BCH 4033- ch.17: pyruvate dehydrogenase and the citric acid cycle

exam 4 content

what is the cytric acid cycle?

the final common pathway from the oxidation of fuel molecules

serves as a source of building blocks for biosynthesis

what is the link between glycolysis and the citric acid cycle?

the oxidative decarboxylation of pyruvate to form acetyl CoA by a 3-enzyme complex

where does the citric acid cycle take place in eukaryotes?

inside the mitochondria

what do most molecules enter the citric acid cycle as?

acetyl CoA

what does the citric acid cycle start with?

the condensation of oxaloacetate and the acetyl unit of acetyl CoA to produce citrate

how is succinyl CoA formed (step 1)?

2 Cs from acetyl CoA leave the cycle as CO2 in successive decarboxylations catalyzed by isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase

how is the nucleoside trisphosphate generated (step 2)?

the thioester bond of succinyl CoA is cleaved by orthophosphate

how is a molecule of oxaloacetate regenerated for the next round (step 3)?

succinate is oxidized to fumarate

fumarate is hydrated to form malate

malate is oxidized

how many pairs of electrons are transferred to NAD+ and FAD?

3 pairs of electrons to NAD+ and 1 pair to FAD

electron carriers then oxidized by ETC to generate 9 molecules of ATP

how many molecules of ATP are generated for each 2 C fragments?

10 molecules

for each that’s completely oxidized to H2O and CO2

why does the citric acid cycle only operate under aerobic conditions?

because it requires a supply of NAD+ and FAD that are regenerated in oxidative phosphorylation

what does the rate of the citric acid cycle depend on?

the need for ATP

part of why it needs aerobic conditions

why is the irreversible formation of acetyl CoA from pyruvate a regulatory point?

because the activity of the pyruvate dehydrogenase complex is stringently regulated by reversible phosphorylation

what 2 enzymes in the eukaryotic citric acid cycle are important for regulation?

isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase

energy-rich molecules decrease activity of these

what mechanisms reduce the rate of formation of acetyl CoA?

isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase mechanisms

when the energy charge of the cell is high and when biosynthetic intermediates are abundant

what happens when the cell has adequate energy available?

the citric acid cycle can provide a source of building blocks for a host of important biomolecules

• ex.) nucleotide bases, proteins, heme groups

depletes cycle of intermediates

what happens when the cycle needs to metabolize fuel?

anaplerotic reactions replenish the cycle intermediates

how does the glyoxylate cycle enable plants and bacteria to subsist on acetate?

it bypasses the 2 decarboxylation steps of the citric acid cycle

where is the glyoxylate cycle prominent?

in oil-rich seeds when it takes place in glyoxysomes

succinate produced can be converted into carbs by combination of citric acid cycle and gluconeogenesis

what is an anaplerotic reaction?

a reaction that leads to the net synthesis, or replenishment, of pathway components

what is the glyoxylate cycle?

a metabolic pathway found primarily in microorganisms and plants that converts 2 carbon acetyl units into succinnate for energy production and biosynthesis