Topic 3-L4 - Catabolism in chemoorganotrophs Flashcards
There are three different terms used for how ATP is generated:
1) Substrate level phosphorylation.
2) Oxidative phosphorylation
3) Photophosphorylation
Substrate level phosphorylation:
ATP generated as a product of a metabolic reaction – energy from an exergonic reaction (energy-rich bone) used to power transfer of phosphate onto ADP to form ATP
Oxidative phosphorylation:
Energy from Electron transfer reactions generate a proton motive force, that is used to generate ATP using ATP synthase
Photophosphorylation:
Energy captured from light is used to generate a proton motive, that is used to generate ATP using ATP synthase
For many chemoorganotrophs, sugars like glucose are a preferred energy source ?
Can other organic compounds can also be used to generate energy ?
Yes
Yes
Many other sugars (disaccharides, polymers, monomers other than glucose) can be converted to
glucose or into an intermediate in glycolysis or the citric acid cycle
Glucose to CO2 – Slow and controlled oxidation, not a single step which would release a tremendous amount of energy all at once.
- not in a single step
- releases ALOT of E
- happens over a series of rxns in which high E substrates are oxidized to lower E molecules, ultimately into CO2
Is glycolysis conserved in all domains of life?
YES
Glycolysis
- doesn’t require O2
- Can be followed by either respiration or fermentation
2 stages for glycolysis
- investment stage and second stage
Investment stage
- requires ATP
- no redox rxns
- starts and ends with 6 carbon mol.
Stage 2 of glycolysis
- ATP generated via substrate-level phosphorylation
- ATP generated when intermediate I (phosphoenolpyruvate - PEP) – high energy
phosphate bond - pyruvate final product
(important metabolite)
Between investment stage and stage 2 of glycolysis
NAD+ reduced to NADH (redox rxn)
Substrate level phosphorylation in glycolysis produces
2 ATP per glucose
NADH (in glycolysis ) is useful for
producing additional
ATP
Glycolysis Lacks redox balance! Produces NADH, but no electron acceptor to regenerate NAD+. Redox balance can be restored using
fermentation or via the citric acid cycle/respiration.
Glycolysis generated
2 pyruvate and 2 NADH per glucose
Oxidation of pyruvate can be used to
generate a great deal more ATP using the citric acid cycle/respiration (preferred pathway for chemoorganotrophs
For organisms who can’t for respiration to oxidize pyruvate, they must use
Fermentation
Before the CAC, pyruvate is converted to
acetyl-COA – acetyl-COA then enters the citric acid cycle
What else can be fed into the CAC?
other organic molecules (lipids, amino acids, etc)
What else can the CAC provide?
provides key metabolic intermediates used anabolic reactions
The CAC is a Sentra hub for metabolism - not just found in
aerobic chemotrophs
Where does CAC take place?
Mitochondria for eukaryotes
Steps for CAC
1) 2C acetyl-CoA + 4C oxaloacetate = 6C citrate
2) via series of oxidations, citrate converted to 4C oxaloacetate which begins another cycle with addition of acetyl-CoA mol.
3) 2 redox rxns occur, no CO2 released from succinate to oxoacetate
4) oxaloacetate can be made from 3C compounds by addition of CO2
Substrate level phosphorylation in CAC produces
1 more ATP per pyruvate (2 per glucose)
CAC Produces how many NADH and FADH?
2 NADH and 1 FADH2 per pyruvate (2x per glucose)
This reducing power (NADH/FADH2) is fed into __________. Why?
electron transport chain
to generate more ATP or anabolic reactions (NADPH).
The electron transport chain is also known as
Respiration
Where does ETC take place
Takes place in the cytoplasmic membrane (inner mitochondrial membrane for eukaryotes)
In ETC, redox balance is
restored and oxidized forms of electron carriers (NAD+) regenerated
How does the ETC work?
1) Electrons passed down a series of electron carriers with increasingly +ve reduction potentials (Eo’) until a final electron
acceptor (‘external’ or ‘terminal’ electron acceptor) is reduced
2) Protons pumped out of the cell in the process – generates the proton motive force (ultimately used to generate ATP)
For aerobic respiration (most efficient) ___ is the terminal electron acceptor in ETC.
O2
For anaerobic respiration, what kind of terminal electron acceptor used?
A wide range of terminal acceptors can also be used (anaerobic respiration)
Electron carriers generated by
glycolysis/CAC
In ETC, NADPH is generally used primarily for
biosynthetic reactions (rather than ATP generation)
In ETC, key electron carriers are
1) NADH Dehydrogenase & Flavoproteins
2) Iron-sulfur proteins
3) Quinones (such as ubiquinone)
4) Cytochromes
NADH Dehydrogenase & Flavoproteins
- Where NADH electrons are deposited (to recycle back to NAD+)
- NADH Dehydrogenase transfers 2
electrons to a flavoprotein - Flavoproteins contain either FAD/
FADH 2 or FMN/FMNH2.
Iron-sulfur proteins
- Iron sulfur clusters are metal cofactors used by many different proteins involved in electron transfers
- ETC complexes often contains multiple Fe/S clusters
- Oxidation state and reduction potential
(Eo’) varies depending on nature of cluster & protein
Quinones (such as ubiquinone)
- they aren’t proteins – small
molecules that move within membrane - Accept 2 electrons, transfer to next
carrier in chain - Often serve to link Fe/S proteins to
cytochromes
Cytochromes
- Cytochromes are proteins that contain heme prosthetic groups (iron
coordinated within organic molecule) - Different proteins/different heme
groups, different reduction potentials - ETC Complexes often contain multiple
cytochromes – typically the last stop
before terminal acceptor
In the ETC, Electrons transferred from
lower reduction potential carriers (such as flavoproteins, Fe/S proteins) to higher reduction potential carriers (such as cytochromes) and then ultimately to their final electron acceptor (O2 for aerobic respiration).
In the final step of ETC, does the final electron acceptor get used up?
YES, needs of continuous source to keep going.
In the ETC, what are complexes (complex l, ll, lll)
multiple proteins (and often multiple electron acceptors). Can include proton pumps that couple energetically favourable electron transfer to proton pumping
Which complexes do e- enter at ?
Complex l and ll
ETC e- pathway through the complex l
Complex I starts with NADH (lower Eo’) – pumps 4 more protons per 2e…generates more energy
ETC e- pathway through the complex ll
Complex 2 starts with FADH2 (higher Eo’), pumps fewer protons
Once e- passed through either complex ll or l,
quinone is reduced – passes electrons
on to complex III
What’s the terminal e- acceptor
O2 (aerobic respiration) , generates H2O
- From NADH to H2O = 10 protons pumped (per NADH)
Individual microbes can operate multiple different electron transport chains with different components – sometimes simultaneously ?
Yes
Common electron examples that aren’t O2 are
NO2^- and SO4^(2-)
Do microbes have to always use a single terminal electron acceptor
NO, Some microbes have the ability to use multiple different terminal
electron acceptors, depending on availability
Can different electron donors be fed into ETC?
Yes
What is ATP synthase and where’s it found
Enzyme located in cytoplasmic membrane (prokaryotes) and in mitochondrial membrane (eukaryotes)
How does ATP synthase produce ATP?
Protons flow back along their
gradient –generates mechanical energy – used to power ADP à ATP (ATP synthesis)
ATP synthase - mechanism
Fo and F1 parts connected by stalk (y) – Fo in membrane – F 1 in cytosol
- Fo : P+ flow through, spins like turbine
- F1 : held in place, drives conformational change
How many H+ pumped and how many ATP made?
3.3 H+ pumped to generate 1 ATP
Is ATP formation in ATP synthase reversible?
YES. ATP hydrolysis can generate PMF
E.g. – fermentative organisms
Oxidative phosphorylation makes a lot of ATP?
YES
aerobic respiration of glucose
- Cell takes in: High energy electron donor (glucose) and excellent e- acceptor (O 2)
- Traps energy of moving those electrons to a lower energy state in the form of ATP
- Spits out fully oxidized CO2 and generates H2O as biproduct
Many chemoorganotrophs are metabolically flexible
Chemoorganotrophs have preferred energy sources (e.g. glucose) that they will use first, if available
- In a pinch (probably quite common) they can use of a wide range of high energy organic molecules
Many bacteria used a pathway called __________ to convert fatty acids to acetyl-CoA, which can then be fed into CAC/respiration
β-oxidation
Can chemoorganotrophs be consumed for E source
Amino acids can be converted to entry points to the citric acid cycle
(e.g., pyruvate).
Catabolite repression
if a better energy source (e.g. glucose) is around, enzymes to use other energy source inhibited/not expressed
The glyoxylate cycle
- Variation on the citric acid cycle
- Used in order to grow on 2-carbon
molecules like acetate/acetyl CoA - Because oxaloacetate (4 carbon)
gets drawn off for biosynthesis –
need to regenerate extra to run
citric acid cycle - Produces less reducing power (less ATP) but provides oxaloacetatebuilding block for synthesis of amino acids, glucose, etc.
E. coli is a facultative anaerobe meaning is can
live/grow with or without O2
E.coli can
- do aerobic respiration, anaerobic respiration & fermentation
- Can assemble different electron transport chains. Under anaerobic conditions, can respire using nitrate (if available). If not available – fermentation as a last resort
Nitrate respiration is less
efficient – pumps fewer protons than with O2
Fermentation
- Chemotrophic metabolism without the use of an external electron acceptor
- Anaerobic!
- Substrate-level phosphorylation can be used to generate ATP
- Redox balance achieved by excretion of reduced fermentation products
Lactic acid fermentation
- Fermentation regenerates NAD+ and so maintains redox balance
- Can continue to break down glucose for ATP…but generates little energy/ATP
- Some bacteria are heterofermentive lactic acid fermenters – they generate mix of lactose + other fermentation products. Can be useful to avoid lactate accumulation
Fermentation pathway used by
Yeast and to produce beer
Ethanol fermentation produces
3C pyruvate produces CO2 (1C) and
ethanol (2C) – both excreted
Ethanol fermentation
- NAD+ replenished
- Saccharomyces cerevisiae – yeast widely used in food/beverage fermentations
Ethanol fermentation…Beer!
- The ethanol generated by yeast alcoholic fermentation used to make
alcoholic beverages - The CO2 generated by alcoholic fermentation used to make dough rise
(alcohol mostly evaporated off when baking the bread) - The CO2 can also be used to naturally carbonate – beer (also sodas like root beer…before modern “forced carbonation technology)
In addition to glucose (via pyruvate), microbes can ferment a wide range of
organic compounds (E.g. fatty acids, amino acids, purine/pyrimidines, etc)
Common theme:
Generate an energy-rich molecule (bond) that can be hydrolyzed to produce ATP, donate electrons to (reduce) a metabolite and excrete to obtain redox balance
Fermentation can vary from a
last resort option to the sole source
of energy - depends on the microbe
Laxative acid fermentation makes
2 lactate and 2 ATP
Aerobic fermentation produces
6 CO2, 6 H2O, 38 ATP
