Lecture Eighteen:
MITOCHONDRIA
& THE CITRIC
ACID CYCLE
(Figures in red are for the 7th Edition)
[Based on
Chapters
17 & 18
of Berg,
Tymoczko,
& Stryer]
 STRUCTURE OF MITOCHONDRIA
 Figure 18-2, page 504 (18-2, page 545)
 Mitochondria are oval-shaped subcellular organelles,
about 2m length and 0.5m diameter
 Two surrounding membranes
 An Outer mitochondrial membrane (OMM)
 An Inner mitochondrial membrane (IMM)
 Extensive and highly-folded
 Therefore __________ compartments in mitochondria
 The intermembrane space
 Between the OMM and IMM
 The __________
 Surrounded by the IMM
 NOTE: Fatty acid oxidation, pyruvate dehydrogenase
complex, the citric acid cycle => located in the _________
 NOTE: Respiratory electron transfer, oxidative
phosphorylation, and solute transfer in and out of
mitochondria => located on the IMM
 The Endosymbiotic Theory of the Evolution of
Mitochondria (and Chloroplasts)
 Suggests mitochondria evolved from free-living
aerobic respiring bacteria (similar to Paracoccus
denitrificans) that entered into an endosymbiotic
association with the ancestors of eukaryotes
 Now unable to live independently since most of
their proteins are synthesised by nuclear genes
 However mitochondria retain their own DNA and
transcription and translation machinery, which resembles
the machinery of bacteria (i.e. the 70S rather than 80S
ribosomes) and synthesises a few of the mitochondrial
proteins
 THE CITRIC ACID CYCLE - BACKGROUND
 Last lecture: Glucose converted to pyruvate
 In glycolysis in the cytosol
 The next steps in generating energy from glucose are:
 The oxidative decarboxylation of pyruvate to form
acetyl CoA by pyruvate dehydrogenase complex
 Acetyl CoA (activated acetyl unit) then completely
oxidised to CO in a series of reactions known as:
2
 The Tricarboxylic Acid Cycle
 or The _______________
 or THE CITRIC ACID CYCLE
 Final common pathway for the oxidation of fuel
molecules (amino acids, fatty acids and carbohydrates)
 Most enter as ________________
 Provides intermediates for biosynthesis
 PYRUVATE DEHYDROGENASE COMPLEX
 Catalyses the oxidative decarboxylation of pyruvate to
form acetyl CoA
 Irreversibly funnels the product of glycolysis into
the citric acid cycle
 Pyruvate + CoA + NAD
+
acetyl CoA + CO + NADH
2
 The pyruvate dehydrogenase complex is a multimeric
assembly of three different kinds of enzymes
 Table 17-1, page 477 (Table 17-1, page 517)
 In E.coli eight trimers of transacetylase form a hollow
cube, with pyruvate dehydrogenase and lipoyl
dehydrogenase enzymes bound to the outside
 With three different enzyme processes ________ closely
together, it makes possible the co-ordinated catalysis
of a complex reaction
 The proximity of one enzyme to another:
 Increases the overall reaction rate
 Minimises side (unwanted) reactions
 Called ____________________
 THE CITRIC ACID CYCLE
 Stoichiometry of the Citric Acid Cycle
 The overall stoichiometry of the cycle is:
 Figure 17-2, page 476 (17-2, page 516)
 The basic reactions of the cycle are as follows:
 Citrate Synthase
 Cycle starts with the condensation of acetyl CoA (C )
2
and oxaloacetate (C ) to give citrate (C )
4
6
 Aconitase
 Citrate is isomerised to isocitrate (____)
 Isocitrate dehydrogenase
 Oxidative decarboxylation produces CO and
2
-ketoglutarate (____)
 -Ketoglutarate dehydrogenase
 Oxidative decarboxylation produces CO and
2
succinyl CoA (____)
 Succinyl CoA synthetase
 Succinyl CoA is cleaved to form succinate and GDP
is converted to GTP
 Succinate dehydrogenase
 Succinate is oxidised to fumarate (C )
4
 Fumarase
 Fumarate is then hydrated to form malate
 Malate dehydrogenase
 Finally malate is oxidised to regenerate
oxaloacetate (____)
 THE CITRIC ACID CYCLE IN MORE DETAIL
 Figure 17-15, page 489 (17-15, page 529)
 This can be summarised:
 Table 17-2, page 489 (Table 17-2, page 529)
+
 Acetyl CoA + 3NAD + FAD + GDP + P + 2H O 
i
2
+
2CO + 3NADH + FADH + GTP + 2H + CoA
2
2
 STEPS IN THE CITRIC ACID CYCLE
[ Figure, Page 482 ] [ Figure, Page 522 ] Formation of Citrate
[ Figure, Page 484 ] [ Figure, page 524 ] Formation of Isocitrate
[ Figure, Page 485 upper ] [ Figure, Page 525 upper ] Formation of -Ketoglutarate
[ Figure, Page 485 lower ] [ Figure, Page 525 lower ] Formation of Succinyl CoA
[ Figure, Page 486 ] [ Figure, page 526 ] Formation of Succinate
[ Figure, Page 487 ] [ Figure, page 527 ] Formation of Oxaloacetate
 This breaks down into the basic steps:
 Two carbon atoms enter the cycle as acetyl CoA
 Two carbon atoms leave the cycle as __________
 There are four oxidation-reduction reactions in the cycle,
+
three pairs of electrons are transferred to NAD and one
pair of electrons to FAD
 In addition one high energy phosphate bond (______) is
formed in the cycle
 As the oxidation of the NADH and FADH forms 9 ATP
2
in oxidative phosphorylation, the complete oxidation of
each 2-carbon acetyl CoA to H O and CO yields a total
2
2
of 10 high-energy phosphate bonds
 THE CITRIC ACID CYCLE IS A SOURCE OF
BIOSYNTHETIC PRECURSORS
 Figure 17-20, page 493 (17-20, page 534)
 It provides intermediates for biosynthesis
 The majority of carbon atoms in ___________ come
from succinyl CoA
 Many amino acids are derived from -ketoglutarate
and oxaloacetate
 NOTE: These intermediates must be replenished if
drawn off for biosynthesis
 This is achieved by anaplerotic (Greek for “fill up”)
reactions
 CONTROL OF THE CITRIC ACID CYCLE
 Figure 17-19, page 492 (17-19, page 532)
 The rate of the citric acid cycle has to meet the demand for
ATP in an animal cell
 This requires careful adjustments
 It is controlled by both
 a) THE ENERGY STATUS OF THE CELL:
 1st Control Point
 Citrate synthase, inhibited by ________
 Regulates entry of 2C units into cycle
 2nd Control Point
 Isocitrate dehydrogenase, stimulated by
ADP, inhibited by NADH and ATP
 3rd Control Point
 -ketoglutarate dehydrogenase inhibited by
high energy charge and it’s products
(succinyl CoA and NADH)
+
 b) The rate of supply of NAD and FAD:
 produced by respiration when the energy status is
low
 So cycle only operates under AEROBIC CONDITIONS
 NOTE:
 Irreversible formation of acetyl CoA from pyruvate
by pyruvate dehydrogenase is controlled by
reversible phosphorylation so acetyl CoA formation
is reduced if energy status of cell is high
 Summary of Lecture Eighteen
 MITOCHONDRIA
 Two membranes => OMM & IMM
 Two soluble regions as a result
 Derived from ancient bacteria
 THE CITRIC ACID CYCLE
 Located in the matrix
 Initiated by pyruvate dehydrogenase complex
 Nine step cyclic process
 Generates CO
2
 Yields 10 high-energy phosphate bonds
 Majority of Carbons for porphyrins
 Many amino acids for protein production
 CONTROLLED BY:
 Energy status of cell:
 At three points
+
 Rate of supply of NAD and FAD