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