Poolman lectures 5-8: Metabolic diversity Flashcards
(26 cards)
What are the three mechanisms of energy coupling that can increase reducing power or achieve redox balance at the expense of energy?
- Coupling a reaction directly to ATP hydrolysis
- Reverse electron transport
- Electron bifurcation
What are two important functions of flavin-based bifurcating enzymes?
- They allow the cell to make highly electronegative intermediates that can drive difficult endergonic reductions and that can also be used to conserve energy through pmf formation.
- They can increase fermentative energy yields
Describe the major differences between assimilative and dissimilative processes.
Assimilative reductions are used to synthesize cellular material, and it requires energy (ATP).
Dissimilative reductions are used to generate energy in anaerobic metabolism and it generates energy by transferring electrons to an external electron acceptor.
What is the purpose of carboxysomes in cyanobacteria, and why
are they not present in the chloroplasts of plants and algae?
Carboxysomes improve the efficiency of RuBisCO dramatically because they
concentrate CO2 and exclude O2 at the site of RuBisCO activity. Chloroplasts of plants and algae don’t have carboxyxomes, because the cyanobacterial ancestor of chloroplasts didn’t need one, since CO2 levels were much higher and O2 levels lower back then.
How many electrons and how much ATP are required to make
one hexose molecule by the Calvin cycle?
12 NADPH (so, 24 electrons) and 18 ATP are required.
How many electrons and how much ATP are required to fix one molecule of N2 into two of NH3?
6 electrons are necessary to fix one molecule of N2 into two NH3, eight are actually being consumed in the process, along with 16 ATP.
Describe two ways in which N2-fixing organisms can protect
their nitrogenase from O2.
- O2 is removed rapidly through respiration
- O2-retarding slime layers on the outer surface of the cell.
What does the enzyme comples nitrogenase do?What is the sequence of electron transfer in nitrogenase?
Nitrogenase catalyzes nitrogen fixation. Nitrogenase consists of two complexes: dinitrogenase, and dinitrogenase reductase.
The sequence of electron transfer in nitrogenase is:
electron donor –> dinitrogenase reductase –> dinitrogenase–> N2
What is a heterocyst and what is its function?
A heterocyst is a differentiated cell in certain cyanobacteria where conditions are anoxic (despite the fact that photosynthesis and O2 production occur in neighboring cells). This is to protect nitrogenase so that it can fix N2 into organic compounds, which are then shared with neighboring photosynthetic cells.
Which type of light-sensitive pigment do plants, cyanobacteria, anoxygenic phototrophs and algea have?
- Plants, algae, cyanobacteria: chlorophyll
- Anoxygenic phototrophs: bacteriochlorophyll
What is the cause of the different wavelength absorptions of different purple bacteria species and what is the ecological significance of this?
Different species of bacteria synthesize photocomplexes of slightly different protein structure, affecting the absorption maxima of bacteriochlorophyll a. By employing different pigments with different absorption properties, this allows different species of bacteria to coexist in the same habitat, without competing with each other.
What is the function of antenna pigments and where are they located?
Antenna pigments are located outside of the photosynthetic reaction centers. They function to absorb light and funnel it to the reaction center, enabling the reaction center to recieve light energy that would otherwise be missed (like in low light conditions for example)
How are the photosynthetic pigments of purple bacteria arranged in the cell?
They are arranged in internal membrane systems that arise from invagination of the cytoplasmic membrane. Membrane vesicles called chromatophores or membrane stacks called lamellae are common membrane arrangements in purple bacteria.
What type of bacteria are you likely to find at the bottom of a microbial mat, or deep in lakes/inland seas and what enables these organisms to live there?
You’re likely to find green sulfur bacteria there. They have structures called chlorosomes, which function as giant antenna to capture even the lowest of light intensities.
Bacteriochlorophyll molecules of chlorosomes are not attatched to proteins. Chlorosomes contain bacteriochlorophyll c, d or e, and light energy absorbed by these antenna pigments is transferred to bateriochlorophyll a (in the reaction center in the membrane) through the FMO protein.
What is the main function of carotenoids?
Carotenoids serve as photoprotective agents that protect the cell from bright light. (It protects the cell from toxic forms of oxygen produced in photooxidation reactions)
What are the main light-harvesting systems of cyanobacteria and red algae?
Phycobiliproteins, which give them their characteristic color.
How do phycobilisomes allow cyanobacteria to grow at lower light intensities than would otherwise be possible?
Because energy transfer from phycobilisomes goes downhill from phycobilisomes to the reaction center.
For example: phycocyanin/ phycoerythrin –> allophycocyanin –> chlorophyll a of PSII
Why is reverse electron transport needed by phototrophs having Q-type reaction centers and not needed by those having FeS-type reaction centers?
Because Q-type reaction centers do not have the reducing power to reduce NAD+. They therefore need to employ reverse electron transport (‘uphill’, against their gradient) driven by the pmf.
FeS reaction centers, on the other hand, naturally have enough energy to NAD+ directly. In green sulfur bacteria, electrons from FeS-type centers directly reduce ferredoxin, which then transfers electrons to NAD(P)⁺ via ferredoxin-NADP⁺ reductase.
What types of reaction centers do purple and green sulfur bacteria have?
- Purple bacteria: Q-type reaction centers
- Green sulfur bacteria: FeS-type reaction centers
Which reaction center does photosystem I have?
And photosystem II?
Photosystem I has an FeS-type reaction center, whereas photosystem II has a Q-type reaction center.
In terms of intermediates, how does the Sox system differ from other sulfide-oxidizing systems?
The Sox system has sulfur dehydrogenase (SoxCD). In the absence of SoxCD, a sulfur atom bound to SoxYZ is added to a growing sulfur granule in the periplasm. The sulfur in the granule can be
reductively activated and transported to the cytoplasm where it is eventually oxidized to sulfite (SO3 2-) by the reverse activity of
DsrAB, an enzyme homologous to the enzyme sulfite reductase
found in sulfate reducing bacteria.
Why is only a very small amount of energy available from the
oxidation of Fe2+ to Fe3+ at acidic pH?
At acidic PH, the reduction potential of the Fe3+/Fe2+ couple is electropositive (+0.77 at PH 2).
Because of this electropositive reduction potential, reducing power (NAD(P)H) must be obtained by reverse electron transport, which costs energy. This is why only a small amount of energy is available.
What is the function of rusticyanin and where is it found in the cell?
Rusticyanin is located in the periplasm, and it accepts the electrons from cytochrome c located in the outer membrane, and transfers these to a periplasmic cytochrome c.
How can Fe2+ be oxidized under anoxic conditions?
