Carbohydrate digestion and metabolism Flashcards
What is the first step of glycolysis?
Phosphorylation of glucose
Which enzyme catalyzes the first step of glycolysis?
Hexokinase
What type of enzyme is the enzyme that catalyzes the first step of glycolysis?
Transferase
First step of glucose: reversible or irreversible?
Irreversible
Which ion does hexokinase need to phosphorylate glucose?
Magnesium ion (Mg^ 2+)
The phosphate used in step one of glycolysis is from where?
ATP
Why does hexokinase need a magnesium ion in order to catalyze the first step of glycolysis?
Mg^ (2+) shields the negative charges of the phosphosyl groups in ATP, making the terminal phosphorus atom an easier target for nucleophilic attack by an –OH of glucose.
Which isozyme of hexokinase is found in the hepatocytes?
Glucokinase
What step happens after phosphorylation of glucose?
Conversion of glucose 6-phosphate to its ketose isomer, fructose 6-phosphate
Most kinases need this ion to catalyze phosphorylation
Magnesium
What step happens after isomeration of glucose 6-phosphate to fructose 6-phosophate?
Fructose 6-phosphate is phosphorylated
Which enzyme phosphorylates fructose 6-phosophate?
Phosphofructokinase-1 (PFK-1)
PFK-1 turns fructose 6-phosphate into which molecule?
Fructose 1,6- bisphosphate
Why is the phosphorylation of fructose 6-phosphate important?
Because it is the first “committed” step in the glycolytic pathway. Glucose 6-phosphate and fructose 6-phosphate have other possible fates, but fructose 1,6-bisphosphate is targeted for glycolysis
What happens next to fructose 1,6- bisphosphate to proceed in glycolysis?
It is cleaved into two triose phosphates.
Which triose phosphates are cleaved from fructose 1,6- bisphosphate?
Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate
What is the difference between glyceraldehyde 3-phosphate and dihydroxyacetone phosphate?
The first is an aldose, the second a ketose
The cleavage of fructose 1,6-bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate: reversible or irreversible?
Reversible
Phosphorylation of fructose 6-phosphate: reversible or irreversible?
Irreversible
What is the last step of the preparatory phase of glycolysis?
Interconversion of the triose phosphates. Dihydroxyacetone phosphate is converted into glyceraldehyde phosphate
How many steps does glycolysis have?
10
ATP investment in glycolysis is for what?
Phosphorylation of glucose 6-phosphate and fructose 6-phosphate
Oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate is the first of two energy-conserving step. T/F
T
In the oxidation of glyceraldehyde 3-phosphate, which molecule is reduced?
NAD+ is reduced to NADH
The phosphate transfer from 1,3-bisphosphoglycerate to ADP is irreversible. T/F
F
What is the last step of glycolysis?
Transfer of phosphoryl group from phosphoenolpyruvate to ADP
What are the products of glycolysis?
4 ATP (2 net), 2 pyruvate, 2 NADH
Which enzyme catalyzes the conversion of phosphoenolpyruvate to pyruvate?
Pyruvate kinase
Where does glycolysis occur?
Cytosol
Which of the following items is true?
A. Fat is converted to glucose
B. Conversion of glucose to pyruvate requires a high amount of energy and is expensive.
C. Glycolysis and gluconeogenesis are both regulated at the same time
D. In the Glucose-alanine cycle, alanine is used as a substrate in glycolysis.
B
Which of the following processes is NOT involved in regulating glycolysis?
A. Covalent modulation, wherein pyruvate kinase is inhibited by high ATP concentration, and activated in the presence of low energy states.
B. Allosteric control where phosphofructokinase assumes either an R or T state governing its affinity to fructose 6 phosphate.
C. Product inhibition, where hexokinase in the liver cells is inhibited by glucose 6 phosphate.
D. Hormonal regulation, wherein gluconeogenesis starts when glucagon increases the intracellular cAMP, causing the phosphorylation and inactivation of pyruvate kinase.
C
How many of gluconeogenesis’ reactions are reverse of glycolytic reactions?
7 out of 10
Before pyruvate is converted to PEP, what must it be converted to?
Oxaloacetate
Which enzyme converts pyruvate to oxaloacetate?
Pyruvate carboxylase
Coenzyme of pyruvate carboxylase
Biotin
Where is pyruvate converted to oxaloacetate?
Mitochondria
Why must pyruvate go to the mitochondria to be converted to oxaloacetate?
Because pyruvate carboxylase, the enzyme that catalyzes this reaction, is found only in the mitochondria
The mitochondrial membrane has no transporters for oxaloacetate. How is it exported to the cytosol?
Oxaloacetate is converted to malate to pass through the mitochondrial membrane in case of pyruvate from glycolysis
Or oxaloacetate is converted to PEP in the mitochondria in case of puruvate in cori cycle
Which enzyme converts oxaloacetate to malate?
Malate dehydrogenase
What happens next after malate leaves the mitochondrion?
It is converted back to oxaloacetate in the cytosol
Oxaloacetate is converted to PEP by which enzyme?
Phosphoenolpyruvate carboxykinase
Which one of the following is not true? During a heart attack and when the cell is oxygen starved, anaerobic glycolysis will:
A. be activated by low oxygen pressure
B. be activated by low ATP to ADP ratio
C. produce ATP by oxidative phosphorylation
D. produce pyruvate as an end product
C
Glycolysis has 2 phases and 4 stages. Which of the following is FALSE regarding glycolysis?
A. Input of 1 molecule of glucose and output of 2 molecules
of pyruvate.
B. 1 cycle produces 10 ATP’s
B
- Glycolysis is regulated through which of the following?
A. Product inhibition
B. Allosteric control
C. Covalent modulation
D. All of the above
D
Which of the following statements on regulation of gluconeogenesis is false?
A. Insulin turns on gluconeogenesis
B. Glucagon turns on gluconeogenesis
C. Acetyl CoA turns on gluconeogenesis
D. Phosphorylation turns on gluconeogenesis
A
Fructose-6-Phosphate → Fructose-1,6-bisphosphate
Enzyme?
Phosphofructokinase (PFK)
Fructose-1,6-bisphosphate –> Fructose-6-Phosphate
Enzyme?
Fructose-1,6-bisphosphatase
Phosphoenolpyruvate -> pyruvate
Enzyme?
Pyruvate kinase
Why is phosphorylation of Glucose to Glucose-6-phosphate
important?
A. It helps keep glucose inside of the cell
B. It activates glucose for glycolysis
C. It allows glucose to get inside the cell
D. It helps store ATP
A
What best describes gluconeogenesis?
A. Gluconeogenesis is a metabolic pathway which breaks down amino acid and turns it into glucose.
B. Gluconeogenesis converts fatty acids into glucose.
C. Gluconeogenesis increases insulin.
D. Gluconeogenesis increases glucagon.
C
What is TRUE about GLUT 4?
A. It is sodium-dependent.
B. it is insulin dependent.
B
Fructose 2,6-bisphosphate is formed by the action of the Enzyme II. Phosphorylation of this enzyme:
A. leads to an increase in F26bP concentration
B. leads to a decrease in F-2,6-BP concentration
C. takes place in the mitochondrion
D. is catalyzed by phosphofructokinase
B
Which of the following best describes the regulation of Enzyme II?
A. Glucagon promotes phosphorylation of Enzyme II.
B. Insulin promotes the phosphorylation of Enzyme II.
C. An enzyme specific protein phosphatase mediates the effect of these two hormones.
D. Calcium is an important secondary messenger for this process
A
Another important regulatory step in glycolysis is the conversion of phosphoenol pyruvate (PEP) to pyruvate. This is likewise important in determining the relationship between glycolysis and gluconeogenesis. Which of the following statements best describes the conversion of
PEP to pyruvate?
A. The conversion of PEP to pyruvate involves a large negative free energy change.
B. The conversion of PEP to pyruvate consumes one ATP.
C. The conversion of PEP to pyruvate occurs inside the mitochondrion.
D. The conversion of PEP NADH2.
A
Which of the following is active when phosphorylated?
A. Glycogen Synthetase
B. Pyruvate Kinase
C. Glycogen Phosphorylase
D. Pyruvate Dehydrogenase
C
Metabolic pathways must be regulated in a way that prevents futile cycles and wasting of energy like in glycolysis and gluconeogenesis, glycogen synthesis (glycogen sythase) and breakdown (glycogen phosphorylase) need to be regulated in a consistent manner. Which of the following statements best describe how this is achieved on glycogen metabolism?
A. glycogen promotes phosphorylation
B. glucagon inhibits
C. Insulin promotes phosphorylation of synthethase and phosphorylase
D. Insulin inhbits phosphorylation
A
Amplification works in metabolic regulation by
A. Allowing a small amount of signal molecule to cause an increased intracellular transduction
B. Stimulating a signal at a long distance
C. An increase in the amount of ligand molecules
D. An increase in the amount of receptor molecules
A
Why is the skeletal muscle glycogen NOT involved in blood glucose regulation?
A. The muscle does not have enough glycogen content for this function.
B. Muscle glycogen content fluctuates widely and cannot provide adequate substrate for glucose formation
C. The muscle does not contain any glucose-6-phosphatase and is therefore unable to release glucose in the blood.
D. Muscle glycogen is mainly used to maintain glucose levels during starvation.
C
Which one of the following organs has the highest demand for glucose as a fuel?
A. Brain
B. Muscle (skeletal)
C. Heart
D. Liver
A
Which of the following is NOT an insulin counter-regulatory hormone?
A. Glucagon
B. Epinephrine
C. Cortisol
D. IGF-1
D
After digestion of pancake that contains flour, milk, and sucrose, the major carbohydrate products entering the blood are which of the following?
A. Glucose B. Fructose and galactose C. Galactose and glucose D. Fructose and glucose E. Glucose, galactose and fructose
E
Which of the following statements does NOT describe gluconeogenesis?
A. Gluconeogenesis happens in the liver and kidneys, and does not occur in the skeletal muscle
B. Not all enzymes involved in gluconeogenesis are in the cytosol.
C. Gluconeogenesis produces glucose from other metabolites for use as fuel source by the brain, testes, erythocytes and kidneys.
D. Gluconeogenesis in human beings describes how glucose is formed from acetyl coA derived from free fatty acids.
D
Which of the following could NOT be utilized in glycolysis as substitute for glucose?
A. Fructose
B. Galactose
C. Mannose
D. Free Fatty Acid
D
Which statement is TRUE regarding carbohydrate metabolism?
A. The pentose phosphate pathway is a metabolic pathway since this oxidizes glucose into nucleotides.
B. Gluconeogenesis makes glucose from non-carbohydrate precursors to maintain glucose levels during starvation
C. In glycolysis, glucose is hyrolyzed to form glucose 6 phosphate in a reaction catalyzed by glucose-6-phosphatase.
D. Complete aerobic conversion of 1 molecule of glucose yields 2 ATP’s and 2 lactate
B
A patient diagnosed to have a tumour in the pancreas underwent CT-guided biopsy. Histopathologic findings revealed an “insulinoma” (insulin-secreting tumour). Which of the following would you expect to see in a patient with such a tumour?
A. Overweight patient
B. Hyperglycemic symptoms
C. Both A and B
D. Neither A nor B
A
A sprinter missed breakfast prior to his morning training. Which pathways are expected to take over for proper muscle contraction?
A. Cori cycle
B. Glucose-alanine cycle
C. Glycogenesis
D. A and B only
D
Which of the following statements BEST describe the central role of glucose in caloric metabolism?
A. Most of our energy stores can be traced back to glucose which is abundant in our diet.
B. Most of the glucose we eat actually ends up as structural components and little as energy.
C. Glucose can be derived from amino acids and fatty acids, but not the other way around.
D. Glucose is able to store the most amount of energy per weight.
A
Which of the following statements BEST describes the importance of glucose homeostasis?
A. Important organs like the brain and the heart use glucose almost entirely.
B. Important organs use glucose because it is the largest amount of energy store in our body.
C. Erythrocytes use glucose because they lack oxygen.
D. Glucose is the only major biomolecule that can be metabolized anaerobically
D
Which of the following statements BESTdescribes glycogen as an energy store?
A Glycogen stores can provide as much as 1 week worth of energy
B Glycogen in the liver are mainly used for replenishing serum glucose
C Glycogen is easily mobilized by being broken down into smaller keto-acids
D Glycogen stores are replenished by breaking down amino acids
B
Which of the following statements BEST describes the enzyme hexokinase?
A. This enzyme is active only when serum glucose levels are high.
B. This enzyme makes it possible to release glucose from extra-hepatic tissues
C. This enzyme is NOT inhibited by high levels of glucose-6-phosphate
D. This enzyme has high activity even if serum glucose levels are low.
D
Which of the following reactions is regulated in glucose metabolism?
A. Entry of glucose into the mitochondrion.
B. Conversion of glucose-6-phosphate to fructose-6-phosphate
C. Conversion of pyruvate to phosphoenol-pyruvate
D. Phosphorylation of fructose-6-phosphate to fructose-2,6-biphosphate
D
Conversion of fructose-6-phosphate (F6P) to fructose-1,6-biphosphate (F-1,6-P) in glycolysis is an example of a substrate cycle. Which of the following statements BEST describes substrate cycles?
A. Substrate cycles are synonymous to futile cycles.
B. Substrate cycles can be wasteful.
C. The same enzyme catalyzes both reactions in a substrate cycle.
D. The TCA cycle is another example of a substrate cycle.
B
Which of the following statements describes the regulation of the F6P – F-1,6-P cycle?
A. Excess F-6-P leads to stimulation of the phosphofructokinase 2 enzyme.
B. Glucagon leads to phosphorylation of phosphofructokinase 1 enzyme.
C. Fructose biphosphatase enzyme 2 is active in the phosphorylated state.
D. Insulin leads to dephosphorylation of phosphofructokinase 1 enzyme.
C
A key molecule in the regulation of this substrate cycle is fructose-2,6-biphosphate (F-2,6-P). Which of the following statements is true about the role of F-2,6-P?
A. F-2,6-P is formed by isomerization of fructose-1,6- biphosphate
B. F-2,6-P prevents the phosphorylation of fructose-6-phosphate
C. Glucagon promotes the biphosphatase activity of enzyme 2
D. Phosphorylation of enzyme 2 leads to more F-2,6-P
C
Which of the following enzymes is active in the phosphorylated state?
A. Phosphofructokinase enzyme 2
B. Pyruvate kinase
C. Glycogen phosphorylase
D. Glycogen synthetase
C
Which of the following accurately describes the fate of glucose?
A. Glucose can not be converted to fatty acids.
B. Glucose from the diet is stored as glycogen mostly in the liver
C. Glucose may be converted to lactate when oxygen is not available
D. Glucose is the sole energy source of erythrocytes and the liver
C
The Phosphoenol Pyruvate (PEP) – Pyruvate pseudocycle is an important initial step in gluconeogenesis which allows the conversion of pyruvate and acetyl CoA to PEP. Which of the following statements is true about the enzymes involved in this pseudocycle?
A. Increased ADP stimulates pyruvate carboxylase
B. Increased ADP inhibits PEP carboxykinase
C. Increased Acetyl CoA inhibits pyruvate carboxylase
D. Glucagon stimulates pyruvate kinase
B
Which of the following accurately describes the regulation of pyruvate dehydrogenase?
A. It is active in the dephosphorylated form.
B. Acetyl CoA inhibits the enzyme
C. Glucagon stimulates the enzyme which helps in conversion of Acetyl CoA to fatty acids
D. Excess ATP stimulates the enzyme
B
Which of the following explains why glycogen breakdown does not occur at the same time as glycogen synthesis?
A. Phosphorylation activates glycogen synthase while it inactivates glycogen phosphorylase
B. Insulin promotes dephosphorylation of glycogen synthase and glycogen phosphorylase
C. Calmodulin inhibits glycogen phosphorylase and stimulates glycogen synthase
D. Glucagon promotes dephosphorylation of glycogen synthase and glycogen phosphorylase
B
What source replenishes our blood glucose levels?
a. Breakdown of amino acids from hepatic portal circulation
b. Breakdown of carbohydrates from diet
c. Breakdown of glycogen from liver and skeletal muscles
B
In the absence of oxygen, why is the formation of lactate important?
It replenishes the cell’s supply of NAD needed to proceed with glycolysis. Pyruvate serves as the acceptor of H from NADH
Which is the major gluconeogenic amino acid?
Alanine
Why is glycerol a precursor of gluconeogenesis?
Because it can be converted to DHAP, an intermediate of gluconeogenesis
In gluconeogenesis, why is GTP required?
It provides energy and phosphate group for the conversion of OAA to PEP
In gluconeogenesis, how much ATP is required per gluose molecule?
4
Three important differences of hexokinase and glucokinase
Glucokinase has higher Km than hexokinase
Glucokinase is not inhibited by glucose 6-phosphate, which inhibits hexokinase
Glucokinase is inhibited by the binding of a regulatory protein specific to the liver, while hexokinase is inhibited by allosteric modification by G6-P
What inhibits hexokinase?
Allosteric binding of glucose 6-phosphate
What inhibits PFK-1?
Allosteric binding of ATP
high levels of citrate,
What is the effect of high levels of AMP to FBPase I?
Inhibition
What is the effect of high levels of ADP and AMP to PFK-I?
Stimulation
When blood glucose levels decreases, which hormone signals the liver to produce and release more glucose and to stop consuming it for its own needs?
Glucagon
When blood glucose is high, which hormone signals the liver to use glucose as fuel and as a precursor for the synthesis and storage of glycogen and triacylglycerol?
Insulin
The fasting hormone
Glucagon
The fed state hormone
Insulin
Which reaction does fructose 2,6-bisphosphate regulate?
Conversion of fructose 6-phosphate to fructose 1,6-bisphosphate
F 2,6-bisphosphate binds to the allosteric site of which glycolytic enzyme?
PFK-1
Effect of allosteric binding of F 2,6-BP to PFK-1
Increase in affinity of PFK-1 for F 6-P
Decrease in affinity of PFK-1 for inihibitors ATP and citrate
Effect of allosteric binding of F 2,6-BP to FBP-1
Reduced affinity of FBP-1 to its substrates
Enzyme that catalyzes phosphorylation of F 6-P to F 2,6-BP
PFK-2
Enzyme that breaks down F 2,6-BP to F 6-P
FBP-2
PFK-2 and FBP-2 are part of a single enzyme. T/F
T
Which secondary messenger activates the phosphorylation of PFK-2/FBP-2?
cAMP
Effect of phosphorylation to PFK-2
Inhibition
What reaction does PFK-2 catalyze?
F 6-P to F 2,6-BP
What reaction does FBP-2 catalyze?
F 2,6-P to F 6-P
Which hormone lowers F 2,6-BP levels?
Glucagon
Which hormone increases F 2,6-BP levels?
Insulin
Effect of dephosphorylation to PFK-2
Activation
Effect of phosphorylation to FBP-2
Activation
Effect of dephosphorylation to FBP-2
Inhibition
Which enzyme is activated by insulin: PFK-2 or FBP-2?
PFK-2
Which enzyme is activated by glucago: PFK-2 or FBP-2?
FBP-2
Phosphorylation activates which enzyme: PFK-2 or FBP-2?
FBP-2
ATP inhibits pyruvate kinase. T/F
T
Long chain fa and acetyl CoA stimulate pyruvate kinase. T/F
F
They inhibit.
Glucagon inhibits pyruvate kinase through phosphorylation of cAMP-dependent protein kinase in the liver. T/F
T
What is the effect of cAMP to the metabolism of carbohydrates in the muscle?
It activates glycogenolysis and glycolysis to provide the fuel needed for fight and flight response.
Which hormone increases cAMP for glycogen break down in the muscle?
Epinephrine
How does the accumulation of acetyl acid from the break down of fatty acids affect pyruvate dehydrogenase and pyruvate carboxylase?
It inhibits pyruvate dehydrogenase and allosterically activates pyruvate carboxylase
In which organs is glycogen primarily found?
Liver and muscle
Which enzyme catalyzes the removal of glycosidic linkage between two glucose residues at a nonreducing end of glycogen?
Glycogen phosphorylase
Glucose residues removed from the nonreducing end are β-D-glucose 6-phosphate. T/F
F
They are α -D-glucose 1-phosphate
How is the phosphorolysis reaction by glucose phosphorylase different from the hydrolysis of amylase?
In phosphorolysis, some of the energy of the bond is preserved in the formation of glucose 1-phosphate. In hydrolysis, there is no energy is preserved as such.
Pyrodoxal phosphate is an essential cofactor in the glycogen phosphorylase reaction. T/F
T
This is an unusual role for pyridoxal phsphate. Its more typical role is as a cofactor in amino acid metabolism
Glucose 6-phosphatase, the enzyme that converts G 6-P to glucose is found in which two organs of the body?
Kidney and liver
Where does glycogen synthesis occur?
All animal tissues, especially in liver and skeletal muscles
In glycogen synthesis, which protein is both the primer on which new chains are assembled and the enzyme that catalyzes their assembly?
Glycogenin
Function of protein glycogenin in glycogenesis
It catalyzes the assembly of new glucose chains, and it is the primer on which new glucose chains are assembled
Glycogen phosphorylase when phosphorylated is activated or deactivated?
Activated
Calcium activates glycogen phosphorylase. T/F
T
Which hormones activate glycogen phosphorylase?
Epinephrine (muscle) and glucagon (liver)
ATP allosterically inhibits glycogen phosphorylase. T/F
T
In the muscle, phosphoprotein phosphatase 1 (PP1) removes the phosphoryl groups from glycogen phosphorylase. Glycogen phosphorylase is then activated or deactivated?
Deactivated
Glycogen phosphorylase in the liver is active when dephosphorylated, unlike that in the muscle. T/F
F
Glycogen phosphorylase in both liver and muscle is active when phosphorylated and deactivated when dephosphorylated.
Glucose allosterically binds to glycogen phosphorylase in the liver resulting in (phosphorylation/dephosophorylation?) to (activate/deactivate?) the enzyme.
Dephosphorylation, deactivate
Glycogen phospohrylase has two forms. Which is the deactivated form, which is the activated form?
Phosphorylase a is activated
Phosphorylase b is deactivated
Of the two forms of glycogen phosphorylase, which is phosphorylated?
Phosphorylase a
Unlike glycogen phosphorylase, glycogen synthase only has a phosphorylated form. T/F
F
Like glycogen phosphorylase, it exists in both phosphorylated and dephosphorylated forms.
Which is the phosphorylated and dephosphorylated forms of glycogen synthase?
Glycogen synthase a is phosphorylated
Glycogen synthase b is dephosphorylated
Which molecule serves as the allosteric ativator of glycogen synthase b?
G 6-P
Which protein kinase phosphorylates glycogen phosphorylase?
PKA
Which is the most important regulatory kinase of glycogen synthase?
Glycogen synthase kinase 3 (GSK3)
GSK3 cannot phosphorylate glycogen synthase until what event?
Until another protein kinase, casein kinase II (CKII) has first phosphorylated the glycogen synthase on a nearby residue (an event called priming)
When glycogen synthase is phosphorylated, it is: activated or deactivated?
Deactivated
Which of the following characterize/ s glucose transporters in general?
A. They have distinct substrate specificities.
B. They transport glucose using the sodium co-transport system.
C. They use facilitated diffusion down glucose concentration gradients.
D. All of the above
D
Which occurs in the second half of glycolysis?
A. Phosphorylation of a hexose
B. Isomerization of a hexose phosphate
C. Formation of triose phosphates from a hexose diphosphate
D. Substrate level phosphorylation of ADP
D
Which of the ff best describes gluconeogenesis?
A. Pyruvate-CoA is an immediate of gluconeogenesis
B. Gluconeogenesis is a complete reversal of glycolysis
C. Alanine and glutamine are precursors of your glucose
D. Gluconeo provides free glucose to your muscles
C
The immediate products of the oxidative phase of Pentose Phosphate Pathway:
A. ribulose-5-phosphate and NAD+
B. ribose-5-phosphate and NADP+
C. xylulose-5-phosphate and NADPH
D. ribulose-5-phosphate and NADPH
D
The absence of which enzyme explains why the muscle cells cannot release glucose for circulation in the blood.
A. glycogen phosphorylase
B. glucose-6-dehydrogenase
C. glucose-6-phosphatase
D. phosphogluconomutase
C
