Download Substrate Level Phosphorylation Substrate level phosphorylation

Chapter 9: Cellular Respiration
Energy
Transfer
In Life
Reaction Coupling
•Catabolic reactions: reactants act as “fuels,” broken down with the
help of enzymes
•Fermentation: sugar degradation without oxygen (anaerobic)
•Cellular Respiration: most efficient and prevalent means of
respiration
•Energy released from respiration can be used for cellular work
(endergonic reactions) or as heat
•Ex: ATP regeneration from ADP + Pi
•Glucose (C6H12O6) – ΔG = - 686 kcal/mol
•spontaneous
•ADP + Pi + 7.3 kcal/mol
•C6H12O6 + 6 O2
ATP
6 CO2 + 6 H2O + Energy (ATP + heat)
Oxidation/Reduction Reactions (Redox)
•The relocation of e- releases E from organic molecules.
•Loss of is e- oxidation.
•Gain of is e- reduction.
LEO the lion goes GER!!!
• e- donor – reducing agent (is oxidized)
• e- acceptor – oxidizing agent (is reduced)
•Note: Redox can happen without a complete transfer of
electrons – can change e-
•Highly electronegative atoms are strong oxidizers (ex. Oxygen)
Redox Reactions
• The substance being reduced actually gets
“bigger” because the increased number of
electrons allows for more bonds
• Glucose oxidation transfers electrons (of
hydrogen) to a lower energy state as it bonds
with oxygen
– Energy released is used in ATP regeneration
Pulling e- away from an atom – requires energy
•This is the activation energy of cell respiration
e- lose energy when then move from a less electronegative atom to
a more electronegative atom (ex. Oxygen)
•This powers ATP regeneration, and creates water
•Hydrogen – low electronegativity
•Oxygen – high electronegativity
•Hydrocarbons – many uphill e- (high ∆G), such as found in our
foods, including lipids and carbohydrates
Hydrocarbons – many uphill e•Excellent fuel source – lots of e- to travel downhill – energy
released.
•Therefore, the oxidation of glucose is an exergonic
reaction
•The energy of
glucose’s electrons is
harvested in a series
of stepwise reactions
relying on NAD+ and
the electron tranport
chain
www.tva.gov
•Glucose is broken down in steps.
•Electrons are removed and transported with protons
•Both are carried by NAD+ - nicotinamide adenine dinucleotide
•NAD+ is an e- acceptor and a proton carrier
•Dehydrogenase: removes 2 hydrogen atoms from a substrate,
thereby oxidizing it
•e- transfer to O2 from NADH – ΔG = - 53 kcal/mol
•NADH holds stored energy that can be used in the future to power
the creation of ATP
Glucose
H+ and e-
NADH
ETC
Relies on proteins in
inner membrane of
mitochondria of
eukaryotes
OXYGEN
Final e- acceptor
(because oxygen is highly
electronegative)
The Stages of Cellular Respiration: A Preview
•Glycolysis
•Turns glucose into 2 pyruvate molecules
•No O2
•Occurs in the cytoplasm
•Relies on Substrate Level Phosphorylation
•Substrate level phosphorylation uses an enzyme to add a substrate’s
phosphate group to ADP
•Catabolic: -∆G
•Dehydrogenases and NAD+
•Citric Acid Cycle
•Uses oxygen
•Occurs in Mitochondrial Matrix
•Substrate Level Phosphorylation
•Catabolic
•Dehydrogenases and NAD+ used, transfer of e- to NAD+ making
NADH
•Oxidative Phosphorylation: accepts e- from NADH and passes them
through a chain of proteins
•Uses oxygen
•Mitochondrial Inner Membrane
•Includes the Electron Transport Chain and Chemiosmosis
•Anabolic – requires an energy input
•Proton Pump and ATP synthase
Glycolysis
•Hexose to Triose
•6C to 3C
•Oxidized to Pyruvate
•Energy Investment Phase
•Glucose becomes G3P,
requiring an input of 2
ATP
•Energy Payoff Phase
•G3P becomes pyruvate,
substrate level
phosphorylation occurs
twice, 2 ATP created
•2 net ATP
Energy Investment Phase
Step 1:
•Hexokinase
•Phosphate ‘traps’ glucose
•Increases reactivity
Step 2:
•Isomerases
Step 3:
•Activated for cleavage
•Phosphofructokinase (PFK) phosphorylates
glucose
•Allosterically regulated: PFK is inhibited by ATP
(ATP is an allosteric inhibitor)
Step 4:
•Cleavage
•Creation of Structural Isomers
Step 5:
•Isomerase
•Active molecule G-3-P
2 ATP have been used
Energy Payoff Phase
Step 6:
•G3P is oxidized
•Very exergonic
•Phosphorylation of oxidized sugar
Step 7:
•Substrate Level
phosphorylation
•Sugar oxidized to an organic
acid
Step 8:
•Phosphate relocated
Step 9:
•Dehydration reaction
•Creation of double bond
•Phosphate bond unstable
Step 10:
•Substrate level phosphorylation
•Net 2 ATP produced.
Glycolysis – a review
•ATP used: 2
•ATP produced: 4
•2 per G3P
•All via substrate level
phosphorylation
•NADH produced: 2
•1 per G3P
The Glycolysis/Citric Acid
Intermediate
•Oxygen Required
•Occurs in the Mitochondrial Matrix
•Active transport of pyruvate into matrix,
Pyruvate is converted to Acetyl Coenzyme A
Fully
oxidized
– very
little E
Very
Reactive
2-C molecule
Sulfur-containing
The Citric Acid Cycle
•Tricarboxylic Acid Cycle
•Krebs Cycle – Hans Krebs – 1930s
•8 Steps
•Specific enzymes
•Cycle – 2 time per glucose (1 time
per pyruvate)
•FAD – flavin adenine dinucleotide
•Electron carrier similar to
NAD+
Step 1:
•2-C + 4-C = 6-C
•Acetyl CoA + Oxaloacetate
•Coenzyme A recycled
Step 2:
•Isomerase
Step 3:
•CO2 released
•NAD+
NADH
Step 4:
•CO2 released
•NAD+
NADH
•Coenzyme A added
Step 5:
•Coenzyme A removed
•GDP
GTP
•Substrate-level
phosphorylation – ATP
Step 6:
•FAD
FADH2
Step 7:
•Hydration reaction
•Bond rearrangement
Step 8:
•OAA regenerated
•NAD+
NADH
The Citric Acid Cycle – A Review
•CO2
•ATP
•Per glucose: 6
•Per glucose: 2
•Per pyruvate: 3
•Per pyruvate: 1
•NADH
•Per glucose: 6
•Per pyruvate: 3
•FADH2
•Per glucose: 2
•Per pyruvate: 1
KREBS CYCLE ANIMATION
Pathway of the Electron Transport Chain
•Inner membrane of the mitochondria: contains a chain of several
complexes (some are protein, others are non-protein)
•Cristae
•4 protein components I- IV
•Prosthetic groups: non-protein components that help transport
e•e- carriers arranged in a ‘downhill’ formation via e- carriers such as
NADH and FADH2
•E- carriers alternate between reduced and oxidized forms
•NADH begins at Protein Complex I
•FADH2 begins at Protein Complex II
The Path of Electrons:
Protein Complex I
•Flavoprotein
•Flavin mononucleotide
•Iron-sulfide
Ubiquinone (Coenzyme Q)
•Non-protein prosthetic group
•Hydrophobic
•Mobile
Protein Complex II
•FAD
•Iron-sulfide
Protein Complex III
•Cytochrome b (heme)
•Iron-sulfide
•Cytochrome c1 (heme)
Cytochrome c
•Not in a protein – prosthetic
group
Protein Complex IV
•Cytochrome a (heme)
•Cytochrome a3 (heme)
Oxygen
•Final electron acceptor
The Electron Transport Chain
•Makes no ATP
•ΔG = -53 kcal/mol (exergonic – power the endergonic creation of
ATP)
•Proton gradient created
Chemiosmosis – Energy coupling
•Inner mitochondrial memebrane
•ATP synthase
•Reverse ion pump
•Endergonic reaction powered by ETC
•Relies on proton-motive force
•Bacteria: use gradient across cell
membrane
•Cells use chemiosmosis to generate
ATP, do active transport and rotate
flagella
•32-34 ATP produced
•Glucose → NADH → ETC → Proton
Motive Force → ATP
ETC animation
•Electron Shuttle (via active transport)
into Mitochondrion from Cytoplasm
varies with different cells
Accounting 101
•1 NADH = generates ~3 ATP
•10 H+ across membrane
•NAD+: liver cells
•3-4 H+ = 1 ATP
•FAD: brain cells
•1 FADH2 = generates ~ 2 ATP
•Total 36-38 ATP produced
•40% efficient (rest is lost as heat)
CYTOSOL
Electron shuttles
span membrane
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
2 NADH
Glycolysis
Glucose
2
Pyruvate
2
Acetyl
CoA
+ 2 ATP
by substrate-level
phosphorylation
Maximum per glucose:
6 NADH
Citric
acid
cycle
+ 2 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 32 or 34 ATP
by substrate-level by oxidative phosphorylation, depending
phosphorylation on which shuttle transports electrons
from NADH in cytosol
About
36 or 38 ATP
Fermentation
•No O2 – anaerobic
•Substrate-level phosphorylation
•As in glycolysis
•NAD+ is needed at start and
therefore must be regenerated
Alcoholic Fermentation
•Yeast – for use in brewing and baking
•Bacteria
Lactic Acid Fermentation
•Bacteria – used in making yogurt
•Fungi
•Muscle cells – use LAF when oxygen is scarce
•Liver recycles lactic acid back to lactate
•Former thinking: Lactic Acid build-up in muscle was the cause of
muscle cramping
•Facultative Anaerobes: aerobic or anerobic
•Obligate Anaerobes: cannot live in the presence of oxygen
Fermentation v. Aerobic Respiration
• Both use glycolysis to oxidize glucose
and other organic fuels to pyruvate
• Fermentation yields 2 ATP via
substrate level phosphorylation
– Aerobic respiration yields as much
as 38 ATP via oxidative
phosphorylation
• NAD+ is the oxidizing agent in
fermentation so oxygen is not
involved. In fermentation, the final
e- acceptor is pyruvate
– Aerobic respiration’s final eacceptor is oxygen
• In fermentation, the energy of
pyruvate is still unavailable to the cell
Glucose
Pyruvate
CYTOSOL
No O2 present
Fermentation
Ethanol
or
lactate
O2 present
Cellular respiration
MITOCHONDRION
Acetyl CoA
Citric
acid
cycle
Evolutionary Significance
•Glycolysis is performed by almost all living things
•Glycolysis does not require organelles
•Probably evolved in ancient prokaryotes before there was oxygen
in the atmosphere
•Oldest bacterial fossils date to 3.5 bya, while scientists believe
oxygen was not present until 2.7 bya
•Heterotroph Hypothesis
•Anaerobic Heterotrophs → Anaerobic Autotrophs
(cyanobacteria) → Aerobic Heterotrophs → Aerobic Autotrophs
Metabolic Pathways - Catabolism
•Glycolysis derives sugar from many sources
•Carbohydrates are digested into simple sugars
•Proteins: are digested into amino acids
•Deamination: removal of the amino group
from amino acids
•Removed amino acids eventually become
ammonia (then uric acid or urea)
•Fats
•Glycerol is converted into G-3-P
•Beta oxidation: changes fatty acids to 2-C
fragments which are then converted into
acetyl-CoA
•Hydrocarbons of fats are an excellent source of
fuel
•1 g of fat oxidized yields twice the ATP of a
carbohydrate
Metabolic Pathways – Anabolism
•Biosynthesis: food molecules are reused to make needed
molecules other than ATP
•Create ½ amino acids
•Nonessential amino acids are made in cells. Essential
amino acids must be obtained in the diet.
•Acetyl CoA is created from fatty acids
•Dihydroxacetone Phosphate – fat precursor for glycolysis
Regulation
•Feedback Inhibition
•Enzyme regulation
•Phosphofructokinase
•Allosteric: contains sites for
inhibitors and activators
•Inhibitors
•ATP
•Citrate: synchronized
the rate of the CAC and
glycolysis
•Activators
•AMP