Anaerobic metabolism Flashcards
oxygens main role is to
be a terminal e acceptor
in the absence of oxygen (2)
use an alternative e acceptor (preferred)
switch to fermentative metabolism (poor energy yields)
In fermentative metabolism production of NADH and FADH2 must be
exactly balanced by reactions that generate NAD+ and FAD
Aerobic E coli metabolism
glycolysis
glycolysis converts glucose to pyruvate (for the TCA)
NADH and FADH2 is generated (fed into the respiratory chain and electrons are passed to O2, protons pumped = ATP)
Anaerobic E coli metabolism
pyruvate is converted to lactate, acetate, ethanol, CO2 and H2
NADH is consumed
electrons are fed to nitrate to make nitrite
lactic acid fermentation (least preferred)
phosphoenolpyruvate to pyruvate to lactate
yields 2 ATP
mixed acid fermentation (most preferred)
pyruvate to acetyl coa by pyruvate formate lyase
formic acid to H2 and CO2
acetyl coa to acetate and ethanol
yields 3 ATP
How do cells detect a shortage of O2?
FNR (1 component system)
ArcB (2 component system)
FNR
fumarate and nitrate reductase regulator
senses O2 directly - one component system
soluble protein that forms a dimer when it is active
it is active under reducing (low O2) conditions as a TF
FNR has an iron sulfur centre, when it is oxidised the Fe-S reacts with O2 and this stops it from dimerising
FNR helps to recruit
the sigma factor 70 RNA pol to the site of anaerobic genes that code for enzymes that allow the cell to use alternative e acceptors,
eg. aspartase, fumarate reductase, nitrite and nitrate reductase
ArcA/B
ArcB is the sensor kinase, activated under anaerobic conditions
ArcA is the RR
Arc does not sense O2 directly - it senses the reduction status of the respiratory chain (under anaerobic conditions, all components of the aerobic resp chain become progressively more reduced - a traffic jam of e)
Arc senses the quinone pool
Arc senses the
quinone pool
which results from a build up of e in the resp chain
senses all the quinones which are in red form - quinol
oxidised quinones inhibit ArcB by oxidising it
How is Arc slightly different to other sensor kinases?
Phosphorelay not phosphotransfer
How does ArcB sense quinones?
they oxidise ArcB causing a conformational change
anaerobically, when ArcB is active it is a dimeric protein and each monomer has 2 special cytosines that exist as free thiols
aerobically, the oxidised quinones convert the cysteines to disulfides, which block the kinase activity of ArcB
under fully oxic conditions, quinones also oxidise another cysteine 241 in the linker domain of ArcB which completely silences its activity
FNR senses O2 directly whereas ArcB senses
the absence of a terminal electron acceptor
this allows a set of conditions where FNR is active but not Arc
when O2 runs out, the cell relies on FNR to try to see if expressing genes for alt e acceptors will help. this means its respiratory chain can keep going (the more efficient metabolism)
When there are no terminal electron acceptors
ArcA-P and FNRred work together to switch the cell over to fermentative metabolism
Hierarchy of e acceptors in the respiratory chain
O2 highest
Fumarate lowest
