Download File

Glycolysis
RESPIRATION (2009)
Glycolysis
Objective
Explain the stepwise breakdown of glucose in cellular Respiration
Function:
• Glycolysis is a partial breakdown of a six-carbon glucose molecule into two,
three-carbon molecules of pyruvate, 2NADH +, 2H+, and 2 net ATP as a result
of substrate-level phosphorylation
•
Glycolysis occurs in the cytoplasm of the cell.
The overall reaction is:
glucose (6C) + 2 NAD+ 2 ADP +2 inorganic phosphates (Pi) yields 2 pyruvate (3C)
+ 2 NADH + 2 H+ + 2 net ATP
• Glycolysis also produces a number of key precursor metabolites
Precursor metabolites produced by glucose
• Glycolysis does not require oxygen and can occur under aerobic and
anaerobic conditions.
• During aerobic respiration, two NADH molecules transfer protons and
electrons to the electron transport chain to generate additional ATPs by
way of oxidative phosphorylation.
• The glycolysis pathway involves 9 distinct steps, each catalyzed by a
unique enzyme.
glucose is
phosphorylated
• Glycolysis does not require oxygen and can occur under aerobic and
anaerobic conditions.
• During aerobic respiration, two NADH molecules transfer protons and
electrons to the electron transport chain to generate additional ATPs by
way of oxidative phosphorylation.
• The glycolysis pathway involves 9 distinct steps, each catalyzed by a
unique enzyme.
glucose is phosphorylated
3. A second phosphate provided by the hydrolysis of a second molecule of ATP is added to the
fructose 6-phosphate to form fructose 1,6-bisphosphate.
4. The 6-carbon fructose 1,6 bisphosphate is split to form two, 3-carbon molecules:
gleraldehyde 3-phosphate and dihydroxyacetone phosphate. The dihydroxyacetone
phosphate is then converted into a second molecule of glyceraldehyde 3-phosphate. Two
molecules of glyceraldehyde 3-phosphate will now go through each of the remaining steps in
glycolysis producing two molecules of each product.
5. As each of the two molecules of glyceraldehyde 3-phosphate is oxidized energy is
released, this energy is used to add an inorganic phosphate (Pi) group to form two
molecules of 1,3-biphosphoglycerate each containing a high-energy phosphate
bond.
•
During these oxidations, two molecules of NAD+ are reduced to form two NADH + 2H+.
6. As each of the two molecules of 1,3-biphosphoglycerate are converted to 3phosphoglycerate, the high-energy phosphate group is added to ADP producing 2
ATP by substrate-level phosphorylation
7. The two molecules of 3-phosphoglycerate are rearranged to form two molecules
of 2-phosphoglycerate
8. Water is removed from each of the two molecules of 2-phosphoglycerate
converting the phosphate bonds to a high-energy phosphate bonds as two
molecules of phosphoenolpyruvate are produced..
9. Two molecules of phosphoenolpyruvate are converted to two molecules of
pyruvate, the high-energy phosphate groups are added to ADP producing 2 ATP by
substrate-level phosphorylation
KREBS CYCLE / CITRIC ACID CYCLE
OBJECTIVES
Outline the Krebs cycle
Explain the significance of the Krebs cycle in ATP formation
The overall reaction for the transition / link reaction is:
2 pyruvate + 2 NAD+ + 2 coenzyme A yields 2 acetyl-CoA-SH + 2 NADH + 2 H+ + 2
CO2
FORMATION OF ACETYL-CoA-SH THROUGH THE TRANSITION
REACTION OR LINK REACTION
• The Link reaction connects glycolysis to the citric acid (Krebs) cycle.
Through a process called oxidative decarboxylation
•
The link reaction converts the two molecules of the pyruvate (3C) from
glycolysis into two molecules of the acetyl Coenzyme A (acetyl-CoA-SH),
[2C] and 2 molecules of carbon dioxide.
•
First, a carboxyl group of each pyruvate is removed as carbon dioxide and
then the remaining acetyl group combines with coenzyme A (CoA-SH) to
form acetyl-CoA-SH.
• As the two pyruvates undergo oxidative decarboxylation, two molecules
of NAD+ become reduced to 2 NADH + 2H+
• The 2 NADH + 2H+ carry protons and electrons to the electron transport
chain to generate additional ATP by oxidative phosphorylation
• Before the pyruvates from glycolysis can feed into the citric acid cycle,
they must undergo a transition reaction. The pyruvate is converted into a
2-carbon acetyl group as the third carbon is lost as CO2. The acetyl group
is attached to coenzyme A to form acetyl-CoA.
•
The 2-carbon acetyl-CoA combines with the 4-carbon oxaloacetate of the
citric acid cycle to form 6-carbon citrate.
•
Citrate is converted (isomerisation) to isocitrate.
•
The 6-carbon isocitrate is oxidized by NAD+ and decarboxylated to
produce reduced NADH and 5-carbon alpha-ketoglutarate. (One carbon is
lost as CO2.)
Krebs Cycle
. The 5-carbon alpha-ketoglutarate is oxidized by NAD+ and decarboylated to
produce reduced NADH and 4-carbon succinyl-CoA. (One carbon is lost as CO2.)
6. Oxidation of succinyl-CoA produces succinate and one GTP that is converted
to ATP.
7. Oxidation of succinate by FAD produces reduced FADH2 and fumarate.
8. Fumarate is converted into malate.
9. Oxidation of malate by NAD+ produces reduced NADH and oxaloacetate.
Two molecules of acetyl-CoA from the link reaction enter the citric acid cycle.
This results in the formation of
• 6 molecules of NADH
• 2 molecules of FADH2
• 2 molecules of ATP
• 4 molecules of CO2
The NADH and FADH2 molecules then carry electrons to the electron transport
system for production of ATPs by oxidative phosphorylation
RESPIRATION NOTES 3 –ELECTRON TRANSPORT CHAIN ECT.
OXIDATIVE PHOSPHORYLATION
OBJECTIVE
• Explain the process of oxidative phosphorylation with reference to the
electron transport chain
Include the roles of hydrogen and electron carriers; the synthesis of ATP and the
role of oxygen. No details of the carriers are required. A summary of ATP
production should be known
• The process of respiration has so far been geared to the production of
NADH + H+ and FADH2
• During the electron transfer chain (ETC) NADH + H+ and FADH2 donate high
energy electrons which are passed along a series of carrier molecules
before combining with O to form water
• The movement of these electrons results of pumping of protons and
synthesis of ATP.
• The carriers are components of the inner mitochondrial membrane and
include four large multi-enzyme protein complexes
• NADH dehydrogenase,
• Succinate Dehydrogenase
•
cytochrome reductase
• cytochrome oxidase In addition to shuttles
• Ubiquinone or Cytochrome Q
• Cytochrome C The NAD molecules then returns to the Krebs Cycle and
Glycolysis to collect more hydrogen.
• FADH binds to complex II, succinate dehydrogenase rather than complex I
NADH dehydrogenase, to release its hydrogen.
• The electrons are passed down the chain of proteins complexes from I to
IV, each complex binding electrons more tightly than the previous one.
• In complexes I, III and IV the electrons give up some of their energy, which
is used to pump protons across the inner mitochondrial membrane by
active transport through the complexes.
• During the oxidation of NADH, electrons enter the electron transport chain
at Complex I NADH dehydrogenase or NADH-Q reductase)
•
As the electrons move through Complex I via a series of redox reactions,
protons are pumped from the mitochondrial matrix into the
intermembrane space.
• The electrons are transferred to the first shuttle molecule, ubiquinone.
• Ubiquinone or Cytochrome Q is reduced to ubiquinol which diffuses to
Complex III.
• As the electrons move through Complex III - Cytochrome c reductase),
protons are again pumped from the mitochondrial matrix to the
intermembrane space.
•
The electrons are transferred to the second shuttle molecule, cytochrome
c.
• The reduced cytochrome c diffuses to Complex IV - cytochrome oxidase.
• Again protons are pumped from the mitochondrial matrix to the
intermembrane space.
•
The ultimate electron acceptor is molecular oxygen which is reduced to
water.
• Altogether 10 protons are pumped across the membrane for every
hydrogen from NADH (or 6 protons for FADH).
•
In complex IV the electrons are combined with protons and molecular
oxygen to form water, the final end-product of respiration. The process is
catalyzed by cytochrome oxidase
•
The oxygen diffused in from the tissue fluid, crossing the cell and
mitochondrial membranes
•
Oxygen is only involved at the very last stage of respiration as the final
electron acceptor, without it the respiratory chain does not function.
ATP Synthase
• The movement of protons from the mitochondrial matrix to the
intermembrane space generates an electrochemical potential across the
inner mitochondrial membrane.
• The proton gradient that is generated across the inner membrane is
required by the enzyme ATP synthase, which contains a proton pump.
•
Following the phosphorylation of ADP, the enzyme must release the ATP.
•
This release is dependent on the presence of a proton gradient.
•
It takes 4 protons to synthesize 1 ATP molecule.
This process is referred to as the chemiosmotic theory
• Some poisons act by making proton channels in mitochondrial membranes,
so giving an alternative route for protons and stopping the synthesis of ATP.
• This also happens naturally in the brown fat tissue of new-born babies and
hibernating mammals:
• Respiration takes place, but no ATP is made, with the energy being turned
into heat instead
Comparison ATP production in respiration and photosynthesis
Summary of Respiration
• We can now see how much ATP is made from each glucose molecule.
ATP is made in two different ways:
• Some ATP molecules are made directly by the enzymes in glycolysis or the
Krebs cycle. This is called substrate level phosphorylation (ADP is being
phosphorylated to form ATP).
• Most of the ATP molecules are made by the ATP synthase enzyme in the
respiratory chain. This requires oxygen it is called oxidative
phosphorylation.
Summary of Products of Respiration
Stage
Mols. produced per
glucose
glycolysis
2 ATP used
4 ATP (2 per triose
phosphate)
2 NADH (1 per triose
phosphate)
2
4
6
Link reaction
2 NADH (1per
pyruvate)
6
5
Krebs Cycle
2 ATP (1 per Acetyl
CoA-SH)
6 NADH (3 per Acetyl
CoA-SH)
2 FADH (1 per Acetyl
CoA-SH)
2
18
4
2
15
3
Total
Final ATP yield
Old method
Final ATP
yield
New method
2
4
5
38
Other substances used to make ATP
•
Triglycerides are broken down to fatty acids and glycerol, both enter the
Krebs Cycle.
32
• A typical triglyceride might make 50 acetyl CoA molecules, yielding 500 ATP
molecules
•
Fats are a very good energy store, yielding 2.5 times as much ATP per g dry
mass as carbohydrates.
• Proteins are not normally used to make ATP, but in times of starvation they
can be broken down and used in respiration.
• They are first broken down to amino acids, which are converted into
pyruvate and Krebs Cycle metabolites and then used to make ATP.
Anaerobic Respiration
Goal: to reduce pyruvate, thus generating NAD+


Where: the cytoplasm
Why: in the absence of oxygen, it is the only way to generate NAD+ and ADP
Alcohol Fermentation - occurs in yeasts in many bacteria
o

The product of fermentation, alcohol, is toxic to the organism
Lactic Acid Fermentation - occurs in humans and other mammals
o The product of Lactic Acid fermentation, lactic acid, is toxic to mammals
o This is the "burn" felt when undergoing strenuous activity




The only goal of fermentation reactions is to convert NADH to NAD+ (to use in
glycolysis).
No energy is gained
Note differences - anaerobic respiration - 2 ATP's produced (from glycolysis), aerobic
respiration - 32 ATP's produced (from glycolysis, Krebs cycle, and Oxidative
Phosphorylation)
Thus, the evolution of an oxygen-rich atmosphere, which facilitated the evolution of
aerobic respiration, was crucial in the diversification of life
Respiratory Substrates and Respiratory Quotient (RQ)
• It is sometimes useful to deduce which substrate is being used in a person’s
metabolism at a specific time.
• This can be done if the volume of oxygen taken in, and the volume of
carbon dioxide given out are measured.
•
From this data the respiratory quotient (RQ) can be calculated:
RQ = Volume of carbon dioxide given off
Volume of oxygen taken in
The values of RQ to be expected vary depending of which substances are broken
down by respiration.
• Carbohydrates (glucose) 1.0
• protein 0.9
• fat (lipids) 0.7
• Under normal conditions the human RQ is in the range of 0.8-0.9, indicating
that some fats and proteins, as well as carbohydrates, are used for
respiration.
•
Values greater than 1.0 are obtained when anaerobic respiration is in
progress.
Measuring respiratory rate can be done by using a respirometer
• The potassium hydroxide solution removes carbon dioxide from the
surrounding air.
• Therefore any carbon dioxide, which is produced by respiration, is
immediately absorbed and it does not affect the volume of remaining air.
• Any changes in volume, which take place, must be due to oxygen uptake.
• A manometer and the calibrated scale measure these changes.
• Tube B acts as a control.