Citric acid cycle and respiratory chain
Pavla Balínová
Mitochondria
Structure of mitochondria:
• Outer membrane
• Inner membrane (folded)
• Matrix space (mtDNA, ribosomes, enzymes of CAC,
β-oxidation of FA, heme synthesis,…)
Function of mitochondria:
• production of acetyl-CoA from pyruvate (PDH reaction)
• production of ATP (by oxidative phosphorylation)
• degradation of FA by β-oxidation
• urea synthesis
• heme synthesis,….
Citric acid cycle (CAC)
tricarboxylic acid cycle, Krebs cycle
• CAC is a set of reactions which form a metabolic
pathway for aerobic oxidation of saccharides, lipids and
proteins.
• Reduced equivalents (NADH, FADH2) are released by
sequential decarboxylations and oxidations of citric
acid. These reduced equivalents are used to respiratory
chain and oxidative phosphorylation to produce ATP
• CAC plays a key role in futher metabolic reactions
(i. e. gluconeogenesis, transamination, deamination or
lipogenesis)
Function of CAC
• Oxidation of CH3-CO- to 2 CO2 → formation of
reduced coenzymes NADH + H+ and FADH2
• CAC is a central junction of an intermediary
metabolism = amphibolic pathway
→ catabolic pathways generate intermediates into CAC
→ anabolic pathways withdraw some intermediates from
CAC (oxaloacetate → gluconeogenesis, succinyl-CoA → synthesis
of porphyrins etc.)
Coenzyme A (CoA)
-CO-CH3
acetyl
Figure was assumed from http://www.lipidlibrary.co.uk/Lipids/coa/index.htm
Figure was assumed from http://www.biocarta.com/pathfiles/KrebPathway.asp
Result of CAC:
Production of 2 mol CO2, 3 mol NADH + H+, 1 mol FADH2,
1 mol GTP
Anaplerotic (support) reactions:
• Pyr + CO2 + ATP → oxaloacetate + ADP + Pi
(pyruvate carboxylase)
• degradation of most amino acids gives the following
intermediates of CAC: oxaloacetate, α-ketoglutarate,
fumarate
• Propionyl-CoA → succinyl-CoA
Regulation of CAC
Regulatory factors of CAC are:
• NADH / NAD+ ratio
• ATP / AMP ratio
• availability of CAC substrates and energy situation within
the cell
Regulatory enzymes of CAC:
Citrate synthase is mainly regulated with availability of
acetyl-CoA and oxaloacetate.
Isocitrate dehydrogenase and α-ketoglutarate
dehydrogenase are inhibited by ↑ NADH / NAD+. On the
contrary, these enzymes are activated by AMP and NAD+.
The activity of CAC is closely linked to the availability of O2.
Transport of acetyl-CoA within the cell
• acetyl-CoA + oxaloacetate → citrate (citrate
synthase in CAC)
• citrate is exported from mitochondria to cytoplasm in
exchange for malate
• citrate is cleaved to acetyl-CoA and oxaloacetate
(citrate lyase) in the cytoplasm
• reduction of oxaloacetate to malate (malate
dehydrogenase = „malic enzyme“ – production of
NADPH + H+)
• malate is returned by antiport into mitochondria or it
is oxidative decarboxylated to pyruvate
Respiratory chain
Reduced coenzymes NADH and FADH2 release H atoms
(e- and H+) in electron transport system.
•location: inner mitochondrial membrane
•composition: enzyme complexes I – IV, 2 mobile carriers
of electrons – coenzyme Q (ubiquinone) and cytochrome c
•function: transport of electrons in series of redox
reactions and H+. Oxygen is a final acceptor of electrons.
H+ are transmitted by complexes I, III and IV. Proton
gradient is used to move ATP-synthase.
Figure was assumed from http://web.indstate.edu/thcme/mwking/oxidative-phosphorylation.html
Figure was assumed from http://www.biocarta.com/pathfiles/h_etcPathway.asp
Oxidative phosphorylation
H+ are ejected from mitochondrial matrix into intermembrane
space by complexes I, III and IV.
These protons create an electrochemical gradient across the inner
mitochondrial membrane. Energy of this gradient is used for
movement of ATP synthase. This enzyme allows protons to flow
back down their concentration gradient across the membrane.
Figure was assumed from http://en.wikipedia.org/wiki/Electron_transport_chain
Respiratory chain + oxidative (aerobic) phosphorylation
Figure was assumed from http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/oxphos.html
ATP-synthase
ATP-synthase consists of 2 subunits:
F0 – in inner mitochondrial membrane (a proton channel)
F1 – catalytic unit (matrix)
Figure was assumed from http://en.wikipedia.org/wiki/ATP_synthase
Uncoupling proteins
Uncoupling proteins (UCP) are compounds which allow
protons to flow across the mitochondrial membrane with
low production of ATP.
Energy of proton gradient is released as heat.
UCP-1 (thermogenin) – brown adipose tissue (newborns)
UCP-2 – mainly white adipose tissue
UCP-3 – skeletal muscles
UCP-4,5 – brain
Hibernating mammals
Uncoupling proteins
(UCP)
UCP-1
Thermogenin
Energy of proton
gradient is released
as heat.
Clinical correlation
Hypoxia
= a lack of O2 in the inner mitochondrial membrane
causes: lack of O2 in breathed air, myocardial infarction,
anemia, atherosclerosis
result: failure of ATP formation
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Cyanide poisoning
Ion CN- binds to the complex IV (Fe3+ in heme of the
cytochrome a) and blocks an electron transport to O2 →
stopping of respiratory chain and synthesis of ATP → a
rapid failure of cellular functions and death
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