chapter 14

Question 1

Six carbon monosaccharides glucose and fructose have ___ hydroxyl groups

Answer 1

5

Question 2

Glucose cyclic formation

Answer 2

-In solution, cyclic form predominate
-Free hydroxyl of C-5 reacts with the aldehyde C-1
-Gives asymmetry to C-5 producing α and β stereoisomers called pyranoses because they resemble pyran
-Systematic names are α-D-glucopyranose and β-D- glucopyranose
-The carbonyl C is the anomeric C

Question 3

Glucose plays a central role in metabolism
-Good fuel- rich in
-Can use it to make
-Can be used to provide
-Good precursor

Answer 3

-Good fuel- rich in potential energy; the standard free energy change is -2840kJ/mol
-Can use it to make polymeric forms such as starch and glycogen- why are these important to the cell?
maintaining a relatively low cytosolic osmolarity
-Can be used to provide energy under aerobic conditions and anaerobic conditions
-Good precursor molecule. Bacteria can use glucose for making the carbon skeletons for every amino acid, nucleotide, coenzyme, fatty acid or other metabolic intermediate that is required for growth
–Photosynthesizers- reduce atmospheric CO2 to trioses then converting them to glucose
–Non photosynthesizers- use 3 and 4 carbon precursors in a process called gluconeogenesis

Question 4

Four Major Pathways 
of Glucose Utilization

Answer 4

Storage
-Can be stored in the polymeric form (starch, glycogen)
-When there’s plenty of excess energy

Glycolysis
–Generates energy via oxidation of glucose
–Short-term energy needs

Pentose Phosphate Pathway
–Generates NADPH via oxidation of glucose
–For detoxification and the biosynthesis of lipids and nucleotides

Synthesis of Structural Polysaccharides
-For example, in cell walls of bacteria, fungi, and plants

Question 5

Glycolysis: Importance
-Almost universal central
-The pathway of the largest
-Sole source of
-Many plant tissues derive most
-Many anaerobic microorganisms are entirely

Answer 5

-Almost universal central pathway of glucose catabolism
-The pathway of the largest flux of carbon
-Sole source of metabolic energy in some cells and tissues such as brain
-Many plant tissues derive most of their energy from glycolysis
-Many anaerobic microorganisms are entirely dependent on glycolysis

Question 6

Sequence of enzyme-catalyzed reactions by which glucose is converted into pyruvate

-Some of the oxidation-free energy is captured by

Research of glycolysis played a large role in the development of modern biochemistry

Answer 6

Sequence of enzyme-catalyzed reactions by which glucose is converted into pyruvate
-Pyruvate can be further aerobically oxidized
-Pyruvate can be used as a precursor in biosynthesis

-Some of the oxidation-free energy is captured by the synthesis of ATP and NADH

Research of glycolysis played a large role in the development of modern biochemistry
-Understanding the role of coenzymes
-Discovery of the pivotal role of ATP
-Development of methods for enzyme purification
-Inspiration for the next generations of biochemists

Question 7

Glycolysis: Overview
-In the evolution of life, glycolysis probably was one of the
-It developed before

-Thus, the task upon early organisms was:
How to extract free energy from glucose anaerobically?

The solution:

Answer 7

-In the evolution of life, glycolysis probably was one of the earliest energy-yielding pathways
-It developed before photosynthesis, when the atmosphere was still anaerobic

-Thus, the task upon early organisms was:
How to extract free energy from glucose anaerobically?

The solution:
-First: Activate it by phosphorylation
-Second: Collect energy from the high-energy metabolites

Question 8

Glycolysis: Overview
-A 6-carbon glucose molecule is broken to
-phase 1 and phase 2

Answer 8

-A 6-carbon glucose molecule is broken to two 3 carbon pyruvate molecules in two phases of 5 steps
-Phase 1- Prep phase, overall, two phosphoryl groups are transferred to the sugar, glucose is converted to fructose and fructose is split to two 3 carbon molecules (glyceraldehyde 3-PO4)
-Phase 2- Payoff phase, inorganic phosphate is added to the glyceraldehyde 3-PO4 then energy is released. The net yield is 2 molecules of ATP and two molecules of NADH per molecule of glucose

Question 9

Chemical Logic of Glycolysis
ATP forms a complex with Mg2+

Answer 9

ATP forms a complex with Mg2+

Mg complexes shield the negative charges and influence the conformation of the phosphate groups in ATP

This makes the terminal phosphorous atom easier to nucleophilic attack by the OH of glucose

Question 10

Step 1:
Rationale

Answer 10

Phosphorylation of Glucose

Irreversible reaction

Hexokinase catalyzes transfer of terminal phosphoryl group from ATP to an acceptor nucleophile

-16.7

Rationale
–Traps glucose inside the cell
–Lowers intracellular glucose concentration to allow further uptake

-This process uses the energy of ATP
-Hexokinase in eukaryotes, and glucokinase in prokaryotes
-Nucleophilic oxygen at C6 of glucose attacks the last (γ) phosphate of ATP
-ATP-bound Mg++ facilitates this process by shielding the negative charges on ATP

Highly thermodynamically favorable/irreversible
–Regulated mainly by substrate inhibition

Question 11

Step 2:
Rationale

Answer 11

Phosphohexose Isomerization

-Change of an aldose to a ketose
-Formation of fructose 6-PO4 is necessary to allow the bond to be broken between C-3 and C-4. the C-1 position needs to be an alcohol and not a carbonyl group

=1.7

Rationale
–C1 of fructose is easier to phosphorylate by PFK
–Allows for symmetrical cleave by aldolase

-An aldose (glucose) can isomerize into a ketose (fructose) via an enediol intermediate
-The isomerization is catalyzed by the active-site glutamate, via general acid/base catalysis

Slightly thermodynamically unfavorable/reversible
–Product concentration kept low to drive forward

Question 12

Step 3
Rationale

Answer 12

Step 3: 2nd Priming Phosphorylation

Formation of the 1,6 bis phosphate is targeted to glycolysis

-14.2

Rationale

-Further activation of glucose
-Allows for 1 phosphate/3-carbon sugar after step 4

First Committed Step of Glycolysis
-fructose 1,6-bisphosphate is committed to become pyruvate and yield energy

-This process uses the energy of ATP

-Highly thermodynamically favorable/irreversible
-Phosphofructokinase-1 is highly regulated
–By ATP, fructose-2,6-bisphosphate, and other metabolites
–Do not burn glucose if there is plenty of ATP

Question 13

Step 4
Rationale

Answer 13

Step 4: Aldol Cleavage of F-1,6-bP
Cleavage to 3 carbon products
=23.8

Rationale
-Cleavage of a six-carbon sugar into two three-carbon sugars
-High-energy phosphate sugars are three-carbon sugars

-The reverse process is the familiar aldol condensation
-Animal and plant aldolases employ covalent catalysis
-Fungal and bacterial aldolases employ metal ion catalysis

-Thermodynamically unfavorable/reversible
–GAP concentration kept low to pull reaction forward

Question 14

Step 5:

Answer 14

Triose Phosphate Interconversion
- 7.5

Rationale:
-Allows glycolysis to proceed by one pathway

Aldolase creates two triose phosphates:
–Dihydroxyacetone Phosphate (DHAP)
–Glyceraldehyde-3-Phosphate (GAP)

-Only GAP is the substrate for the next enzyme
-DHAP must be converted to GAP

-Completes preparatory phase

Thermodynamically unfavorable/reversible
–GAP concentration kept low to pull reaction forward

Question 15

Glucose Carbons in GAP

Answer 15

Only glyceraldehyde 3-PO4 can be further degraded

Question 16

Step 6:

Answer 16

Oxidation of GAP (Payoff Begins)
Oxidation of glyceraldehyde 3-PO4 to give an acyl phosphate

6.3

Rationale:
-Generation of a high-energy phosphate compound
-Incorporates inorganic phosphate
-Which allows for net production of ATP via glycolysis!

-First energy-yielding step in glycolysis
-Oxidation of aldehyde with NAD+ gives NADH
-Active site cysteine
–Forms high-energy thioester intermediate
–Subject to inactivation by oxidative stress

Thermodynamically unfavorable/reversible
–Coupled to next reaction to pull forward

Question 17

Step 7:

Answer 17

1st Production of ATP
-Note that the reaction is reversible, but the concentration of 1,3 bisphosphate glycerate is small
-Substrate-level phosphorylation

-18.5

Rationale:
-Substrate-level phosphorylation to make ATP
1,3-bisphosphoglycerate is a high-energy compound
–can donate the phosphate group to ADP to make ATP

-Kinases are enzymes that transfer phosphate groups from ATP to various substrates

Highly thermodynamically favorable/reversible
–Is reversible because of coupling to GAPDH reaction

Question 18

Step 8:

Answer 18

Step 8: Migration of the Phosphate

4.4

Rationale:
-Be able to form high-energy phosphate compound
-Mutases catalyze the (apparent) migration of functional groups
-One of the active site histidines is post-translationally modified to phosphohistidine
-Phosphohistidine donates its phosphate to O2 before retrieving another phosphate from O3
–2,3-bisphosphoglycerate intermediate
–Note that the phosphate from the substrate ends up bound to the enzyme at the end of the reaction

Thermodynamically unfavorable/reversible
–Reactant concentration kept high by PGK to push forward

Question 19

Step 9:

Answer 19

Dehydration of 2-PG to PEP

-Removal of water from 2-phosphoglycerate.
-Free energy of hydrolysis of phosphoryl gp in 2-phosphoglycerate (-17.6 kJ/mol) and for phosphoenolpyruvate (-61.9 kJ/mol) due to a redistribution of energy within the molecule

7.5

Rationale
–Generate a high-energy phosphate compound

2-Phosphoglycerate is not a good enough phosphate donor
–Two negative charges in 2-PG are fairly close
–But loss of phosphate from 2-PG would give a secondary alcohol with no further stabilization

Slightly thermodynamically unfavorable/reversible
–Product concentration kept low to pull forward

Question 20

Step 10:

Answer 20

2nd Production of ATP

-Irreversible reaction
-Use -30.5 kJ/mol to form ATP from ADP, the rest of energy -31.4kJ/mol used to drive the reaction forward
-Substrate level phosphorylation

-31.4

Rationale
–Substrate-level phosphorylation to make ATP
–Net production of 2 ATP/glucose

-Loss of phosphate from PEP yields an enol that tautomerizes into ketone
-Tautomerization
–effectively lowers the concentration of the reaction product
–drives the reaction toward ATP formation

-Pyruvate kinase requires divalent metals (Mg++ or Mn++) for activity
-Highly thermodynamically favorable/irreversible
—Regulated by ATP, divalent metals, and other metabolites

Question 21

Summary of Glycolysis

Answer 21

Glucose + 2ATP + 2NAD+ + 4ADP + 2Pi  2Pyruvate + 2ADP + 2NADH + 2H+ + 4ATP + 2H2O
Glucose + 2NAD+ + 2ADP + 2Pi  2Pyruvate + 2NADH + 2H+ + 2ATP + 2H2O

-Used: 1 glucose; 2 ATP; 2 NAD+
-Made:
2 pyruvate
Various different fates
4 ATP
Used for energy-requiring processes within the cell
2 NADH
Must be reoxidized to NAD+ in order for glycolysis to continue

-Note that 4 electrons are transferred from two molecules of glyceraldehyde 3- phosphate. These electrons are transferred to the electron transfer chain

Question 22

Regulation of Glycolysis
Glycolysis is heavily regulated
-ATP yield of glycolysis under anaerobic conditions is
-Flux through the pathway is regulated to maintain
-Short term- Rate of glycolysis achieved by complex
-Longer term- regulation by hormones

Answer 22

-Glycolysis is heavily regulated
–Ensure proper use of nutrients
–Ensure production of ATP only when needed
-ATP yield of glycolysis under anaerobic conditions is 2 ATP per molecule of glucose
-ATP yield of glycolysis under aerobic conditions is 30-32 ATP per molecule of glucose
-Flux through the pathway is regulated to maintain constant ATP levels and levels of intermediates that are used in biosynthesis
-Short term- Rate of glycolysis achieved by complex interplay among ATP consumption, NADH regeneration, allosteric regulation of hexokinase, PFK1 and pyruvate kinase
-Longer term- regulation by hormones glucagon, insulin, epinephrine and gene regulation of the glycolytic enzymes

Question 23

Glycolysis in cancer

Answer 23

-When a tumor just begins to form, cancer cells that are further than 100 to 200 μm away from capillaries utilize glycolysis for energy needs.
-Many cancer cells also have less mitochondria than normal cells and cannot produce as much ATP as normal cells from aerobic respiration so they tend to use glycolysis to a larger extent for energy needs
-These cancer cells take up more glucose than normal cells and convert it to pyruvate and lactate in order to recycle NADH
-Cancer cells overproduce an isozyme of hexokinase that in insensitive to feedback inhibition by glucose-6-phosphate that commits the cell to continued glycolysis
-Allow the cells to survive anaerobic conditions till blood supply has caught up

Question 24

Glycolysis occurs at ____ rates in tumor cells

Answer 24

Glycolysis occurs at elevated rates
 in tumor cells

Question 25

Glycose uptake is deficient in

Answer 25

type 1 diabetes mellitus

Question 26

Feeder Pathways for Glycolysis

Answer 26

Glucose molecules are cleaved from glycogen and starch by glycogen phosphorylase
–Yielding glucose-1-phosphate

Disaccharides are hydrolyzed
-Maltose: 2 D-glucose
-Lactose: D-glucose and D-galactose
-Sucrose: D-glucose and D-fructose
-Fructose, galactose, and mannose enter glycolysis at different points

Question 27

Starch breakdown
-In humans, starch is a major source of
-Beginning in the mouth
-Stomach, α amylase inactivated by

Answer 27

-In humans, starch is a major source of carbohydrates in diet.
-Beginning in the mouth, saliva has α-amylase which hydrolyse internal glycosidic linkages to produce short oligosaccharides
-Stomach, α amylase inactivated by low pH but another form secreted by the pancreas continues breakdown to form maltose, maltotriose and limit dextrins containing amylopectin with α1→ 6 branch points

Question 28

Glycogen breakdown

Answer 28

Phosphorylase enzyme catalyzes the attack of an a1→ 4 glcycosidic linkage on the non reducing end of the chain to remove a glucose as glucose 1-phosphate in a reaction called phosphorolysis

Question 29

Conversion of galactose to glucose

Answer 29

Note that there is oxidation of the C-4 on galactose by NAD+ then reduction by NADH

Defects of enzymes in the pathway cause galactosemia

Question 30

Fate of pyruvate under aerobic condtions
-Pyruvate oxidized to
Acetate enters the
-NADH formed by
-What happens in cases where there is limiting O2?

Answer 30

-Pyruvate oxidized to acetate
-Acetate enters the TCA cycle and is oxidized to CO2 and H2O
-NADH formed by dehydrogenation of glyceraldehyde 3 phosphate must be reoxidized to NAD+ by passage of its electrons to O2 in mitochondrial respiration
-What happens in cases where there is limiting O2?
-NAD+ cannot be regenerated and this would leave the cell with no electron acceptor for glyceraldehyde 3- phosphate and glycolysis would stop

Question 31

What is the fate of pyruvate under anaerobic conditions?
-NAD+ must be
-During anaerobic glycolysis, NADH

Answer 31

-NAD+ must be regenerated
-During anaerobic glycolysis, NADH transfers electrons to other acceptors such as lactate or ethanol

Large negative free energy of reaction favors lactate formation

Question 32

Fermentation
-general term for
-Oxygen is
-Very ancient biological mechanism for
-The chemistry of the reaction has been completely
-Differences among species in details of its

Answer 32

-general term for anaerobic degradation of glucose or other organic nutrients to obtain energy conserved as ATP
-Oxygen is not consumed and there is no change in the NAD+/NADH concentration
-Very ancient biological mechanism for obtaining energy
-The chemistry of the reaction has been completely conserved among vertebrates, yeast and plants
-Differences among species in details of its regulation and the fate of the pyruvate that is formed

Question 33

Anaerobic Glycolysis:
Fermentation
-Generation of energy (ATP) without
No net change in
-Regenerates
The process is used in

Answer 33

-Generation of energy (ATP) without consuming oxygen or NAD+
-No net change in oxidation state of the sugars
-Reduction of pyruvate to another product
-Regenerates NAD+ for further glycolysis under anaerobic conditions
-The process is used in the production of food from beer to yogurt to soy sauce

Question 34

Animals undergo 
lactic acid fermentation
-Reduction of pyruvate to lactate,
-During strenuous exercise
-The acidification of muscle prevents
The lactate can be
-Requires a

Answer 34

-Reduction of pyruvate to lactate, reversible
-During strenuous exercise, lactate builds up in the muscle
–Generally less than 1 minute
-The acidification of muscle prevents its continuous strenuous work
-The lactate can be transported to the liver and converted to glucose there
-Requires a recovery time
–High amount of oxygen consumption to fuel gluconeogenesis
–Restores muscle glycogen stores

Question 35

Lactic Acid Fermentation

Answer 35

Change of pyruvate to lactate ensures that there is no net change in NAD+/NADH

Question 36

The Cori Cycle – Lactate cycle
-Lactate (C3H6O3) (as is formed during strenuous muscle activity) is carried in blood to the
-Lactate build up results in
-The conversion back to glucose does not result in a
Note that some energy was

Answer 36

-Lactate (C3H6O3) (as is formed during strenuous muscle activity) is carried in blood to the liver where it is reconverted to glucose (C6H12O6) during recovery from exercise.
-Lactate build up results in acidification and limits the period of heavy muscular activity (no more than a minute)
-The conversion back to glucose does not result in a change in the oxidation level of the molecule since the H:C ratio is the same
-Note that some energy was extracted from the system

Question 37

Fermentation to Produce Ethanol
-Carried out by
-Glucose is changed to
-Two-step reduction of
-Humans do not have
-CO2 produced in the first step is responsible for:
-Both steps require cofactors and coenzymes

Answer 37

-Carried out by yeast and other microorganisms
-Glucose is changed to pyruvate in glycolysis and pyruvate is converted to ethanol and CO2 (Note: not lactate)
-Two-step reduction of pyruvate to ethanol, irreversible
-Humans do not have pyruvate decarboxylase
-We do express alcohol dehydrogenase for ethanol metabolism
-CO2 produced in the first step is responsible for:
–carbonation in beer
–dough rising when baking bread
-Both steps require cofactors and coenzymes
—Pyruvate decarboxylase: Mg++ and thiamine pyrophosphate
—Alcohol dehydrogenase: Zn++ and NAD+

Question 38

Ethanol Fermentation
-First pyruvate is decarboxylated in an
-Second, acetaldehyde is reduced to
-Net equation is:

Answer 38

-First pyruvate is decarboxylated in an irreversible reaction that does not involve oxidation to produce acetaldehyde
-Second, acetaldehyde is reduced to ethanol and NADH is used
-Net equation is:
glucose + 2ADP+ 2Pi → 2 ATP+ 2ethanol + 2 CO2 + 2 H2O

Question 39

TPP is a

Answer 39

common acetaldehyde carrier

TPP-Thiamine pyrophosphate, the coenzyme form of vitamin B1
Pyruvate decarboxylase is dependent on TPP

Question 40

Pentose Phosphate Pathway

Answer 40

-Glucose is needed for other things apart from energy
-One of these is pentose phosphates
The pathway is known by several names:
Pentose phosphate pathway
Phosphogluconate pathway
Hexose monophosphate pathway

-Used to make pentoses for RNA, DNA, ATP, NADH, FADH
-Used to make NADPH needed for reductive biosynthesis (as needed in fatty acid synthesis, cholesterol or steroid hormone synthesis) or to counter the effect of free radicals

Question 41

Pentose Phosphate Pathway

-Note the generation of
-Reactions occur in the

Answer 41

Note the generation of NADPH and the decarboxylation step
Reactions occur in the cytosol like the glycolysis reactions

Question 42

The main products are of pentose phosphate pathway
-NADPH is an
Ribose-5-phosphate is a

Answer 42

-The main products are NADPH and ribose 5-phosphate
-NADPH is an electron donor
–Reductive biosynthesis of fatty acids and steroids
–Repair of oxidative damage

Ribose-5-phosphate is a biosynthetic precursor of nucleotides
–Used in DNA and RNA synthesis
–Or synthesis of some coenzymes

Question 43

Step 1- regulatory

Answer 43

Loss of H = oxidation

Forms an intramolecular ester or lactone

NADP+ forms NADPH Reaction equilibrium is in the direction of NADPH

Question 44

Step 2

Answer 44

-Lactone hydrolysed to free acid by lactonase
-Reaction has a large negative standard free energy and is essentially irreversible

Question 45

Step 3

Answer 45

-Decarboxylation to form a ketopentose
-A second NADPH is generated

Question 46

Step 4

Answer 46

Isomerization to change a
ketose to an aldose

Net reaction:
glucose 6-phosphate + 2NADP+ + H2O → ribose 5-phosphate +CO2 + 2NADPH + 2H +

Question 47

Pentose Phosphate Pathway
How do you go from a 5 carbon sugar to a 6 carbon sugar?

Answer 47

How do you go from a 5 carbon sugar to a 6 carbon sugar?

The non oxidative phase recycles pentose phosphates to glucose 6-phosphate

Question 48

Non-oxidative phase
regenerates G-6-P from R-5-P

Answer 48

Used in tissues requiring more NADPH than R-5-P
Such as the liver and adipose tissue

Question 49

NADPH regulates partitioning into glycolysis vs. pentose phosphate pathway

Answer 49

When NADPH is forming faster than it is being used for biosynthesis and glutathione reduction, [NADPH] rises and inhibits the first enzyme in the pentose phosphate pathway. As a result, more glucose 6-phosphate is available for glycolysis.