatory control of glycolysis by addition of a novel compound to a hepatocyte cell culture system. Your studies demonstrate that addition of the compound decreases the kinase activity of PFK2 while simultaneously increasing the phosphatase activity of the enzyme. This response most closely reflects which of the following naturally occurring controls exerted on PFK2?

Accepted Answer

Phosphorylation by cAMP-dependent protein kinase (PKA) decreases the kinase activity and increases the phosphatase activity of PFK2, similar to the effect of the novel compound.  When PFK2 is phosphorylated, via the action of PKA, its activity shifts from a kinase to a phosphatase. Within hepatocytes, this shift in catalytic activity of PFK2 results in less activation of the rate-limiting enzyme of glycolysis, PFK1, to ensure the liver produces glucose via gluconeogenesis instead of oxidizing the sugar.

Question 8

You are examining the regulatory control of glycolysis by addition of a novel compound to a hepatocyte cell culture system. You discover that addition of the compound results in an increased rate of ATP production from glucose while simultaneously reducing glucose output into the culture medium. Your experimental compound is most likely mimicking which of the following?

Accepted Answer

Fructose 2,6-bisphosphate is a potent activator of phosphofructokinase-1 (PFK-1), a key regulatory enzyme in glycolysis, which increases the rate of glycolysis and ATP production. It also inhibits gluconeogenesis, reducing glucose output.  Fructose-2,6-bisphosphate (F2,6BP) is the most potent allosteric activator of PFK1, resulting in activation of glycolysis. Simultaneously, F2,6BP allosterically inhibits the gluconeogenic enzyme F-1,6-Bpase

Question 9

You are examining the regulatory control of glycolysis by addition of a novel compound to a hepatocyte cell culture system. You discover that addition of the compound results in a decreased rate of ATP production from glucose while simultaneously increasing glucose output into the culture medium. Further studies demonstrate that the changes observed in your culture system with addition of the compound can be mimicked by altering the cytosolic levels of citrate. Your compound is most likely altering the rate of glycolysis
By which of the following mechanisms?

Accepted Answer

Citrate is a known allosteric inhibitor of 6-phosphofructo-1-kinase (PFK1), a key regulatory enzyme in glycolysis. The compound likely mimics this effect, reducing glycolysis and ATP production while increasing glucose output.  Citrate, present in the cytosol, serves as a signal for a high-energy charge and inhibition of the TCA cycle. Under these conditions the rate of glucose oxidation via glycolysis needs to be reduced. Citrate serves as an allosteric inhibitor of the rate-limiting enzyme of glycolysis, PFK1.

Question 10

Cytosolic reducing equivalents (NADH) generated by glycolysis in the liver, enter the mitochondria for generation of ATP by which of the following processes?

Accepted Answer

The electrons from cytosolic NADH are transferred to oxaloacetate to form malate, which is then transported into the mitochondria where it is converted back to oxaloacetate, regenerating NADH inside the mitochondria. This is known as the malate-aspartate shuttle.  The electrons from reduced cytosolic NADH enter the mitochondria via either the malateaspartate shuttle or the glycerol phosphate shuttle. In the context of the malate-aspartate shuttle, the electrons from cytosolic NADH are transferred to oxaloacetate, forming malate via the action of cytosolic malate dehydrogenase. The malate is then actively transported into the mitochondria, where it enters the TCA cycle.

Question 11

You are examining the regulatory control of glycolysis by addition of a novel compound to a hepatocyte cell culture system. You discover that addition of the compound results in an increased rate of ATP production from glucose. Further studies demonstrate that the changes observed in your culture system with addition of the compound can be mimicked by a number of substances that activate the rate-limiting enzyme of glycolysis. Your compound is most likely altering the rate of glycolysis in a manner similar to all of which of the following?

Accepted Answer

AMP and fructose 2,6-bisphosphate are known activators of phosphofructokinase-1 (PFK-1), the rate-limiting enzyme of glycolysis, leading to increased glycolytic flux and ATP production. Inorganic phosphate also supports glycolysis by participating in substrate-level phosphorylation. Allosteric activation of PFK1 would be expected to occur under conditions of low-energy charge. Thus, as the level of ATP drops, there will be a concomitant rise in both AMP and inorganic phosphate levels. Both serve to allosterically activate PFK1. PFK1 is also allosterically activated by fructose-2,6-bisphosphate whose levels also rise in response to changing energy levels.

Question 12

You are examining the effects of various oxygen concentrations on cultures of erythrocytes. You find that at low O₂ pressures there is rapid depletion of a necessary intermediate in the glycolytic pathway. Further analysis of the cells you are culturing indicates that the activity of the lactate dehydrogenase in these cells is extremely low relative to what is expected. Under these hypoxic conditions, which of the following is the most likely rapidly depleted intermediate?

Accepted Answer

Under hypoxic conditions, low lactate dehydrogenase activity would lead to a rapid depletion of NAD⁺ because it is not being regenerated from NADH, which is necessary for glycolysis to continue.

Question 13

You are studying the effects of the addition of a novel compound to the chow of laboratory animals on the overall rate of flux through glycolysis. You discover that when the animals consume the mcompound containing chow, there is an increase in glycolytic flux to pyruvate. Examination of serum hormone levels in these animals indicates a change in the ratios of several key regulators of glucose homeostasis. The consumption of the test compound most likely causes an increase in which of the following hormone ratios, thus best explaining the observed changes in the rate of glycolysis?

Accepted Answer

An increase in the insulin to glucagon ratio promotes glycolysis by enhancing glucose uptake and utilization, which aligns with the observed increase in glycolytic flux.  As the level of insulin rises, there will be a concomitant increase in the activity of phosphodiesterase within responsive cells, such as the liver. The phosphodiesterase will hydrolyze cAMP leading to reduced activity of PKA. As the activity of PKA falls, the level of phosphorylation of 6-phosphofructo-2-kinase, PFK2 will fall which reduces its phosphatase activity. When functioning as a phosphatase, PFK2 hydrolyzes fructose-2,6-bisphosphate (F2,6BP) to fructose 6-phosphate. When PFK2 is not phosphorylated, it acts as a kinase and will, therefore lead to increased levels of F2,6BP. Conversely, as the level of glucagon increases, the activity of PKA increases leading to increased phosphorylation of PFK2. This in turn results in decreased F2,6BP levels due to the phosphatase activity of PFK2.

Question 14

In the production of ATP during glucose metabolism in erythrocytes, which of the following is the immediate donor of the phosphoryl group to ADP?

Accepted Answer

Phosphoenolpyruvate (PEP) donates a phosphoryl group to ADP to form ATP in the glycolytic pathway, specifically in the reaction catalyzed by pyruvate kinase. The phosphate donors during glycolysis are 1,3-bisphosphoglycerate and phosphoenolpyruvate. These high-energy intermediates of glycolysis function the same regardless of the cell in which glycolysis is occurring.

Question 15

You are studying the effects of the addition of a novel compound to the chow of laboratory animals on the overall rate of flux through glycolysis. You discover that when the animals consume the compound containing chow, there is an increase in glycolytic flux to pyruvate. Examination of cells from the livers of these animals demonstrates that when the compound enters the cells, it causes a rapid decrease in the ATP:ADP ratio. This effect most likely results in which of the following?

Accepted Answer

A decrease in the ATP:ADP ratio indicates a higher demand for ATP, which stimulates glycolysis to produce more ATP. This results in increased activity of phosphofructokinase-1, a key regulatory enzyme in glycolysis, thereby increasing the glycolytic pathway flux to meet the energy demand.  As the level of ATP falls, the level of ADP and AMP will increase. AMP serves as an allosteric activator of the rate-limiting enzyme of glycolysis, PFK1. Thus, as the level of ATP falls, the rate of flux through glycolysis will increase.

Question 16

A healthy 28-year-old woman exhibits a peak serum glucose concentration of 220 mg/dL within 30 minutes of the consumption of 75 g of glucose. Two hours after ingestion, her serum glucose concentration decreases to 100 mg/dL. The uptake and trapping of glucose within liver cells at high serum glucose concentrations is facilitated by which of the following enzymes?

Accepted Answer

Glucokinase is an enzyme that facilitates the uptake and trapping of glucose in liver cells by phosphorylating glucose to glucose-6-phosphate, especially at high serum glucose concentrations. Hepatic glucokinase has a high K_m for glucose in order to ensure that the liver does not trap glucose before sufficient levels are obtained in the blood. As the level of blood glucose increases, glucokinase will become more active resulting in phosphorylation of more glucose thus, trapping it in the cell leading to a gradual reduction in blood glucose levels.

Question 17

The hydrolysis of which of the following compounds is characterized by a standard free energy change (ΔG_0′) that is more negative than that for the reaction: ATP → ADP + P_i?

Accepted Answer

Phosphoenolpyruvate (PEP) has a very high-energy phosphate bond, and its hydrolysis releases more energy than the hydrolysis of ATP, resulting in a more negative ΔG°'.

Question 18

The Warburg effect states that cancer cells carry out glucose metabolism in a manner distinct from the glycolytic process of cells in normal tissues. Cancer cells tend to "ferment" glucose into lactate even in the presence of sufficient oxygen to support mitochondrial oxidative phosphorylation. Which of the following enzymes is most responsible for the observations accounting for the Warburg effect?

Accepted Answer

Pyruvate kinase is responsible for converting phosphoenolpyruvate to pyruvate in glycolysis, and its altered activity in cancer cells contributes to the Warburg effect by promoting lactate production even in the presence of oxygen.  In mammals, 2 genes encode a total of 4 pyruvate kinase (PK) isoforms. The PKM gene, so called originally due to initial characterization in muscle tissues, encodes the PKM1 and PKM1 isoforms. Most tissues express either the PKM1 or PKM2. PKM1 is found in many normal differentiated tissues, whereas PKM2 is expressed in most proliferating cells, including in all cancer cell lines and tumors tested to date. PKM2 is much less active than PKM1 but is allosterically activated by the upstream glycolytic metabolite fructose 1,6-bisphosphate (FBP). PKM2 is also unique in that, unlike other PK isoforms, it can interact with phosphotyrosine in tyrosine-phosphorylated proteins such as those resulting from growth factor stimulation of cells. The interaction of PKM2 with tyrosine-phosphorylated proteins results in the release of FBP leading to reduced activity of the enzyme. Low PKM2 activity, in conjunction with increased glucose uptake, facilitates the diversion of glucose carbons into the anabolic pathways that are derived from glycolysis. So, for proliferating cells such as cancers, even though it may seem counterproductive to prevent complete oxidation of glucose solely for ATP production, the demands for carbon incorporation into biomass clearly supersedes the needs for ATP production from glucose. Of metabolic significance to proliferating cells is that they must avoid ATP production in excess of demand to avoid allosteric inhibition of PFK1. Therefore, the inhibition of PKM2 by binding to tyrosine-phosphorylated proteins, following growth factor stimulation, may serve to uncouple the ability of cells to divert the carbons from nutrients (such as glucose) into biosynthetic pathways from the production of ATP. This may, in fact, be the underlying reason why PKM2 activity has evolved to be decreased in rapidly dividing cells.

Question 19

You are studying the effects of the addition of a novel compound to the chow of laboratory animals on the overall rate of flux through glycolysis in skeletal muscle. You discover that when the animals consume the compound containing chow, there is a decrease in glycolytic flux to pyruvate while at the same time there is increased glucose output to the blood. Cells isolated from muscle biopsy tissue from these animals respond to addition of the compound with a dramatic rise in the levels of cAMP. Given this effect of the compound, which of the following will occur first?

Accepted Answer

The compound increases cAMP levels, which will first bind to the regulatory subunits of PKA.  The primary function of cAMP is to bind to the regulatory subunits of PKA, which leads to the release of the catalytic subunits.

Question 20

A 9-month-old child is presented to the emergency room by his parents who report that he has been vomiting and has severe diarrhea. The episodes of vomiting began when the parents started feeding their child cow's milk. The infant exhibits signs of failure to thrive, weight loss, hepatomegaly, and jaundice. Laboratory tests show elevated blood galactose, hypergalactosuria, and metabolic acidosis with coagulation deficiency. These clinical and laboratory findings are most consistent with a deficiency in which of the following enzymes?

Accepted Answer

The symptoms and laboratory findings are consistent with classic galactosemia, which is caused by a deficiency in the enzyme UDP-galactose uridyltransferase (GALT). This enzyme deficiency leads to the accumulation of galactose-1-phosphate and galactose, causing the symptoms described.  This patient is suffering from classic (type 1) galactosemia. Type 1 galactosemia occurs as a consequence of mutations in the gene encoding GALT. Classic galactosemia manifests by a failure of neonates to thrive. Vomiting and diarrhea occur following ingestion of milk, hence individuals are termed lactose intolerant. Clinical findings include impaired liver function (which if left untreated leads to severe cirrhosis), elevated blood galactose, hypergalactosemia, hyperchloremic metabolic acidosis, urinary galactitol excretion, and hyperaminoaciduria. Unless controlled by exclusion of galactose from the diet, these patients can go on to develop blindness and fatal liver damage. Blindness is due to the conversion of circulating galactose to the sugar alcohol galacitol, by an NADPH-dependent aldose reductase that is present in neural tissue and in the lens of the eye.

Quiz 10: Carbohydrates: Glycolysis and Glucose Homeostasis

You have a patient exhibiting severe episodes of hypoglycemia. Measurement of circulating insulin and glucagon levels indicate that they are normal and responsive to cycles of feeding and fasting. Which of the following enzyme defects would most likely explain the hypoglycemia?

You are treating an 11-year-old female type 1 diabetic patient who forgot to eat after she self-administered her afternoon insulin. Blood work demonstrates that her serum glucose level is 1 mM (18mg/dL) . Which of the following statements correctly defines the metabolic situation in this patient?

Under conditions of anaerobic glycolysis, the NAD₊ required by glyceraldehyde-3-phosphate dehydrogenase is supplied by a reaction catalyzed by which of the following enzymes?

Cytosolic reducing equivalents (NADH) generated by glycolysis in the liver, enter the mitochondria for generation of ATP by which of the following processes?

In the production of ATP during glucose metabolism in erythrocytes, which of the following is the immediate donor of the phosphoryl group to ADP?

The hydrolysis of which of the following compounds is characterized by a standard free energy change (ΔG_0′) that is more negative than that for the reaction: ATP → ADP + P_i?

Quiz 10: Carbohydrates: Glycolysis and Glucose Homeostasis

You have a patient exhibiting severe episodes of hypoglycemia. Measurement of circulating insulin and glucagon levels indicate that they are normal and responsive to cycles of feeding and fasting. Which of the following enzyme defects would most likely explain the hypoglycemia?

You are treating an 11-year-old female type 1 diabetic patient who forgot to eat after she self-administered her afternoon insulin. Blood work demonstrates that her serum glucose level is 1 mM (18mg/dL) . Which of the following statements correctly defines the metabolic situation in this patient?

Under conditions of anaerobic glycolysis, the NAD+ required by glyceraldehyde-3-phosphate dehydrogenase is supplied by a reaction catalyzed by which of the following enzymes?

Cytosolic reducing equivalents (NADH) generated by glycolysis in the liver, enter the mitochondria for generation of ATP by which of the following processes?

In the production of ATP during glucose metabolism in erythrocytes, which of the following is the immediate donor of the phosphoryl group to ADP?

The hydrolysis of which of the following compounds is characterized by a standard free energy change (ΔG0′) that is more negative than that for the reaction: ATP → ADP + Pi?

Under conditions of anaerobic glycolysis, the NAD₊ required by glyceraldehyde-3-phosphate dehydrogenase is supplied by a reaction catalyzed by which of the following enzymes?

The hydrolysis of which of the following compounds is characterized by a standard free energy change (ΔG_0′) that is more negative than that for the reaction: ATP → ADP + P_i?