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,.
Transport of compounds through inner mitochondrial membrane Transporters: malate <--------------> citrate malate <--------------> alfa-ketoglutarate Pyr <--------------> OH - OH - <--------------> P i P i <--------------> malate ATP <--------------> ADP glutamate <-----------> aspartate Shuttles: malate aspartate shuttle glycerolphosphate shuttle Carnitine transport system
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, FADH 2 ) 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 CH 3 -CO- to 2 CO 2 formation of reduced coenzymes NADH + H + and FADH 2 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-CH 3 acetyl Figure was assumed from http://www.lipidlibrary.co.uk/lipids/coa/index.htm
Citric acid cycle Figure was assumed from http://www.biocarta.com/pathfiles/krebpathway.asp
2 mol CO 2 3 mol NADH + H + 1 mol FADH 2 1 mol GTP One turn of CAC produces: Anaplerotic (support) reactions: Pyr + CO 2 + ATP oxaloacetate + ADP + P i (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 O 2.
Transport of acetyl-coa within the cell Mitochondrion: acetyl-coa + oxaloacetate citrate (enzyme citrate synthase in CAC) citrate is exported from mitochondria to cytoplasm in exchange for malate (antiport) ----------------------------------------------------------- Cytoplasm: citrate is cleaved to acetyl-coa and oxaloacetate (enzyme citrate lyase) in the cytoplasm reduction of oxaloacetate to malate (malate dehydrogenase = malic enzyme NADPH + H + is produced) malate is returned into mitochondria or it is oxidative decarboxylated to pyruvate
Transport of reducing equivalents into mitochondria MALATE- ASPARTATE SHUTTLE malate dehydrogenase Figure was adopted from textbook: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2 aspartate aminotransferase (AST)
Transport of reducing equivalents into mitochondria glycerol-3-phosphate dehydrogenase GLYCEROLPHOSPHATE SHUTTLE Figure was adopted from textbook: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Respiratory chain Reduced coenzymes NADH and FADH 2 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 and H + in series of redox reactions. Oxygen (O 2 ) 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
Respiratory chain + oxidative (aerobic) phosphorylation H + are ejected from mit. matrix into intermembrane space by complexes I, III and IV. These protons create an electrochemical gradient across the inner mit. 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 adopted from http://cellular-respiration.wikispaces.com/oxidative+phosphorylation
ATP-synthase ATP-synthase consists of 2 subunits: F 0 in inner mitochondrial membrane (a proton channel) F 1 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, hibernating mammals) UCP-2 mainly white adipose tissue UCP-3 skeletal muscles UCP-4,5 brain
Uncoupling proteins (UCP) UCP-1 Thermogenin Energy of proton gradient is released as heat.
Clinical correlation Hypoxia = a lack of O 2 in the inner mitochondrial membrane causes: lack of O 2 in breathed air, myocardial infarction, anemia, atherosclerosis result: failure of ATP formation Cyanide poisoning Ion CN - binds to the complex IV (Fe 3+ in heme of the cytochrome a) and blocks an electron transport to O 2 stopping of respiratory chain and synthesis of ATP a rapid failure of cellular functions and death