Download Keigo Tanaka Chapter 9 – Cellular Respiration: Harvesting

Keigo Tanaka
Chapter 9 –
Cellular Respiration: Harvesting Chemical Energy
Catabolic pathway
Fermentation
Cellular Respiration
•
C6H12O6 + 6O2 ---> 6CO2 + 6H2O + Energy (ATP + heat)
Redox reactions
•
Oxidation-reduction
•
Adding e- reduces + charge
•
Reduction is e- gain
•
Oxidation is e- loss
•
Reducing agent:
e- donor
•
Oxidizing agent:
e- acceptor
•
Energy is released because the e- lose potential energy when they end up closer to an
electronegative atom.
Oxidizing agent in respiration
•
NAD+ (nicotinamide adenine dinucleotide)
•
Removes electrons from food (series of reactions)
•
NAD + is reduced to NADH
•
Enzyme action: dehydrogenase
•
Oxygen is the eventual e- acceptor
Electron transport chains
•
Electron carrier molecules (membrane proteins)
•
Shuttles electrons that release energy used to make ATP
•
Sequence of reactions that prevents energy release in 1 explosive step
•
Electron route: food---> NADH ---> electron transport chain ---> oxygen
Keigo Tanaka
Cellular respiration
Respiration is a cumulative function of three metabolic stages:
1. Glycolysis: cytosol; degrades glucose into pyruvate (see pg 162-3)
2. Kreb’s Cycle: mitochondrial matrix; pyruvate into carbon dioxide (see pg 165)
3. Electron Transport Chain: inner membrane of mitochondrion; electrons passed to oxygen (see
pg 166 )
Glycolysis (Summary)
•
1 Glucose ---> 2 pyruvate molecules
•
Energy investment phase: cell uses ATP to phosphorylate fuel
•
Energy payoff phase: ATP is produced by substrate-level phosphorylation and NAD+ is reduced
to NADH by food oxidation
•
Net energy yield per glucose molecule: 2 ATP plus 2 NADH; no CO2 is released; occurs
aerobically or anaerobically
•
Glycolysis generates ATP by substrate-level phosphorylation.
•
Energy investment phase: cell uses ATP to phosphorylate fuel
•
Energy payoff phase: ATP is produced by substrate-level phosphorylation and NAD+ is reduced
to NADH by food oxidation
•
Net energy yield per glucose molecule: 2 ATP plus 2 NADH; no CO2 is released; occurs
aerobically or anaerobically
Glycolysis in Detail (Energy Investment Phase)
1. Hexokinase transfers a phosphate from ATP to glucose
2. Phosphoglucosoisomerase rearranges glucose-6-phosphate to convert it to its isomer fructose-6phosphate
3. Phosphofructokinase transfers a phosphate from ATP to glucose
4. Aldolase cleaves the sugar molecule into dihydroxyacetone phosphate and glyceraldehyde-3phosphate (isomers)
5. Isomerase converts dihydroxyacetone phosphate into glyceraldehyde-3-phosphate
Glycolysis in Detail (Energy Payoff Phase)
Keigo Tanaka
6. Triose phosphate dehydrogenase first oxidizes sugar by the transfer of e- and H+ to NAD+ and then it
uses the energy released to attach a phosphate group to the oxidized substrate
7. Phosphoglykerokinase the phosphate added to the substrate is transferred to ADP resulting in two
molecules of 3-phosphoglycerate
8. Phosphoglyceromutase relocates the remaining phosphate
9. Enolase causes a double bond to form in the substrate by extracting a water molecule to form
phosphoenolpyruvate (PEP)
10. Pyruvate Kinase transfers a phosphate from PEP to ADP producing pyruvate
Glycolysis to the Citric Acid (Krebs) Cycle
•
Before the citric acid cycle can begin Pyruvate must first be converted to acetyl CoA, which links
the cycle to glycolysis
1. Pyruvate’s fully oxidized carboxyl group (-COO-) is given off as a molecule of CO2
2. The remaining two carbon fragment is oxidized to form acetate (ionized form of acetic acid) and
the enzymes are transferred to NAD+, storing the energy in the form of NADH
3. Coenzyme A is attached to the acetate by an unstable bond to form Acetyl CoA
Kreb’s Cycle (Summary)
•
If molecular oxygen is present…….
•
Each pyruvate is converted into acetyl CoA (begin w/ 2):
-> NADH;
coenzyme A (from B vitamin),
•
From this point, each turn 2 C atoms enter (pyruvate) and 2 exit (carbon dioxide)
•
Oxaloacetate is regenerated (the “cycle”)
•
For each pyruvate that enters: 3 NAD+ reduced to NADH;
(riboflavin, B vitamin); 1 ATP molecule
CO2 is released;
NAD+ -makes molecule very reactive
1 FAD+ reduced to FADH2
Kreb’s Cycle in Detail
1. Acetyl CoA adds its two-carbon acetyl group to oxaloacetate, producing citrate
2. Citrate is converted to its isomer, isocitrate, by removal of one water and the addition of
another
3. Isocitrate loses a CO2 molecule and the resulting compound is oxidized, reducing NAD+ to NADH,
and forming alpha-ketogluterate
Keigo Tanaka
4. Another CO2 molecule is lost, and the resulting compound is oxidized, reducing NAD+ to NADH;
the remaining molecule is then attached to coenzyme A by an unstable bond forming succinyl
CoA
5. CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, and then to
ADP, forming ATP (substrate-level phosphorylation) and succinate
6. Two hydrogens are transferred to FAD, forming FADH2 and oxidizing succinate to fumarate
7. The addition of a water molecule rearranges bonds in the substrate forming malate
8. The substrate is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate so it can be
used in the cycle again
Electron transport chain
•
Cytochromes carry electron carrier molecules (NADH & FADH2) down to oxygen
•
Chemiosmosis:
•
ATP synthase:
produces ATP by using the H+ gradient (proton-motive force) pumped
into the inner membrane space from the electron transport chain; this enzyme harnesses the
flow of H+ back into the matrix to phosphorylate ADP to ATP (oxidative phosphorylation)
energy coupling mechanism
Review: Cellular Respiration
•
Glycolysis:
2 ATP (substrate-level phosphorylation)
•
Kreb’s Cycle:
2 ATP (substrate-level phosphorylation)
•
Electron transport & oxidative phosphorylation:
NADH (acetyl CoA) = 6ATP
6 NADH (Kreb’s) = 18 ATP
•
38 TOTAL ATP/glucose
2 NADH (glycolysis) = 6ATP
2 FADH2 (Kreb’s) = 4 ATP
2
Related metabolic processes
•
Fermentation:
1. Alcohol - pyruvate to ethanol
2. Lactic acid - pyruvate to lactate
•
Facultative anaerobes (yeast/bacteria)
•
Beta-oxidation: A metabolic sequence of lipid catabolism that breaks down fatty acids into twocarbon fragments that can enter the citric acid cycle as acetyl CoA
Keigo Tanaka
Fermentation
•
In the absence of oxygen, many cells use fermentation to produce ATP by substrate-level
phosphorylation. In fermentation, pyruvate serves as an electron acceptor for oxidizing NADH
back to NAD+, which can then be reused in glycolysis. The two common end products of
fermentation are ethanol and lactate (ionized form of lactic acid).
The catabolism of various foods
•
Monomers of many organic molecules can be used as fuel for cellular respiration.
Control of Cellular Respiration
•
Allosteric enzymes at certain points in the respiratory pathway respond to inhibitors and
activators that help set the pace of glycolysis and the citric acid cycle. Phosphofructokinase, the
enzyme that catalyzes step 3 of glycolysis is one such enzyme. It is stimulated by AMP (derived
from ADP) but is inhibited by ATP and by citrate. This feedback regulation adjusts the rate of
respiration as the cell’s catabolic and anabolic demands change.