2. CELLULAR RESPIRATION
Electron donor (NADH)
from carbon compound
Ubiquinone Electron acceptor
Glycolysis TCA cycle Electron transport chain
NADH dehydrogenase
enzyme
e- e-
Terminal reductase
enzyme
• Oxygen (aerobic respiration)
• NO3
2-, H2S, SO4
2- (anaerobic
respiration)
FERMENTATION Glycolysis
Alcohol or lactate is produced using
organic compound as electron donor
as well as electron acceptor
3. TRANSITIONING BETWEEN RESPIRATION AND FERMENTATION
Respiration electron acceptors
in
in Ratio of reduced quinone to oxidized quinone
in Conc. of electron donors for respiration
in NADH: NAD+ ratio
Regulatory systems have evolved that respond to variations in the levels of molecules
associated with altered flux through the electron transport chains, resulting in
adjustments to the expression of genes utilized in respiration and redox balance
Srr, Arc sensors sense this ratio
Rex (transcriptional regulator)
Responds to alterations in
the NADH pool
4. Arc AB
Transition between respiratory and fermentative growth in E.coli is regulated by
ArcAB Two component regulatory system
Arc B
Arc A
Dimeric transmembrane sensor histidine kinase
Cytosolic response regulator
Under anaerobic conditions Under aerobic conditions
• Stimulates ArcB kinase activity
(Auto phosphorylation)
• Reduction in the ratio of
oxidized ubiquinone to
reduced menaquinone
• Increased ratio of oxidized
ubiquinone to reduced
menaquinone
• Inhibits ArcB kinase activity
• P- ArB phosphorylation of ArcA
• P-ArcA binds operator DNA of promoter
regions of target genes
5. The ArcB histidine kinase senses the redox status of the membrane soluble quinone pool through
redox-active cysteine residues located in the cytosolic PAS domain. Upon cysteine oxidation and
disulfide bond formation, ArcB autophosphorylation activity is low. Reduction of the disulfide
bond results in increased ArcB kinase activity, and subsequent phosphorylation of the cytosolic
response regulator ArcA. Phosphor-ArcA regulates transcription of downstream target genes via
binding to operator DNA and can function as both a transcriptional repressor and activator
• Cyseines in thiol form stimulates ArcB
kinase activity
• Cysteines in disulphide bond results in
decreased autophosphorylation
6. • Silencing of ArcB activity during the switch to anaerobiosis required the
presence of oxidized ubiquinone
• Activity of kinase activity during the switch to anaerobiosis required the
presence of reduced menaquinone
• Regulation of ArcB kinase activity is also stimulated by the fermentation
byproducts D-Lactate, Pyruvate and acetate
• Majority of the Arc regulon consists of genes utilized in the adaption to
microaerobic and fermentative growth
7. Srr AB two component regulatory system
Master regulator aiding the transition between respiratory and fermentative
growth in Staphylococcus aureus
Electron flux through respiratory pathways decrease during transition from
respiratory to fermentative growth (Reduced menaquinone)
What happens?
Srr AB system increases the transcription of genes involved in
fermentation and decrease transcription of genes encoding TCA cycle
8. Srr B
Srr A
Chromosomal histidine kinase
Response regulator
Under anaerobic conditions
Autophosphorylation of Srr B
P-SrrB phosphorylates the
response regulator Srr A
P- SrrA induces a conformational change
that alters the affinity for target DNA
Strains that grow fermentatively or
with inactivation of terminal oxidase
result in Srr B stimulation
9. Rex – a transcriptional regulator
INCREASE IN THE RATIO OF NADH : NAD+ Meaning respiration is disrupted
When NADH levels are low When NADH levels are high
Rex transcriptional regulator binds
to the operator of target gene and
repress the transcription of genes
involved in fermentation
Rex binds to NADH and prevents
itself from binding to DNA and
allow the transcription of genes
involved in fermentation
Rex homolog in Bacillus subtilis (YidH) – Regulates the transcription of
lactate dehydrogenases, formate or nitrite transporters
Rex – a major role in fermentation pathway
10. During O2 respiration, when the NADH:NAD+ ratio is low, NAD+ displaces NADH from
Rex, which then binds to the operators of target genes and represses transcription.
During decreased respiration under microaerobic conditions, an increase in the
NADH:NAD+ ratio results in binding of NADH to Rex and the dissociation from DNA
11. PRIORITIZING THE USAGE OF O2 AS ELECTRON ACCEPTOR
O2 SENSING TRANSCRIPTIONAL REGULATOR (contain prosthetic groups to sense oxygen)
FNR (Fumarate Nitrate reductase) protein
• Fe cluster utilizing O2 responsive protein
• Master regulator of the transition between aerobic and anaerobic
growth
• Controls the transcription of genes involved in anaerobic respiration
when O2 is absent
FNR activates the expression of genes encoding NO3
-, NO2
- and fumarate
reductase systems which can be used in the absence of oxygen
12. [4Fe-4S]2+ cluster binding promotes FNR homodimerization and enhances site-specific
DNA binding. In the presence of O2, the [4Fe-4S]2+-FNR is oxidized resulting in rapid
conversion to [2Fe-2S]2+ -FNR. Oxidized FNR is a monomer that has decreased affinity
for DNA and is inactive in direct gene regulation. Upon continued exposure to O2, the
[2Fe-2S]2+ cluster is further degraded to form apo-FNR which is inactive in DNA binding
13. PRIORITIZING THE USAGE OF TERMINAL ELECTRON ACCEPTORS OTHER THAN O2
NITRATE SENSORS (Nar and Nre system)
NO3
-, NO2
- dependent gene expression in E.coli is regulated by Nar two
component regulatory system
Nar system contains 2 two component regulatory systems
• Nar XL
• Nar PQ
• Nar Q senses NO3
-/ NO2
- concentration and autophosphorylates, which inturn
phosphorylates Nar P
• Nar X senses NO3
- concentration and autophosphorylates, which inturn
phosphorylates Nar L
Nar activates expression of genes involved in anaerobic respiration and fermentation
14. INTEGRATION OF SIGNALS
IN THE ABSENCE OF O2
Nar GHJI operon is induced by holo-FNR
NITRATE REDUCTASE
IN THE PRESENCE OF NO3
-
Phosphorylated Narl binds upstream of FNR binding location
of narGHJI operon and increases its transcription
IN THE PRESENCE OF FUMARATE DcuSR Two component system is activated
Increased levels of phosphorylated DcuR
Activates the transcription of frd operon that codes for FNR
(which utilizes fumarate as terminal electron acceptor)
