Citric Acid Cycle & Oxidative
Phosphorylation
• The citric acid cycle,
formerly known as the
Kreb cycle, begins in
the mitochondria as
the 2 molecules of
pyruvate enter
through active
transport are further
broken down to yield
the remaining energy
stored in the bonds.
• Before the cycle can begin the
pyruvate molecules must be
converted to Acetyl CoA.
– Pyruvate's -COO- (carboxyl group)
is removed producing the 1st
molecule of CO2
– Acetate (CH3CO-) a 2 carbon
compound is produced.
– Coenzyme A (from a B vitamin) is
added to the molecule via an
unstable -S- bond
• 2 molecules of CO2 are produced (1
each)
• 2 molecules NADH are produced (1
each) which are later used in the
citric acid cycle
Acetyl CoA
Citric Acid Cycle (1-4)
•
8 steps make up the citric acid cycle in which
the energy in the final 2 carbons are
extracted.
– Acetyl CoA is attached to an
Oxaloacetate (4C) molecule to form
citrate (6C).
• CoA is released.
– Citrate is converted to isocitrate (6C)
moving a -OH group (via the removal &
addition of water)
– a-ketoglutarate (5C) is produced.
• CO2 is produced as a carbon is oxidized
• NAD+ is reduced to NADH.
– Succinyl CoA (4C) is produced as the
coenzyme renters into the cycle.
• CO2 is produced as a carbon is oxidized
• NAD+ is reduced to NADH.
Citric Acid Cycle (5-8)
– A PO4 group displaces the CoA
and is transferred to GDP to
form GTP.
• GTP then transfers the PO4 to
ADP forming ATP and
Succinate.
– 2 H are transferred to FAD to
form FADH2 (energy carrying
molecule like NADH) and
Fumarate is formed.
– H2O is added to form Malate.
– Malate is oxidized to form
Oxaloacetate
• producing a NADH from NAD+ .
Citric Acid Summary
Glycolysis
Acetylation
Citric Acid Cycle
Oxidative Phosphorylation
• The remaining energy stored in the carrier molecules NADH and FADH2 are
extracted in the electron transport chain to produce the majority of the
ATP in the membranous matrix of the mitochondria.
– As the carriers (NAD & FAD) move down the chain they alternate between
oxidized and reduced states passing the e- to increasingly electronegative
carriers. (exergonic fall of electrons down the chain)
1. Flavoprotein accepts the H from NADH and becomes reduced (NAD+ is
oxidized)
2. Fe*S accepts the H and becomes reduced as flavoprotein becomes oxidized
3-5 Ubiquinone is the next molecule (only non protein) which continues the
redox chain down through a series of cytochrome molecules
The final cytochrome (c) passes the used e- & H+ to 1/2O to form H2O
1.
2.
3.
4.
Makes no ATP
Manages electrons and removes them to be
used in next step
Pushes H+ ions to the inner membrane
establishing the ion gradient (from NADH &
FADH2)
Forms H2O at the end of the chain
ATP Synthesis
• ATP is synthesized by the protein (enzyme) ATP synthase
that is ubiquitous in the mitochondrial membrane.
– ATP synthase harvests the energy of the proton gradient
(proton motive force) to drive a redox reaction and produce ATP
via oxidative phosphorylation.
• Uses existing ion gradient (H+ ions)
• called chemiosmosis
– simply put...uses H+ ions stored in the form of a gradient to do cellular work
– Movement of ions through the inner channel causes the
catalytic head to spin exposing sites for the production of ATP
from ADP
Cellular Respiration Summary
• Glycolysis
– changes glucose-6-phosphate to 2 pyruvate
molecules
• 2 NADH
• 2 ATP
• Citric Acid Cycle
– changes pyruvate into Acetyl CoA then to citrate
– cycles ultimately modifying citrate into the
beginning molecule Oxaloacetate
• 8 NADH (2 from process of changing pyruvate to
Acetyl CoA)
• 2 FADH2
• 6 CO2 (2 from process of changing pyruvate to
Acetyl CoA)
• 2ATP
• Oxidative Phosphorylation
– uses energy stored in the electron carriers and
the proton-motive force to product ATP
• 32 or 36 ATP (~ 2.5 - 3.3 can be created from ea
NADH, 1.5 - 2 from FADH2)
– if completely efficient it could create 38 ATP
– ~ 40% of the energy in the glucose molecule is
ultimately harnessed
– DG = -686kcal/mol
Variables:
1. the amount of H+ creating
the proton-motive force is
approximate and variable
2. the ratio of NAD+ or FAD
used as carriers (brain
uses more FAD, heart &
liver uses more NAD)
3. some of the protonmotive force is used to
drive the mitochondrion's
uptake of substrates
type of organism
plants use light as energy
to drive the production
of ATP
prokaryotes use the
plasma membrane to
create the ion gradient
and need the gradient
for endo & exocitosis as
well