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Pyruvate Oxidation, Krebs Cycle

pyruvate oxidation - stage 2 

  • occurs in only in mitochondria of eukaryotes
  • 1st forms acetyl-CoA from pyruvate, then oxidizes acetyl-CoA in Krebs cycle
  • single "decarboxylation" reaction that cleaves off one of the carbons on pyruvate (producing acetyl group and CO2)
  • catalyzed in mitochondria by multienzyme complex
  • pyruvate dehydrogenase - enzyme that removes CO2 from pyruvate; has 60 subunits
  • pyruvate + NAD+ + CoA (coenzyme A) >> acetyle-CoA + NADH + CO2
  • acetyl-CoA - produced by a large number of metabolic processes
    • key point for many catabolic processes in eukaryotes
    • used for fatty acid synthesis instead of Krebs cycle when ATP levels are high

Krebs cycle - stage 3 

  • 9 reactions; oxidation of acetyle-CoA
  • takes place in mitochondria matrix
  • combines acetyle-CoA (2-carbon molecule) w/ oxaloacetate (4-carbon molecule) to extract electrons and CO2 to power proton pumps for ATP
  • step A - priming; 3 reactions rearrange chemical groups in acetyl-CoA to prepare the 6-carbon molecule for energy extraction
  • step B - energy extraction; 4/6 reactions oxidize and remove electrons
  • reaction 1 - condensation
    • acetyle-CoA combines w/ oxaloacetate to form citrate
    • irreversible reaction; inhibited when ATP concentration is high
  • reaction 2/3 - isomerization
    • repositions hydroxyl group by taking away H2O and adding it back to a different carbon
    • forms isocitrate from citrate
  • reaction 4 - 1st oxidation
    • oxidized to yield pair of electrons that make NADH from a NAD+
    • decarboxylated to split off a CO2 to form a-ketoglutarate (5-carbon molecule)
  • reaction 5 - 2nd oxidation
    • a-ketoglutarate decarboxylated into succinyl group, which bonds to coenzyme A to form succinyl-CoA
    • CO2 removed
    • oxidized to yield pair of electrons that make NADH from a NAD+
  • reaction 6 - substrate-level phosphorylation
    • bond between succinyl group (4-carbon molecule) and CoA cleaved to phosphorylate GDP into GTP
    • GTP readily converts into ATP
    • succinyl-CoA becomes succinate
  • reaction 7 - 3rd oxidation
    • succinate oxidized into fumarate
    • energy produced not enough for NAD+, so FAD turned into FADH2 instead
    • FAD part of inner mitochondrial membrane, can't diffuse within the organelle
  • reaction 8/9 - oxaloacetate regeneration
    • H2O added to fumarate, making malate
    • malate oxidized to form oxaloacetate and 2 electrons to form NADH from NAD+
  • by end of Krebs cycle, ener
Subject X2: 

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