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Metabolism: Glycolysis,
TCA Cycle, and the
ETC-Oxidative Phosphorylation
Metabolism
The sum of the chemical changes that convert
nutrients into energy and the chemically
complex products of cells
Hundreds of enzyme reactions organized into
discrete pathways
Substrates are transformed to products via many
specific intermediates
Metabolic maps portray the reactions
A Common Set of Pathways
Organisms show a marked similarity in their
major metabolic pathways
Evidence that all life descended from a common
ancestral form
There is also significant diversity
Autotrophs use CO2; Heterotrophs use organic
carbon; Phototrophs use light; Chemotrophs use
Glc, inorganics use S and obtain chem energy
through food generated by phototrophs.
The Sun is Energy for Life
Phototrophs use light to drive synthesis of
organic molecules
Heterotrophs use these as building blocks
CO2, O2, and H2O are recycled
Metabolism
Metabolism consists of catabolism and
anabolism
Catabolism: degradative pathways Usually energy-yielding!
“destructive metabolism”
FUELS -> -> CO2 + H2O + useful energy
Anabolism: biosynthetic pathways energy-requiring!
“constructive metabolism” Useful energy + small molecules --> complex molecules
Organization in Pathways
Pathways consist of sequential steps
The enzymes may be:
Separate
Form a multienzyme complex
A membrane-bound system
New research indicates that multienzyme
complexes are more common than once
thought
Catabolism and Anabolism
Catabolic pathways converge to a few end
products
Anabolic pathways diverge to synthesize many
biomolecules
Some pathways serve both in catabolism and
anabolism and are called amphibolic pathways
Digestion of food polymers: enzyme-catalyzed hydrolysis
Glycolysis: glucose catabolism generate ATP without consuming oxygen (anaerobic)
Citric Acid Cycle: metabolism of acetyl-CoA derived from pyruvate, fatty acids, and amino acids acetyl oxidized to CO2 operates under aerobic conditions reduction of coenzymes NAD+ and FAD; energy used to produce ATP
Oxidative phosphorylation: reduction of molecular oxygen by NADH and FADH2 energy of reduced compounds used to pump protons across a cell membrane potential energy of electrochemical gradient drives phosphorylation of ADP to ATP
Comparing Pathways
Anabolic & catabolic pathways involving
the same product are not the same
Some steps may be common to both
Others must be different to ensure that
each pathway is spontaneous
This also allows regulation mechanisms to
turn one pathway and the other off
METABOLIC REGULATION
Regulated by controlling:
1. Amounts of enzymes
2. Catalytic activities
3. Accessibility of substrates
The ATP Cycle
ATP is the energy currency of cells
In phototrophs, light energy is transformed into
the chemical energy of ATP
In heterotrophs, catabolism produces ATP,
which drives activities of cells
ATP cycle carries energy from photosynthesis or
catabolism to the energy-requiring processes of
cells
WHY ATP?
Free energy is released when ATP is
hydrolyzed.
This energy drives reactions that need it
(eg. muscle contraction)
Recall coupled reactions
ATP has a higher phosphoryl transfer
potential
Redox in Metabolism
NAD+ collects electrons released in catabolism
Catabolism is oxidative - substrates lose
electrons, usually H- ions
Anabolism is reductive - NADPH provides the
electrons for anabolic processes, and the
substrates gain electrons
LEO - GER
RECURRING MOTIFS IN METAB
Certain compounds keep on recurring or
appearing in metabolic reactions and their
functions are the same in the processes
Metab looks complicated but reactions are
actually limited and repeating.
ACTIVATED CARRIERS
These species help carry out the
metabolic reactions, even nonfavorable
ones, at times
Example: ATP (activated carrier of
phosphoryl groups)
Activated carriers of electrons for fuel
oxidation: e- acceptors!
Aerobic systems: O2 is the final e-
acceptor, but this does not occur
directly
Fuels first transfer e- to carriers:
pyridine molecules or flavins.
NAD+:
nicotinamide
adenine
dinucleotide
Activated carriers of electrons for fuel
oxidation: e- acceptors!
FAD: Flavin
adenine
dinucleotide
Activated carrier of electrons for
reductive biosynthesis: e- donors!
NADPH: common
electron donor
R is phosphate
group
Activated carrier of two-carbon
fragments
COENZYME A: carrier of acyl groups
Activated carrier of two-carbon
fragments
VITAMINS
Many vitamins are "coenzymes" -
molecules that bring unusual chemistry to
the enzyme active site
Vitamins and coenzymes are classified as
"water-soluble" and "fat-soluble"
The water-soluble coenzymes exhibit the
most interesting chemistry
Key Reactions in Metabolism
1. REDOX reactions
Electron carriers are needed!
2. LIGATION reactions
Bond formation facilitated by ATP cleavage
3. ISOMERIZATION reactions
4.GROUP TRANSFER
5.HYDROLYTIC reactions
Bond cleavage by addition of H2O
6.ADDITION of functional groups to
double bonds or REMOVAL of
groups to form double bonds
Uses lyases
GLYCOLYSIS
Glycolysis
1897: Hans and Eduard Buchner (Sucrose
cell-free experiments; fermentation can
take place outside of living cells)
METABOLISM became simple chemistry
Glycolysis: “Embden-Meyerhof pathway”
The all-important Glucose
The only fuel the brain uses in non-
starvation conditions
The only fuel red blood cells can use
WHY? Evolutionary: probably available for primitive
systems
The products and their fates
AKA Embden-Meyerhof-Parnas Pathway
Involves the oxidation of glucose
Products:
2 Pyruvate
2 ATP
2 NADH
Cytosolic
Glycolysis
Glycolysis
Anaerobic
The entire
process does not
require O2
Glycolysis: General Functions
Provide energy in the form of ATP
Generate intermediates for other pathways: Hexose monophosphate pathway
Glycogen synthesis
Pyruvate dehydrogenase Fatty acid synthesis
Krebs’ Cycle
Glycerol-phosphate (TG synthesis)
Specific functions of glycolysis
Red blood cells (RBCs) Rely exclusively for energy
Skeletal muscle Source of energy during exercise, particularly high
intensity exercise
Adipose tissue Source of glycerol-P for TG synthesis
Source of acetyl-CoA for FA synthesis
Liver Source of acetyl-CoA for FA synthesis
Source of glycerol-P for TG synthesis
Regulation of Cellular Glucose Uptake
Brain & RBC: The GLUT-1 transporter has high affinity for glucose
and is always saturated. Ensures that brain and RBC always have glucose.
Liver: The GLUT-2 glucose transporter has low affinity and
high capacity. Uses glucose when fed at rate proportional to glucose
concentration
Muscle & Adipose: The GLUT-4 transporter is sensitive to insulin
Glucose Utilization
Phosphorylation of glucose
Commits glucose for use by that cell
Energy consuming
Hexokinase: muscle and other tissues
Glucokinase: liver
Regulation of Cellular Glucose
Utilization in the Liver Feeding
Blood glucose concentration high
GLUT-2 taking up glucose
Glucokinase induced by insulin
High cell glucose allows GK to phosphorylate glucose for use by liver
Post-absorptive state Blood & cell glucose low
GLUT-2 not taking up glucose
Glucokinase not phophorylating glucose
Liver not utilizing glucose during post-absorptive state
Regulation of Cellular Glucose
Utilization in the Liver
Starvation
Blood & cell glucose concentration low
GLUT-2 not taking up glucose
GK synthesis repressed
Glucose not used by liver during starvation
Regulation of Cellular Glucose
Utilization in the Muscle
Feeding and at rest High blood glucose, high insulin
GLUT-4 taking up glucose
HK phosphorylating glucose
If glycogen stores are filled, high G6P inhibits HK, decreasing glucose utilization
Starving and at rest Low blood glucose, low insulin
GLUT-4 activity low
HK constitutive
If glycogen stores are filled, high G6P inhibits HK, decreasing glucose utilization
Regulation of Cellular Glucose
Utilization in the Muscle
Exercising Muscle (fed or starved)
Low G6P (being used in glycolysis)
No inhibition of HK
High glycolysis from glycogen or blood
glucose
Regulation of Glycolysis
Regulation of 3 irreversible steps
PFK-1 is rate limiting enzyme and
primary site of regulation.
Regulation of Glycolysis
Most important regulation hub!
Regulation of PFK-1 in Muscle
Allosterically stimulated by AMP High glycolysis during exercise
Allosterically inhibited by ATP
High energy, resting or low exercise
Citrate Build up from Krebs’ cycle
May be from high FA beta-oxidation -> hi acetyl-CoA
Energy needs low and met by fat oxidation
Regulation of PFK-1 in Liver
Inducible enzyme
Induced in feeding by insulin
Repressed in starvation by glucagon
Allosteric regulation
Like muscle w/ AMP, ATP, Citrate
Activated by Fructose-2,6-bisphosphate
Fermentation
Anaerobic respiration!
Produces ATP without oxygen.
No ETC is present since there is no oxygen
NAD+ gets recycled by use of an organic hydrogen acceptor like lactate or ethanol.
Common in prokaryotes and very useful to humans.
Fermentation
Two type lactic acid and alcohol fermentation.
A build up of lactate in your muscles from over exerting yourself and not taking in enough oxygen causes soreness.
Alcohol fermentation has a by product of CO2 and ethanol which is used to make alcoholic beverages. Yeast and fungus go through alcohol fermentation.
The release of CO2 by yeast is what causes bread to rise.
Alcohol Fermentation pyruvate is
converted to
ethanol in two
steps.
Alcohol
fermentation
by yeast is
used in
brewing and
winemaking.
Lactic Acid Fermentation pyruvate is reduced directly
by NADH to form lactate
Lactic acid fermentation by
some fungi and bacteria is
used to make cheese and
yogurt
The waste product,
lactate, may cause
muscle fatigue, but
ultimately it is
converted back to
pyruvate in the liver.
The Tricarboxylic Acid (TCA) Cycle
Also known as the Krebs Cycle and Citric Acid
Cycle
The citric acid cycle is the final common
pathway for the oxidation of fuel molecules:
amino acids, fatty acids, & carbohydrates.
Most fuel molecules enter the cycle as acetyl
coenzyme A
This cycle is the central metabolic hub of the
cell
The Tricarboxylic Acid (TCA) Cycle
The citric acid cycle oxidizes two-carbon units
Entry to the cycle and metabolism through it
are controlled
It is the gateway to aerobic metabolism for any
molecule that can be transformed into an
acetyl group or dicarboxylic acid,
It is also an important source of precursors for
building blocks
Overview of the TCA Cycle 1. The function of the cycle is the harvesting of high-
energy electrons from carbon fuels
2. The cycle itself neither generates ATP nor includes O2
as a reactant
3. Instead, it removes electrons from acetyl CoA & uses
them to form NADH & FADH2 (high-energy electron
carriers)
4. In oxidative phosphorylation, electrons from NADH &
FADH2 flow through a series of membrane proteins
(electron transport chain) to generate a proton gradient
Overview of the TCA Cycle
5.These protons then flow back through ATP
synthase to generate ATP from ADP & inorganic
phosphate
6.O2 is the final electron acceptor at the end of the
electron transport chain
7.The citric acid cycle + oxidative phosphorylation
provide > 95% of energy used in human aerobic
cells
Fuel for the Citric Acid Cycle
Thioester bond to acetate
-mercapto-ethylamine
Pantothenate
69
Mitochondrion
Oxidative decarboxilation of pyruvate, & citric acid cycle take place in the matrix, along with fatty acid oxidation
Site of oxidative phosphorylation
Permeable
Mitochondrion
TCA Cycle: Overview
Input: 2-carbon units
in the form of Acetyl-
CoA
Output: 2 CO2, 1 GTP,
& 8 high-energy
Electrons in the form
of reducing elements
Cellular Respiration
8 high-energy electrons from carbon fuels
Electrons reduce O2 to generate a proton gradient
ATP synthesized from proton gradient
Acetyl-CoA: Link between glycolysis and TCA
Acetyl CoA is the fuel for the citric acid cycle
Pyruvate Dehydrogenase:
AKA PDH
The enzyme that links glycolysis with other pathways
Pyruvate + CoA + NAD -> AcetylCoA + CO2 + NADH
The PDH Complex
Multi-enzyme complex Three enzymes
5 co-enzymes
Allows for efficient direct transfer of product from
one enzyme to the next
The PDH Reaction E1: pyruvate dehydrogenase
Oxidative decarboxylation of pyruvate
E2: dihydrolipoyl transacetylase Transfers acetyl group from TPP to lipoic acid
E3: dihydrolipoyl dehydrogenase Transfers acetly group to CoA, transfers electrons from reduced
lipoic acid to produce NADH
Regulation of PDH
Muscle
Resting (don’t need)
Hi energy state
Hi NADH & AcCoA
Inactivates PDH
Hi ATP & NADH & AcCoA
Inhibits PDH
Exercising (need)
Low NADH, ATP, AcCoA
Coenzymes
Vitamin B1
FAD
FAD FADH2
NAD
Step 1: Citrate formation
Enzyme: Citrate synthase
Condensation reaction Hydrolysis reaction
Dehydration Hydration
Step 2: Isomerization of citrate to isocitrate
Enzyme: Aconitase
1st NADH produced! 1st CO2 removed
Step 3: Isocitrate to α-ketoglutarate
Enzyme: Isocitrate dehydrogenase
2nd NADH produced! 2nd CO2 removed!
Step 4: Succinyl-CoA formation
Enzyme: α-ketoglutarate dehydrogenase
GTP produced • Equivalent to ATP! • GTP + ADP GDP + ATP
Step 5: Succinate formation
Enzyme: Succinyl CoA synthetase
FADH2 produced!
Step 6: Succinate to Fumarate
Enzyme: Succinate dehydrogenase
Step 7: Fumarate to Malate
Enzyme: Fumarase
3rd NADH produced
Step 8: Malate to Oxaloacetate
Enzyme: Malate dehydrogenase
The TCA Cycle
Summary of the Reactions in TCA
Regulated primarily by
ATP & NADH concentrations
control points:
Pyruvate
dehydrogenase
isocitrate
dehydrogenase
- ketoglutarate
dehydrogenase
Control of the TCA Cycle
Biosynthetic roles of the TCA cycle
OXIDATIVE PHOSPHORYLATION
2006-2007
What’s the point?
The point is to make
ATP!
ATP
ATP accounting so far…
Glycolysis 2 ATP
Kreb’s cycle 2 ATP
Life takes a lot of energy to run, need to
extract more energy than 4 ATP!
What’s the point?
A working muscle recycles over
10 million ATPs per second
There is a better way!
Electron Transport Chain
series of molecules built into inner mitochondrial
membrane
along cristae
transport proteins & enzymes
transport of electrons down ETC linked to
pumping of H+ to create H+ gradient
yields ~30-32 ATP from 1 glucose!
only in presence of O2 (aerobic respiration)
O2 That sounds more like it!
Mitochondria
Double membrane
outer membrane
inner membrane
highly folded cristae
enzymes & transport
proteins
intermembrane space
fluid-filled space between
membranes
Oooooh! Form fits function!
Electron Transport Chain
Intermembrane space
Mitochondrial matrix
Q
C
NADH dehydrogenase
cytochrome bc complex
cytochrome c oxidase complex
Inner
mitochondrial
membrane
G3P Glycolysis Krebs cycle
8 NADH 2 FADH2
Remember the Electron Carriers?
4 NADH
Time to break open the bank!
glucose
Electron Transport Chain
Intermembrane space
Mitochondrial matrix
Q
C
NADH dehydrogenase
cytochrome bc complex
cytochrome c oxidase complex
Inner
mitochondrial
membrane
But what “pulls” the electrons down the ETC?
electrons flow downhill to O2 oxidative phosphorylation!
O2
Electrons flow downhill
Electrons move in steps from
carrier to carrier downhill to O2
each carrier more electronegative
controlled oxidation
controlled release of energy
make ATP instead of fire!
H+
ADP + Pi
H+ H+
H+
H+ H+
H+ H+ H+ We did it!
Set up a H+
gradient
Allow the protons
to flow through
ATP synthase
Synthesizes ATP
ADP + Pi ATP
ATP Are we there yet?
“proton-motive” force
Chemiosmosis
The diffusion of ions across a membrane
build up of proton gradient just so H+ could flow
through ATP synthase enzyme to build ATP
Chemiosmosis links the Electron Transport Chain to ATP synthesis
So that’s the point!
Peter Mitchell
Proposed chemiosmotic hypothesis
revolutionary idea at the time
1920-1992
proton motive force
True story.
H+
H+
O2 +
Q C
32 ATP 2
Pyruvate from
cytoplasm
Electron
transport
system
ATP synthase
H2O
CO2
Krebs
cycle
Intermembrane
space Inner
mitochondrial
membrane
1. Electrons are harvested and carried to the transport system.
2. Electrons provide energy to pump protons across the membrane.
3. Oxygen joins with protons to form water.
2H+
NADH
NADH
Acetyl-CoA
FADH2
ATP
4. Protons diffuse back in down their concentration gradient, driving the synthesis of ATP.
Mitochondrial
matrix
2 1
H+
H+
O2
H+
e-
e-
e-
e-
Cellular respiration
2 ATP ~2 ATP 2 ATP ~34 ATP + + +
Pathway Substrate-Level
Phosphorylation
Oxidative
Phosphorylation
Total
ATP
Glycolysis 2 ATP 2 NADH = 4 - 6
ATP 6 - 8
CoA 2 NADH = 6 ATP 6
Krebs Cycle 2 ATP
6 NADH = 18
ATP
2 FADH2 = 4 ATP
24
TOTAL 4 ATP 32 ATP 36 - 38
Cellular respiration
Summary of cellular respiration
(carbohydrate metabolism)
Oxidative phosphorylation is the process
of making ATP from the reducing elements
NADH and FADH2, with the help of O2 and
the electron transport chain
The electron transport chain is the
structural complex that enables oxidative
phosphorylation to take place
Summary of cellular respiration
Where did the glucose come from?
Where did the O2 come from?
Where did the CO2 come from?
Where did the CO2 go?
Where did the H2O come from?
Where did the ATP come from?
What else is produced that is not listed in this equation?
Why do we breathe?
C6H12O6 6O2 6CO2 6H2O ~40 ATP + + +
ETC backs up
nothing to pull electrons down chain
NADH & FADH2 can’t unload H
ATP production ceases
cells run out of energy
and you die!
Taking it beyond…
What is the final electron acceptor in
Electron Transport Chain?
O2
So what happens if O2 unavailable?
WHOA!