chapter 14 Flashcards
Six carbon monosaccharides glucose and fructose have ___ hydroxyl groups
5
Glucose cyclic formation
-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
Glucose plays a central role in metabolism
-Good fuel- rich in
-Can use it to make
-Can be used to provide
-Good precursor
-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
Four Major Pathways of Glucose Utilization
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
Glycolysis: Importance
-Almost universal central
-The pathway of the largest
-Sole source of
-Many plant tissues derive most
-Many anaerobic microorganisms are entirely
-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
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
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
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:
-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
Glycolysis: Overview
-A 6-carbon glucose molecule is broken to
-phase 1 and phase 2
-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
Chemical Logic of Glycolysis
ATP forms a complex with Mg2+
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
Step 1:
Rationale
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
Step 2:
Rationale
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
Step 3
Rationale
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
Step 4
Rationale
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
Step 5:
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
Glucose Carbons in GAP
Only glyceraldehyde 3-PO4 can be further degraded
Step 6:
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
Step 7:
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
Step 8:
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
Step 9:
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
Step 10:
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
Summary of Glycolysis
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
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
-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
Glycolysis in cancer
-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
Glycolysis occurs at ____ rates in tumor cells
Glycolysis occurs at elevated rates in tumor cells
Glycose uptake is deficient in
type 1 diabetes mellitus
Feeder Pathways for Glycolysis
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
Starch breakdown
-In humans, starch is a major source of
-Beginning in the mouth
-Stomach, α amylase inactivated by
-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
Glycogen breakdown
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
Conversion of galactose to glucose
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
Fate of pyruvate under aerobic condtions
-Pyruvate oxidized to
Acetate enters the
-NADH formed by
-What happens in cases where there is limiting O2?
-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
What is the fate of pyruvate under anaerobic conditions?
-NAD+ must be
-During anaerobic glycolysis, NADH
-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
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
-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
Anaerobic Glycolysis:
Fermentation
-Generation of energy (ATP) without
No net change in
-Regenerates
The process is used in
-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
Animals undergo
lactic acid fermentation
-Reduction of pyruvate to lactate,
-During strenuous exercise
-The acidification of muscle prevents
The lactate can be
-Requires a
-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
Lactic Acid Fermentation
Change of pyruvate to lactate ensures that there is no net change in NAD+/NADH
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
-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
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
-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+
Ethanol Fermentation
-First pyruvate is decarboxylated in an
-Second, acetaldehyde is reduced to
-Net equation is:
-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
TPP is a
common acetaldehyde carrier
TPP-Thiamine pyrophosphate, the coenzyme form of vitamin B1
Pyruvate decarboxylase is dependent on TPP
Pentose Phosphate Pathway
-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
Pentose Phosphate Pathway
-Note the generation of
-Reactions occur in the
Note the generation of NADPH and the decarboxylation step
Reactions occur in the cytosol like the glycolysis reactions
The main products are of pentose phosphate pathway
-NADPH is an
Ribose-5-phosphate is a
-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
Step 1- regulatory
Loss of H = oxidation
Forms an intramolecular ester or lactone
NADP+ forms NADPH Reaction equilibrium is in the direction of NADPH
Step 2
-Lactone hydrolysed to free acid by lactonase
-Reaction has a large negative standard free energy and is essentially irreversible
Step 3
-Decarboxylation to form a ketopentose
-A second NADPH is generated
Step 4
Isomerization to change a
ketose to an aldose
Net reaction:
glucose 6-phosphate + 2NADP+ + H2O → ribose 5-phosphate +CO2 + 2NADPH + 2H +
Pentose Phosphate Pathway
How do you go from a 5 carbon sugar to a 6 carbon sugar?
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
Non-oxidative phase
regenerates G-6-P from R-5-P
Used in tissues requiring more NADPH than R-5-P
Such as the liver and adipose tissue
NADPH regulates partitioning into glycolysis vs. pentose phosphate pathway
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.
