Carbohydrate Metabolism

Question 1

The Cellular Energy Currency

1. What is Catabolism the degradation of?
2. What is the energy conversion strategy cells use to oxidize glucose?
3. What is ATP?
4. What are the 3 main categories of types of cellular work that require energy?

Answer 1

1. Catabolism is the degradation of fuel molecules which provides energy for cellular energy-requiring functions.
2. Small amounts of energy are released at several points in the pathway. This energy is harvested and stored in bonds of ATP.
3. ATP = universal energy currency OR adenosine triphosphate.
4. • Biosynthesis: Synthesis of Metabolic Intermediates and Macromolecules
     - Active Transport: Movement of Ions and Molecules
     - Motility.

Question 2

Adenosine Triphosphate

1. How many kcals does a complete combustion of a mole of glucose yield?
2. ATP serves as a “go-between” molecule that couples what two things?
3. What does ATP capture energy as?
4. What does hydrolysis of the anhydride bonds provide?

Answer 2

1. 686 kcals.
2. Exergonic (energy releasing) catabolism reactions and endergonic (energy requiring) anabolic reactions.
3. ATP “captures” energy as phosphoanhydride bonds.
4. Provides energy for anabolism.

Question 3

ATP: The Molecule

1. What is ATP and what 3 things is it composed of?
2. What is the role of the phosphoester bond?
3. What are second and third groups joined by?

Answer 3

1. ATP is a nucleotide, a molecule composed of:
   - Nitrogenous base
   - 5-carbon sugar
   - One, two, or three phosphoryl groups
2. Phosphoester bond joins the first phosphoryl group to the 5-carbon sugar ribose.
3. Joined by phosphoanhydride bonds = high-energy bonds.

Question 4

ATP: Hydrolysis of the Phosphoanhydride Bond

1. What happens when these phosphoanhydride bonds are broken? What can the result be used for?
2. What does the hydrolysis of the phospohoanhydride bond do in the B-D-Glucose reaction?

Answer 4

1. Large amounts of energy are released. This energy can be used for cellular work. Can drive cellular processes, such as Phosphorylation of glucose or fructose.
2. This releases the phosphoryl group from ATP in the reactant, causing both the phosphoryl group to attach to the #6 carbon forming B-D-Glucose-6-phosphate and ADP in the product.

Question 5

Overview of Catabolic Processes

1. During the catabolic process, what happens to carbohydrates, fats, and proteins?
2. What is the most readily used energy source?
3. What is the cycle of the breaking down of carbohydrates into ATP? (List all steps in order)

Answer 5

1. Carbohydrates, fats and proteins can be degraded to release energy.
2. Carbohydrates.
3. Food > Carbohydrates > Simple sugars ? Glycolysis > ATP

OR

Food > Carbohydrates > Simple sugars ? Glycolysis > Pyruvate > Acetyl CoA > Citric Acid Cycle > Oxidative phosphorylation > ATP.

Question 6

Stage 1: Hydrolysis of Dietary Macromolecules into Small Subunits

1. What is the purpose of Stage 1 in catabolism?
2. What are polysaccharides degraded to? Where does this process begin and with what? Where is this process continued?
3. What are proteins digested into? Where does this process begin and with what? What occurs next?
4. What are fats broken into? Where does this process begin and with what? What occurs next?

Answer 6

1. To degrade food molecules into component subunits.
2. Polysaccharides are degraded into monosaccharides. Begins in the mouth with amylase action on starch. Continues in small intestine with pancreatic amylase to form monosaccharides.
3. Digested into amino acids. Begins in the stomach with acid hydrolysis. Serine proteases act in the small intestine.
4. Fats are broken into fatty acids and glycerol. Begins in small intestine with fat globules. Disperse with bile salts. Degrade with pancreatic lipase.

Question 7

Overview of Digestive Processes

1. What do the salivary glands secrete, and what purpose does it serve?
2. What does the stomach secrete and for what reason?
3. What does the pancreas secrete?
4. What is the role of the liver and gallbladder?
5. How do amino acids and hexoses enter cells?
6. How do fatty acids and glycerols move?
7. What is a necessary intermediate for the hydrolysis reactions of carbohydrates, proteins, and fats?

Answer 7

1. Salivary glands secrete amylase which digests starch.
2. Stomach secretes HCl which denatures proteins, and pepsin which digests.
3. Serine proteases and lipases.
4. The liver and gallbladder deliver bile salts.
5. Amino acids/hexoses enter via active transport.
6. Fatty acids/glycerol move via passive transport.
7. Glycosidase, peptidase, and lipase respectively.

Question 8

Stage 2: Conversion of Monomers into a Form That Can Be Completely Oxidized

1. What are the small subunits assimilated into in Stage 2?
2. The two major pathways of energy metabolism are glycolysis and the citric acid cycle:
   - What do sugars enter glycolysis as? What are sugars converted to?
   - What two things enters the citric acid cycle?

Answer 8

1. Assimilated into the pathways of energy metabolism.
2. • Glycolysis: Sugars enter here as glucose or fructose. Sugars are converted to acetyl CoA and enter citric acid cycle.

• Citric Acid Cycle: Proteins enter here as the carbon skeleton of amino acids. Fatty acids enter here after conversion to acetyl CoA.

Question 9

Stage 3: Complete Oxidation of Nutrients and the Production of ATP

1. What does Acetyl CoA carry?
2. What 3 things occur once acetyl CoA enters the citric acid cycle?

Answer 9

1. Carries acetyl groups, 2-carbon remnants of the nutrients
2. • Electrons and hydrogen atoms are harvested.
     - Acetyl group is oxidized to produce CO2.
     - Electrons and hydrogen atoms harvested are used to produce ATP during oxidative phosphorylation.

Question 10

Glycolysis (Embden-Meyerhof Pathway)
1. What is glycolysis the pathway of?
2. What does glycolysis begin with as the substrate?
3. What organisms can use glucose?
4. Does glucose require oxygen?
5. Where does glycolysis occur?
6. What is the 10 step pathway catalyzed by?
PRODUCTS:
7. What does substrate-level phosphorylation give? Where is the phosphoryl group transferred to and from what?
8. What does NADH carry?
9. What is the fate of pyruvate dependent on?

Answer 10

1. Pathway of carbohydrate catabolism.
2. Begins with D-glucose as the substrate
3. All organisms can use glucose as an energy source.
4. No.
5. Occurs free in the cytoplasm.
6. Catalyzed by enzymes.
7. Substrate-level phosphorylation gives 4 ATP. A phosphoryl group is transferred to ADP from 1,3-bisphosphoglycerate and phosphoenolpyruvate.
8. NADH carries hydride anions with two electrons
9. Pyruvate: the fate depends on cellular conditions

Question 11

Glycolysis Overview

1. What is the anaerobix oxidation of glucose reaction to give two molecules of pyruvate?
2. Why is pyruvate used in follow-up reactions?
3. Why must NADH be reoxidized?

Answer 11

1. Glucose + 2 ADP + 2 Pi + 2 NAD+&raquo_space;> 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
2. To sustain glycolysis.
3. So that glycolysis can continue.

Question 12

Glycolysis Reaction 1 and 2

1. In Reaction 1, what phosphorylates the substrate glucose? What is the product?
2. In Reaction 1, what is the source of the phosphoryl group? What does the expenditure of this source early in the pathway work as?
3. In Reaction 2, what happens to the product of Reaction 1? What enzyme causes this result?
4. In Reaction 2, which carbon in the product becomes exposed? What does this exposed carbon convert?

Answer 12

1. Substrate glucose is phosphorylated by hexokinase. Product is glucose-6-phosphate.
2. ATP. Works as energy “debt” necessary to get the pathway started.
3. Product of reaction 1 is rearranged to the structural isomer fructose-6-phosphate by enzyme phosphoglucose isomerase.
4. Product has an “exposed” C-1, no longer part of the ring structure. Converts and aldose to a ketose.

Question 13

Glycolysis Reaction 3

1. In Reaction 3, what substrate is phosphorylated and by what?
2. What is the product of this reaction? What is the source of the phosphoryl group? What does the expenditure of this source early in the pathway work as?

Answer 13

1. Substrate fructose-6-phosphate is phosphorylated by phosphofructokinase.
2. Product is fructose-1,6-bisphosphate. Source of phosphoryl group is ATP. Again the expenditure of ATP early in the pathway works as energy “debt” necessary to get the pathway started.

Question 14

Glycolysis Reactions 4 and 5

1. In Reaction 4, what happens to the product of reaction 3? What enzyme causes this. What two things are formed in Reaction 4?
2. In Reaction 5, what is Dihydroxyacetone phosphate rearranged into? What enzyme causes this?

Answer 14

1. Product of reaction 3 is split into two 3-carbon intermediates. Caused by the enzyme aldolase. This forms Glyceraldehyde-3-phosphate AND Dihydroxyacetone phosphate.
2. Dihydroxyacetone phosphate is rearranged into a second glyceraldehyde-3-phosphate by the enzyme triose phosphate isomerase.

Question 15

Glycolysis Reaction 6

1. In Reaction 6, what substrate is oxidized and into what?
2. What is reduced in Reaction 6? What is transferred and to where?
3. What is the first step in glycolysis?
4. What is the product of Reaction 6? What is the strength of the bond the new phosphate group is attached with?
5. How many times does steps 6 and onward occur?

Answer 15

1. Substrate glyceraldehyde-3-phosphate is oxidized to a carboxylic acid by glyceraldehyde-3-phosphate dehydrogenase.
2. Reduces NAD+ to NADH. Transfers an inorganic phosphate group to the carboxyl group.
3. First step in glycolysis to “harvest” energy.
4. Product is 1,3-Bisphosphoglycerate. New phosphate group attached with a “high-energy” bond.
5. This and all subsequent steps occur twice for each G-3-P.

Question 16

Glycolysis Reactions 7 and 8

1. In Reaction 7, in what form is energy harvested? What high energy phosphate group is transferred to ADP and by what? What 2 things are formed?
2. In Reaction 8, what is isomerized into what and by what enzyme? Which carbon is the phosphate group moved to?

Answer 16

1. Harvest energy in the form of ATP. 1,3-Bisphosphoglycerate high energy phosphate group is transferred to ADP by phosphoglycerate kinase. This forms 3-Phosphoglycerate and ATP.
2. 3-Phosphoglycerate is isomerized into 2-phosphoglycerate by the enzyme phosphoglycerate mutase. Moves the phosphate group from carbon-3 to carbon-2 .

Question 17

Glycolysis Reaction 9 and 10

1. In Reaction 9, what does the enzyme enolase catalyze? What is the energy rich/highest energy phosphorylated compound in metabolism that is formed in Reaction 9?
2. In Reaction 10, what does Phosphoenolpyruvate serves as? What enzyme causes the transfer? What is made and released?
3. [Reaction 10] What is the final product of glycolysis? Why is Reaction 10 a coupled reaction?

Answer 17

1. Enolase catalyzes dehydration of 2-phospholgycerate. Phosphoenolpyruvate.
2. Phosphoenolpyruvate serves as donor of the phosphoryl group transferred to ADP by pyruvate kinase making ATP and releasing water.
3. Pyruvate is the final product of glycolysis. Hydrolysis of the phosphoester bond provides energy for the formation of the phosphoanhydride bond of ATP.

Question 18

Fermentation

1. From glycolysis, why does pyruvate remain? What happens to NADH? In aerobic conditions, where does both occur?
2. Under what conditions are fermentation reactions used to accomplish this? What type of reactions are fermentation reactions?
3. What are the two fermentation pathways?

Answer 18

1. Pyruvate remains for further degradation and NADH must be reoxidized. In aerobic conditions, both will occur in cellular respiration.
2. Under anaerobic conditions, fermentation reactions accomplish this. Fermentation reactions are catabolic reactions occurring with no net oxidation.
3. Lactate fermentation and Alcohol fermentation.

Question 19

Lactate Fermentation

1. What is Lactate fermentation? What microorganisms use lactate fermentation? What types of food can be produced from lactate fermentation?
2. In the lactate fermentation reaction, what is produced and what is degraded? What intermediate is used?

Answer 19

1. Lactate fermentation is the anaerobic metabolism that occurs in exercising muscle. Bacteria use lactate fermentation. Production of yogurt and cheese.
2. This reaction produces NAD+ and degrades pyruvate. Lactate dehydrogenase is used as the intermediate.

Question 20

Alcohol Fermentation

1. What does yeast ferment anaerobically? What is used to accomplish this?
2. What does Pyruvate decarboxylase remove? What is produced as a result?
3. What does alcohol dehydrogenase catalyze the reduction of? What is released during this?

Answer 20

1. Yeast ferment sugars of fruit and grains anaerobically, using pyruvate from glycolysis
2. Pyruvate decarboxylase removes CO2 from the pyruvate producing acetaldehyde.
3. Alcohol dehydrogenase catalyzes reduction of acetaldehyde to ethanol, releasing NADH in the process

Question 21

Gluconeogenesis: The Synthesis of Glucose

1. What type of starting materials does gluconeogenesis make glucose from? What are the 3 starting materials? In what temporary conditions are two of these starting materials only used in?
2. Where does this process primarily occur?

Answer 21

1. From noncarbohydrate starting materials:
   - Lactate
   - Glycerol
   - Most amino acids (not leucine, lysine)
   - Glycerol and amino acids are used only in starvation conditions.
2. Gluconeogenesis primarily occurs in the liver.

Question 22

Comparison of Glycolysis
and Gluconeogenesis
1. While basically opposite processes, are glycolysis and gluconeogenesis reversals of each other?
2. What are the 3 nonreversible steps of glycolysis that must be bypassed with new routes (when it comes to gluconeogenesis)?

Answer 22

1. No. They are not a simple reversal of each other.
2. • Pyruvate > Phosphoenolpyruvate
     - Fructose-1,6-bisphosphate > Fructose-6-phosphate
     - Glucose-6-phosphate > Glucose

Question 23

Cori Cycle
1. In the Cori cycle, where lactate taken from and where is it transfered? What is it converted to first? What is it converted to last? Where does it return to?

Answer 23

1. Lactate from skeletal muscle is transferred to the liver. Converted to pyruvate then glucose. This glucose can be returned to the muscle.

Question 24

Glycogen Synthesis and Degradation / Structure of Glycogen

1. What is the sole source of energy for mammalian red blood cells and the major source for brain? By what 3 things is it supplied?
2. What is glycogen? What does it exist as?

Answer 24

1. Glucose is the sole source of energy for mammalian red blood cells and the major source for brain. It is supplied:
   - In the diet
   - Via glycogenolysis
   - By gluconeogenesis
2. Glycogen is a highly branched α(1>4) and α(1>6) polymer of glucose. Exists as granules found in cytoplasm of liver and muscle cells.

Question 25

Glycogenolysis

1. What two things (and their respective glandular organ) is glycogen degradation controlled by?
2. What are 3 steps of glycogenolysis?

Answer 25

1. Glycogen degradation is controlled by Glucagon (Pancreas - responds to low blood sugar) and Epinephrine
   (Adrenal gland - responds to stress/threat).
2. • Step 1: Glycogen phosphorylase catalyzes removal of an end glucose as glucose-1-phosphate.
     - Step 2: Debranching enzyme catalyzes removal of the last glucose at an α(1>6) branch as glucose.
     - Step 3: Phosphoglucomutase converts glucose-1-phosphate to glucose-6-phosphate.

Question 26

Glycogenesis: Glycogen
Synthesis
1. What does insulin stimulate synthesis of? What are the two chemical reactions for this? What enzyme are used?
2. What must be activated to add to the growing glycogen chain? What is the reaction for this?

Answer 26

1. Stimulates synthesis of glycogen.
   - glucose + ATP > glucose-6-P + ADP + H+. Enzyme = glucokinase.
   - glucose-6-P > glucose-1-P
   Enzyme = phosphoglucomutase
2. UDP-Glucose.
   - G-1-P + UTP > UDP-glucose + PPi

Question 27

Glycogenesis vs Glycogenolysis

1. Define Hyperglycemia and hypoglycemia.
2. What does insulin stimulate? What does it elevate? What does it activate? What does it inhibit?
3. What does glucagon stimulate? What does it inhibit?
4. Does glycogen synthesis and degradation compete?

Answer 27

1. High blood sugar. Low blood sugar.
2. Insulin stimulates glucose uptake. Elevates glucokinase. Activates glycogen synthetase. Inhibits glycogen phosphorylase.
3. Stimulates glycogen phosphorylase. Inhibits glycogen synthetase.
4. No.