Hard- S11 PDH CAC Anaplerotic

Iron-Sulfur Center in Aconitase

• Iron-sulfur center acts in both substrate binding and catalysis (dehydration and hydration)
• Aconitase regulates iron uptake and metabolism in the cell

Isocitrate Dehydrogenase

• Isocitrate dehydrogenase is an oxidation coupled to a hydride transfer to NAD+
• The enzyme uses a Mn2+ ion cofactor, enhancing the electron withdrawing power of the carbonyl and facilitating decarboxylation
• Carbon lost as CO2 is not originally from carbons in acetyl-CoA

α-KG Dehydrogenase Complex

• α-ketoglutarate dehydrogenase complex performs oxidative decarboxylation, splits the carbon-carbon bond, releases CO2, and reduces NAD+ to NADH
• Has a structure similar to PDH complex and also works similarly
• Has E1, E2, E3 and coenzymes TPP, lipoyllysine, CoA, FAD, and NAD+
• The energy of the oxidation is conserved in the form of a thioester bond

Similar Reactions

• Thioester bond is a high-energy bond with very negative free-energy of hydrolysis
• α-KG dehydrogenase complex and PDH complex share similarities

Origin of Carbon Atoms in CO2

• The carbons in CO2 originate from oxaloacetate, not from pyruvate
• Net complete oxidation of glucose occurs after two pyruvates go through the citric acid cycle

Succinyl-CoA Synthetase

• Succinyl-CoA synthetase performs phosphorylysis of the thioester bond, followed by phosphoryl transfer to GDP, producing succinate plus GTP
• Transfer of phosphate is mediated by His residue in the active site
• GTP is as good as ATP, with free conversion of NTPs in the cell

Succinate Dehydrogenase

• Succinate dehydrogenase converts succinate to fumarate, with reducing power transferred to FAD
• FADH2 yields 1.5 ATP
• Rate controlling enzymes: citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase

Regulation of Activity

• Regulation of activity: activated by substrate availability, inhibited by product accumulation
• Allosteric inhibition or activation by other intermediates

Other Regulation Mechanisms

• Citrate synthase is also inhibited by succinyl-CoA
• α-ketoglutarate is an important branch point for amino acid metabolism
• Succinyl-CoA communicates the flow at this branch point to the start of the cycle
• Inhibition of isocitrate dehydrogenase leads to accumulation of isocitrate, pushing the equilibrium towards citrate
• Accumulated citrate leaves mitochondria and inhibits phosphofructokinase-1 in glycolysis