chapter 13 Brock - metabolic diversity of microorganisms Flashcards
Distinguish between oxygenic and anoxygenic photosynthesis…
In p/syn, ATP is generated from light and then consumed in the reduction of CO2.
Oxygenic - oxygen is produced (eg cyanobacteria)
Anoxygenic - oxygen is not produced (eg purple and green bacteria)
Photosynthetic pigments?
Chlorophylls and bacteriochlorophylls reside in membranes where the light reactions of p/syn are carried out.
Antenna chlorophylls harvest light energy and transfer it to a reaction centre.
Accessory pigments?
eg carotenoids and phycobilins absorb light and transfer the energy to reaction centre chlorophyll - this broadens the wavelengths usable in p/syn.
Carotenoids also have photoprotective role, preventing photooxidative damage to cells.
ATP generation and CO2 fixation in anoxygenic phototrophs…
Electron transport reactions occur in the p/syn reaction centre of anoxygenic phototrophs, forming a PMF and ATP.
Reducing power for CO2 fixation comes from substances like H2S.
NADH production in purple bacteria requires reverse electron transport.
Just one photosystem in anoxygenic phototrophs.
In oxygenic photosyn…
H2O donates electrons to drive CO2 fixation, and O2 is a by-product.
There are two separate but interconnected photosystems in oxygenic phototrophs: PSI and PSII
Tell me about autotrophy…
autotrophy is supported in most phototrophic and chemolithotrophic bacteria by the Calvin cycle - key role of RubisCO.
Carboxysomes contain crystalline RubisCO - they concentrate CO2, the key substrate for this enzyme.
What are the autotrophic pathways for green sulfur and green nonsulphur bacteria?
green sulphur - reverse citric acid cycle
green nonsulphur - hydroxyproprionate cycle
Energy conservation in chemolithotrophs?
chemolithotrophs oxidise inorganic electron donors to conserve energy and obtain reducing power.
Energy conservation occurs from respiratory processes that generate a PMF. Most chemolithotrophs can grow autotrophically.
What about chemolithotrophic hydrogen bacteria?
use H2 as an electron donor, reducing O2 to H2O.
Enzyme hydrogenase is required to oxidise H2, and H2 also supplies reducing power for the fixation of CO2 in these autotrophs.
And sulphur chemolithotrophs?
Reduced sulphur compounds such as H2S, S2O3 2- (thiosulphate ion) and S zero are electron donors for energy conservation.
The electrons enter ETC, yielding a PMF.
Sulphur chemolithotrophs are also autotrophs and fix CO2 by the Calvin cycle.
And chemolithotrophic iron bacteria?
oxidise Fe2+ as an electron donor.
What if there are no external electron acceptors?
organic compounds can be catabolised anaerobically only by fermentation. Most fermentations require that an energy-rich organic compound be formed that can yield ATP by substrate-level phosphorylation. Redox balanced is achieved by the production of fermentation products.
Explain syntrophy
Two organisms cooperate to degrade a compound that neither can degrade alone. In this process H2 produced by one organism is consumed by the partner. H2 consumption affects the energetics of the reaction carried out by the H2 producer, allowing it to make ATP where it otherwise could not.
What’s the most widely used electon acceptor in energy-yielding metabolism?
Oxygen.
However other organic and inorganic compounds can be used as electron acceptors - lower energy yield in anaerobic respiration but can proceed in environments where oxygen is absent.
Tell me about nitrate’s common role as an electron acceptor
Anaerobic respiration.
Nitrate reduction is catalysed by the enzyme nitrate reductase, reducing NO3- to NO2-.
Many bacteria that use NO3- in anaerobic resp. produce gaseous nitrogen compounds (NO, N2O or N2) as final end products of reduction (denitrification).
sulphate-reducing bacteria are obligately anaerobic
they reduce sulphate (SO4 2-) to H2S in a process in which SO4 2- must first be activated to adenosine phosphosulphate (APS).
Some species cannot reduce sulphate but can reduce sulphur zero to H2S (Desulfuromonas)
What do acetogens reduce?
Acetogens are strict anaerobes - they reduce CO2 to acetate, usually with H2 as the electron donor.
Acetate formed via acetyl-CoA pathway - found widely in obligate anaerobes for either autotrophy or acetate oxidation.
What’s methanogenesis?
The production of CH4 from CO2 and H2 or from acetate or methanol by strictly anaerobic methanogenic Archaea. Needs several coenzymes, and energy conservation linked either to PMF or sodium motive force.
What other substances apart from inorganic N and S and CO2 act as electron acceptors for anaerobic respiration?
eg Fe3+, Mn4+, fumarate, some organic and chlorinated organic cpds, and H+
What’s methanotrophy?
the use of CH4 as both carbon source and electron donor, and the enzyme methane monooxygenase is the key enzyme in the aerobic catabolism of methane.
RubisCO is the acronym for…
ribulose bisphosphate carboxylase - a key enzyme of the Calvin cycle
Calvin cycle
the biochemical pathway for CO2 fixation in many autotrophic organisms.
autotroph
an organism that uses CO2 as its sole carbon source
acetyl-CoA pathway
a pathway of autotrophic CO2 fixation and acetate oxidation widespread in obligate anaerobes including methanogens, acetogens, and sulphate-reducing bacteria.
chlorosome
a cigar-shaped structure present in the periphery of cells of green sulphur and green nonsulphur bacteria adn containing the antenna bacteriochlorophylls (c, d or e)
phycobilisome
an aggregate of phycobiliproteins - phycobiliproteins are the antenna pigment complexes in cyanobacteria that contain phycocyanin and allophycocyanin or phycoerythrin coupled to proteins.
reaction centre
a photosynthetic complext containing chlorophyll or bacteriochlorophyll and several other components, within which occur the initial electron transfer reactions of photosynthetic electron flow
reverse citric acid cycle
a mechanism for autotrophy in green sulphur bacteria adn a few other autotrophic Bacteria and in some Archaea.
reverse electron transport
the energy-dependent movement of electrons against the thermodynamic gradient to form a strong reductant from a weaker electron donor.
secondary fermentation
a fermentation in which the substrates are the fermentation products of other organisms
thylakoids
membrane stacks in cyanobacteria or in the chloroplast of eukaryotic phototrophs.
