Carbohydrate Metabolism II Flashcards
Overview of Glucose Metabolism: Glycolysis and Gluconeogenesis
Gluconeogenesis occurs mainly in the _____.
Synthesis of glucose from pyruvate utilizes many of the
same enzymes as _____.
Three Glycolysis reactions have such a large negative ΔG
that they are essentially _____:
_____ (or Glucokinase)
_____
_____.
These steps must be _____ in gluconeogenesis.
liver glycolysis irreversible hexokinase phosphofructokinase pyruvate kinase bypassed
1st Bypass Reaction: Formation of Phosphoenolpyruvate (PEP)
• PEP cannot be directly formed from pyruvate:
– Formed through a two-step process via _____.
– OAA is an intermediary of the _____.
oxaloacetate (OAA)
TCA cycle
1st Bypass Reaction: Formation of Phosphoenolpyruvate (PEP)
• Step 1: Pyruvate is carboxylated by a _____ to form OAA:
– Pyruvate carboxylase is a biotin-dependent _____
enzyme.
– Requires the hydrolysis of one molecule of _____.
• Step 2: OAA is decarboxylated to form PEP by a
_____: – Requires the hydrolysis of one molecule of _____.
– PEPCK is located in the _____, in _____, or both. – It is widely distributed in tissues.
PEP - _____ molecule
pyruvate carboxylase
mitochondrial
ATP
phosphoenolpyruvate carboxykinase (PEPCK) GTP cytosol mitochondria high energy
2nd Bypass Reaction: Formation of Fructose 6-phosphate
_____ hydrolyzes Pi from fructose 1,6-bisphosphate to form fructose 6-phosphate
- – not a _____ of the _____ reaction
- – ATP is not produced when the phosphate is removed; Pi is release by _____
Fructose 6-phosphate is converted to glucose 6-phosphate by the same isomerase used in glycolysis (_____)
fructose 1,6-bisphosphatase reversal phosphofructokinase-1 PFK1 hydrolysis phosphoglucoisomerase
3rd Bypass Reaction: Formation of Glucose
• Glucose 6-phosphatase hydrolyzes Pi from glucose 6- phosphate, and free _____ is released into the blood:
– Not a reversal of the _____ reaction.
– _____ is not produced when the phosphate is removed; Pi is released by _____
glucose
glucokinase
ATP
hydrolysis
Glycolysis vs. Gluconeogenesis
Glycolysis accomplishes a negative ΔG while yielding reducing equivalents (2 _____) and 2 net _____.
• Gluconeogenesis requires the use of 4 _____ and 2 _____ to achieve its negative ΔG.
NADH
ATP
ATP
GTP
Gluconeogenesis and Glycolysis: a Futile Cycle?
Glycolysis and Gluconeogenesis both achieve _____, therefore both are _____.
If both pathways were simultaneously active in a cell, it would constitute a _____ that would waste energy.
An apparent “futile cycle” is actually the site of a finely _____ mechanism.
negative deltaG
spontaneous
“futile cycle”
regulated
Reciprocal Regulation of Gluconeogenesis and Glycolysis
- To prevent the waste of a tuile cycle, glycolysis and gluconeogenesis are _____
- reciprocal allosteric regulation by _____:
- -Phosphofructokinase (glycolysis)
- –_____ by ATP and _____ by AMP
- -Fructose-1,6-bisphosphatase (gluconeogenesis):
- –_____ by AMP
- high cellular ATP/AMP:
- -glucose is not _____ to make ATP
- low ATP/AMP:
- -the cell does not expend _____ in the synthesis of glucose
reciprocally regulated adenine nucleotides inhibited stimulated inhibited degraded energy
first mechanism that regulates both enzymes involved in the negative free energy = _____
this is considered to be a _____ regulation
allosteric regulation by adenine nucleotides
“local”
Reciprocal regulation of Gluconeogenesis and Glycolysis
• Systemic regulation in liver cells by the cAMP cascade:
• Makes glucose available for release to the bloodstream:
– Inhibition of _____.
– Stimulation of _____.
Triggered by low _____.
Mediated by _____.
_____ of enzymes and regulatory proteins by Protein Kinase A (cAMP-
–Dependent Protein Kinase): Pyruvate Kinase:
Glycolysis enzyme that is _____ when phosphorylated.
– CREB (cAMP response element-binding protein):
Activates transcription of the _____ gene, leading to increased _____.
– Phosphofructokinase 2 makes and degrades an allosteric regulator, _____.
glycolysis gluconeogenesis blood glucose glucagon phosphorylation inhibited PEP carboxykinase gluconeogenesis fructose-2,6-bisphosphate
Reciprocal regulation of Gluconeogenesis and Glycolysis
Reciprocal regulation by fructose-2,6-bisphosphate (F2,6P):
F2,6P stimulates _____.
F2,6P allosterically _____ the Glycolysis enzyme PFK-1 (see CHO lecture 1, slide 47).
F2,6P activates the transcription of the _____ gene, the liver variant of Hexokinase that phosphorylates Glc to G6P, the input to _____.
F2,6P allosterically inhibits the gluconeogenesis enzyme _____.
glycolysis activates glucokinase glycolysis fructose-1,6-bisphosphatase
Reciprocal regulation of Gluconeogenesis and Glycolysis
Summary of effects of the glucagon-cAMP cascade in the liver:
Gluconeogenesis is _____.
Glycolysis is _____.
Glycogen breakdown is _____.
Glycogen synthesis is _____.
_____ is formed for release to the blood.
stimulated inhibited stimulated inhibited free glucose
Physiological Significance of Gluconeogenesis
• Failure of gluconeogenesis:
– Usually _____.
– Hypoglycemia leads to _____ and coma.
– Glucose is also necessary to maintain TCA cycle intermediates for _____.
• Excessive gluconeogenesis:
– May lead to _____ in critically ill patients.
– Due (in part) to excess of the stress hormone _____.
• Energy cost:
– _____ on very low carbohydrate diets.
– The continual demand for glucose leads to gluconeogenesis from _____.
– ATP required for gluconeogenesis is provided by increased oxidation of _____.
fatal
brain dysfunction
fat metabolism
hyperglycemia
cortisol
weight loss
amino acids
fatty acids
Part II: The Tricarboxylic Acid Cycle (TCA; Krebs Cycle; Citric Acid Cycle)
• _____ of proteins, fats, and carbohydrates.
• Two stages of cellular respiration.
– Stage 1: Oxidation of fuels.
— Stage 1a: oxidation to _____.
— Stage 1b: oxidation of acetyl groups to
_____ with energy release as reduced
electron carriers _____ and _____.
• Stage 2: ATP generation from electrons
carried by NADH and FADH2 to reduce _____ to _____.
catabolism acetyl CoA CO2 NADH FADH2
O2
H2O
Alternative Fates of Pyruvate and Regeneration of NAD+
A. Pyruvate is metabolized in the _____, and the reducing power of NADH is used to synthesize ATP in the _____.
B. Fermentation to lactate in vigorously contracting muscle, in erythrocytes, in some other cells, and in some microorganisms: _____ and _____ form lactate, regenerating _____.
C. In yeast and other microorganisms, pyruvate is converted to _____.
TCA (Krebs) cycle
mitochondria
NADH
pyruvate
NAD+
ethanol
The TCA and oxidative phosphorylation take place in mitochondria
Glycolysis occurs in the _____
Formation of acetyl COA and the TCA cycle > both take place in the _____ (not associated with the membrane; _____ proteins)
cell
matrix of the mitochondria
soluble
TCA cycle: Important points
• Function: to fully \_\_\_\_\_ derived from carbohydrates, fats and proteins to CO2. • Carbon atoms delivered to the TCA cycle in the form of \_\_\_\_\_. • This oxidation produces: \_\_\_\_\_: • GTP • NADH • FADH2 \_\_\_\_\_ used for biosynthetic processes, e.g. fatty acids, amino acids • \_\_\_\_\_
The TCA is an _____ pathway in _____.
The cycle is regulated by the state of cellular energetics and _____.
TCA cycle intermediates must be _____ to continue the cycle for energy production, as well as to provide substrates for biosynthetic pathways.
oxidize carbon atoms acetyl CoA high-energy molecules metabolic intermediates CO2
aerobic energy-producing
mitochondria
O2 availability
maintained
Formation of Acetyl-Coenzyme A from Pyruvate
Pyruvate:
- Transported into _____ by a transporter
- -Decarboxylated to acetyle-CoA by the _____
Pyruvate dehydrogenase:
- multi-enzyme complex containing multiple copies of _____ enzymes
- several coenzymes:
- – _____
- – coenzyme A (pantothenic acid)
- – _____
- – thiamine pyrophosphate (thiamine)
The release of _____ provides a powerful driving force for the reaction.
mitochondria
pyruvate dehydrogenase complex
three
NAD+ (niacin)
lipoamide FAD (riboflavin)
CO2
Coenzyme A has a _____; the vitamin _____ is bonded to the nt; linker region: _____ (in reduced form ends in a thiol group)
in blue is the acetyl part (binds to the _____ group); everything else remains the same (vitamin b5 and the nt.)
nucleotide portion
B5
beta-mercapto ethylamine
thiol
Conversion of pyruvate to acetyl-CoA: a key branch point of metabolism
Product inhibition by Acetyl CoA and NADH: when these products _____, decarboxylation of pyruvate is _____
The reaction is sensitive to the energy charge:
- _____ by ATP, acetyl-CoA, NADH, fatty acids
- _____ by AMP, CoA, NAD+, Ca2+
accumulate
blocked
inhibited
stimulated
Step 1: Condensation of Acetyl-CoA with oxaloacetate (OAA) to form _____
- Mediated by the enzyme _____.
- This reaction is inhibited by _____.
- Citrate is used in synthesis of _____ and _____.
citrate citrate synthase ATP cholesterol FA
Step 2: Citrate is isomerized to _____.
isocitrate
Step 3: Oxidation and decarboxylation of isocitrate to form _____
- _____ and _____ are produced.
- The reaction is inhibited by _____ and stimulated by _____.
- α-KG is a precursor for glutamate, glutamine, purines.
alpha-ketoglutarate NADH CO2 ATP ADP
Step 4: α-KG is converted to _____
• _____ and _____ are produced.
• Mediated by a dehydrogenase: _____.
• Succinyl-CoA and NADH _____ the reaction.
• Succinyl-CoA is a precursor of _____ and chlorophyll biosynthesis
succinyl-CoA NADH CO2 alpha-KG dehydrogenase inhibited heme
Step 5: Conversion of succinyl-CoA to _____
• Mediated by the _____ enzyme.
• GDP is phosphorylated to form _____.
• _____ – direct transfer of Pi to GDP.
Succinyl-CoA + Pi + GDP –> Succinate + _____ + CoA-SH
succinate succinate thiokinase GTP substrate level phosphorylation GTP
Step 6: Succinate is oxidized to form _____
• _____ is produced.
• Mediated by the _____.
fumarate
FADH2
succinate dehydrogenase
Step 7: fumarate is hydrated to form _____
Step 8: Malate is oxidized to regenerate _____; one more _____ is produced
malate
OAA
NADH
Regulation of the TCA cycle
- In general, the pathway will be inhibited when _____ levels are high
- Conversely, the pathway will be stimulated when _____ levels are low and _____ levels are high
ATP
ATP
AMP/ADP
Overall stoichiometry of the TCA cycle
AcetylCoA + 3 NAD + + FAD + GDP + Pi + 2 H2O –> 2 CO2 + 3 NADH + 3 H + + FADH2 + GTP + CoA-SH
- 2 _____ are formed for each Acetyl CoA used.
- 1 _____ is produced by substrate-level phosphorylation.
- 3 _____ and 1 _____ carry high-energy electrons which will enter the _____ to produce ATP.
CO2 GTP NADH FADH2 electron transport chain
TCA Cycle and Gluconeogenesis
- Gluconeogenesis enzyme _____ (pyruvate + CO2 -> OAA):
- Allosterically _____ by acetyl-CoA.
• [OAA] tends to be _____ for the TCA cycle.
• When gluconeogenesis is active in liver, OAA is
diverted to form _____.
• OAA depletion hinders acetyl-CoA _____ into the
TCA cycle.
• The increase in _____ activates Pyruvate
Carboxylase to make OAA.
pyruvate carboxylase activated limiting glucose entry [acetyl-CoA]
Summary of the TCA Cycle
• _____, energy-producing pathway in _____.
• Produces 1 _____, plus 3 _____ and 1 _____ which provide electrons for
electron transport chain.
• Regulated by the state of _____ and O2 availability:
— the end products ATP and NADH are _____, and the substrates NAD+ and ADP are _____.
- The production of acetyl-CoA is _____ allosterically by metabolites that signal a sufficiency of metabolic energy (ATP, acetyl-CoA, NADH, and fatty acids) and _____ by metabolites that indicate a reduced energy supply (AMP, NAD, CoA).
- Cycle intermediates must be _____.
aerobic mitochondria GTP NADH FADH2 cellular energetics inhibitory stimulatory
inhibited
stimulated
maintained
Part III: Energetics and Oxidative Phosphorylation
• Oxidative phosphorylation couples oxidation of _____ to _____.
• Coupling through the production of a _____ across the _____ mitochondrial membrane.
• The rate of oxidative phosphorylation is tightly controlled by the level of _____ and the availability of _____.
reduced coenzymes
ATP formation
proton gradient
inner
ADP
oxygen
The oxidative phosphorylation system is located on the _____ of the mitochondrion
• The electron transport chain, a series of enzymes, coenzymes and electron transport proteins, are embedded in the _____.
• The apparatus is organized _____ to facilitate electron transfer and ATP synthesis.
TCA = \_\_\_\_\_ in matrix ETC = enzymes associated with the \_\_\_\_\_ (ATP synthase)
inner membrane membrane spatially soluble inner mitochondrial membrane
Electron Transport Chain: The Chemiosmotic Theory
- Chemiosmotic theory: the energy of electron flow is conserved by the pumping of _____ across the membrane, producing an _____ gradient, the _____.
- Electrons are transferred along the chain.
- Electron transfer is coupled to transport of protons into the mitochondrial _____ space.
- Proton gradient provides energy for _____.
protons electrochemical gradient proton-motive force intermembrane ATP synthesis
energy that is transmitted by the redox reactions (comes from the oxidation of those reduced nt’s)
everytime there is a _____ > transports H+ from _____ into intermembrane space
everytime a _____ of e- goes through the entire chain (xfer of _____ H+) > chemiosmotic theory
redox rxn
matrix
pair
3
Electron movement through the electron transport chain
Builds up a high concentration of H+ in _____. When electrons enter _____, they combine with
oxygen to form water.
_____ and _____ are involved with reducing the three complexes
intermembrane space
cytochrome oxidase
NADH
FADH2
Electron movement through the electron transport chain
• Redox potential scale:
— Electrons pass from carriers with a _____ reduction potential to those with a _____ (more positive)
potential.
• Free-energy scale:
— _____ in free energy as a pair of electrons moves through the chain.
• The energy released as electrons flow through three of the complexes is sufficient to power the pumping of H+ ions across the membrane, establishing a _____.
lower
higher
reduction
proton-motive force
Ubiquinone (Coenzyme Q10) accepts _____ electrons and _____ protons
• Complete reduction of ubiquinone requires _____ electrons and _____ protons, and occurs in _____ steps.
two two two two two
Cytochrome oxidase accepts electrons from _____ and forms H2O
• Additional site of coupling to proton pump.
• _____ of O2 uptake in humans goes to this complex.
• _____ and _____ binding to complex stops electron transport.
cytochrome c
90%
cyanide
azide
Transfer of protons to intermembrane space creates an electrochemical proton gradient
• Proton pumping across the membrane:
• Increases [H+] outside the membrane (pH
gradient).
• Creates a difference in _____ potential
(+ outside, - inside) across the membrane.
• Protons will follow the electrochemical
gradient from _____ to the _____.
• ATP synthase uses the energy of the
_____ (proton-motive force) to produce ATP.
There is both an electrical gradient created in addition to a _____ gradient
electrical outside inside electrochemical gradient concentration
Electrochemical proton gradient across inner mitochondrial membrane allows ATP synthase to generate ATP
ATP synthase creates a _____ pathway across the membrane, allowing protons to flow down their electrochemical gradient.
hydrophilic
ATP synthase
- The F0 portion:
- _____: channel for H+ flow.
- Stalk: _____, driven by the H+ gradient.
- The F1 portion: _____.
- ATP synthase is an energy-generating _____.
transmembrane proton carrier
rotor
ATP synthase
molecular motor
ATP Synthase
enzyme allows H+ to cross membrane; the F1 part of the enzyme turns _____ each time one H+ crosses the hydrophilic channel
difference in gradient/charge/cxn > xformed into _____ energy, but physically turning F1 _____
require _____ H+ ions for a complete turn > only _____ can synthesize a molecule of ATP
120 degrees mechanical 120 degrees 3 one complete turn
The ATP synthase reaction is _____
• If the electrochemical proton gradient drops, there will not be sufficient energy to drive ATP production.
• ATP will by hydrolyzed and the concentration of protons inside and outside the membrane will reach
_____.
reversible
equilibrium
Uncoupling agents insert into the _____ membrane, making it _____ to protons
• Uncoupling agents:
• Allow H+ to flow into matrix without passing through the _____.
• Uncouples electron transport from _____.
- A number of poisons are uncoupling agents (e.g. _____, once used as a weight-loss drug).
- _____: 20% of energy is dissipated.
inner permeable ATP synthase ATP synthesis dinitrophenol basal H+ leak
Uncoupling Proteins and Thermogenesis
• Naturally occurring proteins (uncoupling proteins) allow H+ to flow into matrix bypassing the _____.
• The energy of the electrochemical gradient is dissipated as _____.
• In response to cold, increased by _____ and _____ to generate heat.
- Brown adipose tissue is specialized for this process of _____.
- Tissues containing brown fat serve as _____.
ATP heat thyroid hormone epinephrine non-shivering themogenesis "biological heating pads"
Quantitation of Energy Yields
Stoichiometry
• The quantitative relationship between NADH oxidation, H+ pumping and ATP synthesis varies with conditions.
• We will use the following:
For each intramitochondrial:
NADH ==> _____ ATP FADH2 ==> _____ ATP
• The efficiency with which oxidation energy of carbohydrates is converted into ATP bond energy is often greater than _____.
• This is considerably greater than the efficiency of _____ (e.g. internal combustion) _____ (10-20%).
2.5
1.5
40%
thermal
engines
Final ATP Yield per molecule of glucose: _____
30 ATP
Summary
• Energy production is THE _____ biological process.
• Aerobic organisms “burn” foodstuffs to produce _____ and _____ (NADH
and FADH2).
- Oxidative phosphorylation is the mechanism for _____ oxidation of reduced coenzymes to ATP formation.
- Coupling is accomplished through the production of a _____ across the inner mitochondrial membrane.
- The rate of oxidative phosphorylation is tightly controlled by the availability of _____ and _____.
essential
ATP
reduced pyridine nucleotides
coupling
proton gradient
ADP
oxygen
