lecture 5: anaerobic respiratory ETCs

what does pyruvate formate lyase (PFL) do

splits pyruvate into formate and acetyl CoA

what inhibits PFL

oxygen

what can the formate from PFL be used for

can be fed into the anaerobic ETC via formate dehydrogenase (FDH) to help build a H⁺ gradient

what can the acetyl CoA from PFL be used for

• can be converted into acetyl-phosphate by phosphotransacetylase

• can also enter the repressed Krebs cycle

the repressed Krebs cycle

E.coli represses alpha-ketoglutarate dehydrogenase, turning the cycle into 2 branches:

• the oxidative branch

• the reductive branch

the oxidative branch

• runs from citrate to alpha-ketoglutarate

• the main job is to provide precursors (alpha-ketoglutarate) for glutamate and other amino acids

the reductive branch

• E.coli reverse the bottom half of the cycle so it moves from oxaloacetate to succinate

• it consumes NADH which helps the cell recycle NAD+ to keep glycolysis moving

give examples of exogenous terminal acceptors other than oxygen

• nitrate

• nitrite

• fumarate

• trimethylamine N-oxide

• Dimethyl sulfoxide

examples of electron donors

• formate

• H2

• NADH

• lactate

examples of quinones

• menaquinone (MK/MKH2)

the larger the gap between the donor and acceptor…

the more protons the cell can pump

fumarate as a terminal electron acceptor

• Nuo is the only active H⁺ pump and transfers 2e^- from NADH to menaquinone to pump 4H⁺ into the periplasm

• fumarate reductase (Frd) uses the e^- from the menaquinol to reduce fumarate into succinate

• the H⁺ from the oxidation of MKH₂ are released into the cytoplasm so they do not contribute to the PMF

formate-dependent nitrate reduction (NAR)

• formate dehydrogenase (Fdn) strips e^-s from formate, converting it into CO₂, releasing 2H⁺ into the periplasm

• NAR takes e^-s from MKH₂ and reduces nitrate into nitrite

• NAR consumes 2H⁺ from the cytoplasm to create a redox loop

formate-dependent nitrate reduction (NAP)

• formate dehydrogenase transfers e^-s to MK

• periplasmic nitrate reductase (NAP) reduces nitrate into nitrite and consumes H⁺s from the periplasm

why is NAP used

it has a much higher affinity for nitrate and is used to dump excess e^-s quickly to maintain metabolic balance (even if the payoff is much smaller)