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BIO3093 energy metabolism > energy metabolism > Flashcards

energy metabolism Flashcards

1
Q

chemiosmotic hypothesis

A

Coupling of phosphorylation to electron and hydrogen transfer by a chemiosmotic mechanism

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2
Q

proton motive force

A

proton electrochemical gradient ^p
defined by 2 factors:
proton gradient ^pH
membrane potential ^psi

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3
Q

different types of energy transducing membranes

A

mitochondrial inner membrane
bacterial inner membrane
photosynthetic bacterial membrane
chloroplast inner membrane

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4
Q

morphology of mitochondria energy transducing membranes

A

matrix is N
inner membrane space is P

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5
Q

morphology of chloroplast energy transducing membranes

A

light-driven proton pumping from N (stroma) to P (thylakoid lumen)
thylakoid membrane is arranged in stacked folds

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6
Q

morphology of bacterial energy transducing membranes

A
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7
Q

proton current

A

flow of protons through a system

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8
Q

respiratory control

A

the equilibrium between [ADP] and [ATP] or pH can be disturbed to promote the shift of equilibrium in a favourable direction

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9
Q

eqn: observed mass action

A

[ATP]/[ADP]

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10
Q

when [ATP]/ADP] is at equilibrium it is called

A

K equilibrium constant

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11
Q

Gibbs energy increases as

A

[ATP]/[ADP] is further displaced from K

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12
Q

4 stages of eukaryotic aerobic respiration

A

glycolysis
oxidative decarboxylation of pyruvate > Acetyl CoA
citric acid/krebs cycle
oxidative phosphorylation

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13
Q

morphology of photosynthetic bacterial energy transducing membranes

A

invaginated cytoplasmic membrane
can pinch of to form chromatophores at high pressures

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14
Q

proton gradient across a membrane establish…

A

N- (-ve) and P- (+ve) phase compartment

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15
Q

proton gradients are coupled to…

A

ATP synthesis

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16
Q

7 mitochondrial respiratory proteins (in spatial order)

A

Complex 1
Complex 2
UQ
Complex 3
Cytochrome C
Complex 4
ATP synthase

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17
Q

redox carriers in mit. ETC

A
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18
Q

bacterial Complex 2 proper name, structure & function

A

Succinate/ ubiquinone oxidoreductase
4 subunits: 2 hydrophilic (flavoprotein & iron-sulphur protein), 2 hydrophobic membrane proteins (heme b-containing and ubiquinone binding site)
3D packed as trimer
e- transfer occurs within monomers

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19
Q

bacterial SdhB (hydrophilic) subunit of Complex 2 contains

A

3 iron sulphur clusters
[2Fe-2S], [4Fe-4S] and [3Fe-4S]

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20
Q

bacterial SdhA (hydrophilic) subunit of Complex 2 contains

A

covalently attached FAD cofactor and substrate binding site

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21
Q

what is unusual about bacterial Complex 2 SdhB [2Fe-2S] cluster?

A

coordination with 3 Cys & 1 Asp

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22
Q

ubiquinone binding site of bacterial Complex 2 structure

A

cleft between SdhB, C & D close to [3Fe-4S]
side chains of Tyr 83 & Trp 164 are direct ligands to ubiquinone

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23
Q

role of heme B in bacterial Complex 2

A

electron sink for those not passed to UQ
high redox potential attracts e-
close to UQ binding site

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24
Q

structure & function of mitochondrial Complex 2

A

Two hydrophilic subunits
Flavoprotein (FP)
Iron-sulphur protein (IP)
Two hydrophobic membrane subunits
CybL and CybS, that contain one heme b and provide two ubiquinone binding sites.

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25
Q

name & describe advantage of the 2 mit. UQ binding sites

A

Qp and Qd
favours the transfer of two electrons from reduced FADH2 to ubiquinones with high efficiency as it increases stability of bound UQ over that increased time period of e- transfer

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26
Q

3 distinct quinone redox carriers across kingdoms

A

UQ in mitochondria
menaquinone in anaerobic bacteria
plastoquinone in chloroplasts

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27
Q

proper name of Complex 3

A

Q-cytochrome c oxidoreductase

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28
Q

Q-cycle

A

coupling of e- transfer from Q > cyt c to transmembrane proton transport

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29
Q

in Q cycle, UQ is reduced to

A

semi-quinone anion Q-

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30
Q

Q-cycle mechanism summary

A

2molecules of QH2 are oxidised > 2 molecules of Q.
1 molecule of Q is reduced > QH2.
2 molecules of cytochrome c are reduced.
4 protons are released to the cytoplasmic side.
2 protons are removed from the matrix side.

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31
Q

Q-cycle inhibitors & modes of action

A

Myxothiazol – blocks reactions at the QP (Qo) site
Antimycin – acts on the QN (Qi) site preventing the formation of the relatively stable UQ.-.
Stigmatellin – inhibits electron transfer to the Rieske protein.

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32
Q

gated control of Complex 3 by Rieske ISP

A

Reduction of the Rieske centre causes the ISP to move to a new docking position close to cytochrome c1.
after E- > c1, ISP has reduced affinity for c1 heme and moves back > Q site.
ISP is bound to c1 is too far from Qp site to accept e-
so 2nd e- must > bL heme.

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33
Q

Complex 4 proper name

A

cytochrome c oxidase

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34
Q

5 stages of oxygen reduction in Complex 4

A
  1. formation of oxy species
  2. formation of P species
  3. formation of F species
  4. formation of hydroxyl
  5. condensation
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35
Q

2 proton channels speculated to be responsible for proton pumping

A

K (conserved aspartate) and D (lysine)
these aminos have their side chains projecting into the respective channels
not found in all mitochondrial and bacterial enzymes

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36
Q

supercomplex

A

arrangement of ETC proteins into a supramolecular structure that aids e- and proton transfer

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37
Q

redox potential

A

ability of redox couple to attract e-
negative E0 = lower affinity for e- than standard (more likely to form ions)
positive E0 = higher affinity for e- than standard (less likely to form ions)

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38
Q

what happens when the gain of e- changes the pKa of 1 or more ionizable groups?

A

reduction is accompanied by gain or 1 or more protons

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39
Q

when NAD+ undergoes 2e- reduction it gains _ protons

A

1

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40
Q

when UQ undergoes 2e- reduction it gains _ protons

A

2

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41
Q

when cyt c undergoes 2e- reduction it gains _ protons

A

0

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42
Q

how do prosthetic groups increase e- transfer speed?

A

if they are in contact they can act as e- tunnels

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43
Q

usually, the max separation between prosthetic groups forming an e- tunnel is

A

14 Angstrong

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44
Q

e- flow from centres of _ E0 to centres of _ E0

A

low > high E0

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45
Q

in what part of complex IV does the biochemistry take place

A

2 core subunits
ancillary proteins have little/unidentified effect on biochem

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46
Q

which complex IV proton channel is responsible for chemical proton uptake for catalysis

A

K with conserved lysine

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47
Q

which complex IV proton channel is responsible for proton uptake/pumping for pmf

A

D for aspartate

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48
Q

bacterial etc location

A

P = periplasm
N= matrix
ETC proteins embedded in cytoplasmic membrane

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49
Q

bacterial etc differs from mit etc

A

shorter and more flexible
i.e. no complex 3, all e- are delivered directly to and accepted from quinone pool

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50
Q

Complex IV differences in bacteria compared to mit.

A

-different structure haem of haem-copper centre
-lower affinity for oxygen, so when [O2] gets too low it switches to a cyt b oxidase with higher O2 affinity and no proton pump

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51
Q

which of the two is membrane embedded: NAP or NAR?

A

NAR

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52
Q

which of the two is more bioenergetically efficient: NAP or NAR

A

NAR, uses 2 protons from N to reduce NO3- to NO2-

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53
Q

cofactors, by subunit, of NAR

A

NarG MGD & [4Fe-4S]
NarH 3x [4Fe-4S] 1x [3Fe-4S]
NarL 2x bis-coord cyt c

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54
Q

cofactors, by subunit, of NAP

A

NapA MGD-Cys & [4Fe-4S]
NapB 2x bis-coord cyt c
NapC 4x bis-coord cyt c

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55
Q

what pathways occur in chloroplasts?

A

respiration, photosynthesis, other biosynthetic pathways

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56
Q

amyloplast

A

plastid specialised for starch synthesis and storage

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57
Q

light independent reactions of photosynthesis name and function

A

Calvin-Benson-Bassham cycle
fixation of CO2 to make sugars, with help from ATP and NADPH from light dependent reactions

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58
Q

light independent reactions of photosynthesis name and function

A

light energy harvested to convert NADP+ and ADP > NADPH and ATP

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59
Q

key enzyme of CBB cycle

A

Rubisco

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60
Q

give the 2 products of photosynthesis

A

starch & sucrose

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61
Q

how is the electrochemical gradient created in chloroplasts

A

H+ pumped > thylakoid lumen during e- transport
this is used for ATP synthesis

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62
Q

electron shuttle proteins in thylakoid membranes

A

plastoquinone and plastocyanin

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63
Q

how is NADP+ reduced in the thylakoid

A

PSI reduces ferredoxin
reduced Fd will reduce NADP+ with catalysis from ferredoxin-NADP reductase

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64
Q

order of photosynthetic proteins

A

PSII
b6f
PSI
FNR
F-ATPase

65
Q

chloroplast assembly/proteins is coded by _ DNA

A

nuclear and chloroplast

66
Q

PSII structure

A

Mn-containing oxygen evolving complex (OEC) held in Mn stabilising protein (MSP)
core: D1 and D2 proteins. D1 tyr is essential for e- transport
various chlorophyll binding proteins
associated with light harvesting complexes (LHC)

67
Q

what is D1 repair and why is it necessary?

A

PSII polypeptides, particularly D1 get oxidative damage by P680+ and 1^O2 radicals
Stn8 kinase activated by reduced PQ(H2) pool phosphorylates the damaged polypeptides.
the complex is disassembled (modular structure) and only D1 is resynthesised and integrated back in.

68
Q

cofactor of ferredoxin-NADP reductase

A

flavin a.k.a FAD

69
Q

how is photosynthetic activity tuned to FNR availability?

A

In the dark at low stromal pH, binding is strong and FNR is sequestered and stabilised on the thylakoids.
In the light, stromal pH increases, releasing FNR to catalyse NADP reduction.

70
Q

Photon flux density units

A

umol quanta m-2 s-1

71
Q

photon flux density/irradiance =

A

flux of radiant energy per unit area

72
Q

as irradiance increases, excess light

A

also increases

73
Q

other conditions that can increase excess light

A

stressful conditions that limit CO2 fixation like low temp, drought etc

74
Q

long term adjustment to light level fluctuations

A

thickness of leaf, less light = less cells required for photosynthesis, so thinner leaf.

75
Q

PPFD or photosynthetic photon flux density =

A

amount of quanta in the photosynthetic range

76
Q

photosynthetic wavelength range

A

400-700 nm

77
Q

leaves grown at higher PPFD have a _ light saturated CO2 assimilation rate because of _

A

higher
because of thicker leaves through which high light intensity can penetrate and higher [rubisco] and other enzymes in CBB cycle

78
Q

leaf movement purpose

A

short term adjustment to gain more light or avoid light
light perceived by blue light absorbing phototropin photoreceptors

79
Q

chloroplast movement purpose

A

short term adjustment, directed by cytoskeleton which chloroplasts are tethered to to gain more light or avoid
light perceived by blue light absorbing phototropin photoreceptors

80
Q

phototropin photoreceptors

A

absorb blue light
contain flavin cofactor
located outside chloroplast
PHOT1 & PHOT2

81
Q

coordination between _ and _ is required to adjust to environmental changes

A

nucleus and chloroplast because they each contain genes for essential protein subunits

82
Q

2 mechanisms for rapid response to excess energy excitation

A
  1. state transitions to balance PSII and PSI excitation
  2. preventing excitation energy absorbed by LHCs from reaching photosystem reaction centres. excess energy re-radiated as heat via non-photochemical quenching
83
Q

NPQ mechanism

A
  1. photochemistry
  2. formation of triplet excited chlorophyll
    which results in singlet oxygen and photo-oxidative stress
  3. fluorescence emission
  4. dissipation as heat
84
Q

triplet excited chlorophyll =

A

pair of electrons are split between two orbitals of exceeding energy with unpaired spin (spin in same direction)

85
Q

singlet excited chlorophyll =

A

pair of electrons are split between two orbitals of exceeding energy with paired spin (spin in opposite directions)

86
Q

singlet ground chlorophyll =

A

pair of electrons in same orbital with paired spin (spin in opposite directions)

87
Q

damage caused by reactive singlet oxygen

A

damage to thylakoid membrane, particularly reaction centre components
oxidises highly unsaturated fatty acids in membrane producing lipid radicals and hyperoxides

88
Q

SOSG =

A

singlet oxygen sensor green
fluorescent probe to visualise singlet oxygen production

89
Q

SOSG mechanism =

A

-SOSG > leaves
-becomes fluorescent when oxidised by singlet oxygen
-this form an endoperoxide

90
Q

measuring NPQ by chlorophyll fluorescence analysis (2 methods)

A
  1. illuminate with blue light and measure red fluorescence at wavelength specific to PSII
  2. illuminate with white light that pulses at fixed frequency and measure PSII red fluorescence emitted at same frequency. (modulated fluorimetry)
91
Q

extent of fluorescence emission (F) from chlorophyll depends on

A

photosynthetic activity
i.e. if photochemistry is slow, F increases
or if excitation energy is dissipated by NPQ, F decreases

92
Q

quantum efficiency

A

mol O2 produced /mol quanta absorbed

93
Q

ratios between fluorescence signals can be used to calculate

A

quantum efficiency
photochemical quenching
non-photochemical quenching

94
Q

photochemical quenching

A

estimate of redox state of plastoquinone PSII acceptor

95
Q

NPQ comprises 3 components

A

qE
qI
qT

96
Q

qE =

A

energy dependent quenching
rapidly reversible
main component (usually)

97
Q

qI =

A

photoinhibitory quenching
from inactivation of PSII reaction centres
important in severe excess light

98
Q

qT =

A

state transitions
movement of LHCII between PSII (granal stacks) and PSI (unstacked stromal lamellae)

99
Q

2 components of qE

A

both activated by acidification of thylakoid lumen as light intensity and H+ pumping increases
1. xanthrophyll cycle
2. PsBs protein

100
Q

xanthrophyll cycle

A

VDE (activate by low pH) converts violaxanthin to zeaxanthin
less polar Z associates with LHCs and absorbs excess excitation energy, efficiently radiates this as heat
low light = pH increase, = VDE activity decrease, = Z to V by enzymes

101
Q

PsbS protein

A

associates with PSII and LHC to facilitate structural changes when protonated

102
Q

PsbS protein is not essential for qE but is needed for

A

rapid response to large light intensity increase

103
Q

how is NPQ a problem for crop photosynthesis

A

NPQ takes a long time to relax after high light intensity events
while NPQ is high, CO2 fixation is limited because of decreased e- transfer rate
can fix by increasing ZEP (Z > V) activity

104
Q

photoprotective role of carotenoids

A

in algae: absorb excess light before it enters chlorophyll

105
Q

qT mechanism

A

PSII gets overexcited so STN7 kinase activates and phosphorylates LHCII
surface charge change induces LHCII movement > PSI
excess excitation of PSI decreases STN7 activity allowing dephosphorylation and movement > PSII

106
Q

how is excitation imbalance between PSII and PSI sensed to activate STN7

A

redox state of plastoquinone acts as sensor
activates STN7 when reduced

107
Q

how can Plastoquinone measure PSII and PSI relative activity

A

lies between them
PQ redox state controls expression of photosynthesis-associated genes in chloroplast and nucleus

108
Q

how was cyclic electron transport shown to act as a protectant for PSI in high and fluctuating light intensity

A

mutation to PGR5 in Arabidopsis causes sensitivity to fluctuation of light intensity
Yamamoto and Shiknai 2019
PGR5 = proton gradient regulating protein

109
Q

other electron sink proteins/reactions

A

flavodoxins
flavodiiron proteins
alternative oxidase (AOX)
mehler reaction

110
Q

flavodoxin mechanism

A

-alternative low potential PSI e- acceptor in photosynthetic bacteria and algae
-flavin mononucleotide e- carrier (less susceptible to ROS damage than FeS)
-slower turnover
-functional when expressed in plants

111
Q

flavodiiron mechanism

A

-additional PSI e- acceptor
-cyanobacteria, algae and vascular non-flowering plants
accpet e- from PSI and reduce O2 to water
-evidence based off mutants

112
Q

complex I name and general structure

A

-NADH:ubiquinone oxidoreductase
-membrane embedded arm, periplasmic arm
-periplasmic arm contains Fe-S wire
-membrane embedded arm contains proton pumps, discontinuous helices that form dipoles
-may undergo conformational change to bring UQ binding site close enough for e- transfer from Fe-S
-cage of 30 supernumerary subunits around core units in mammalian cells protect from oxidative damage
-cage does not protect subunit that binds NADH so possibly this is replaced once damaged

113
Q

complex 1 function

A

take 2e- from NADH and pass onto UQ
pumps 4H+

114
Q

what is photoprotection

A

process of removing excess light energy absorbed by chloroplasts to prevent damage to photosystems and photosynthetic inhibition.

115
Q

photon flux density

A

amount of photosynthetically active photons (400-700nm) hitting a surface per unit area per unit time.
umol quanta m-2 s-1

116
Q

how do leaves grown under high light intensity for long periods of time differ from leaves grown under low light intensity for long periods of time and why?

A

high intensity = thicker leaves to make use of high incoming light intensity; also exhibit higher CO2 assimilation rate and photosynthesis
lower intensity = thinner and lower CO2 assimilation rate and photosynthesis

117
Q

how is photosynthetic CO2 assimilation measured

A

passing air through leaf chamber clipped to leaf surface and measuring the change in [CO2] with an infrared gas analyser

118
Q

what protein is important for light sensing and photosynthetic efficacy

A

phototropin senses blue light with a flavin cofactor
PHOT1 & PHOT2
involved in movement of chloroplasts in response to short term light fluctuation

119
Q

short term adjustments of chloroplasts in response to high and low light intensity

A

high = chloroplasts move to side of leaves to shade one another and prevent excess excitation energy

low = chloroplasts move to top of leaves to harness as much light energy as possible per chloroplast

mediated by phototropin

120
Q

phenotypic effect of PHOT2 mutant

A

leaf cells unable to move chloroplasts and susceptible to photoinhibition

121
Q

adjustment to prevailing environmental conditions requires… (photosynthesis)

A

retrograde signalling

122
Q

what is retrograde signaling

A

general = signal travels backward from target to original source (signals made from nuclear genome)
specific = co-ordination between expression of photosynthetic genes in chloroplast and nucleus

123
Q

how does retrograde signaling work and what is the point

A
  • metabolic conditions in chloroplast generate signals
  • signals -> nucleus
  • signals influence nuclear gene expression
  • enables the correct number of photosynthetic machinery to be synthesised and from nuclear genome for use in chloroplasts
124
Q

2 molecular pathways for rapid response to excess excitation energy

A
  1. state transitions to balance PS1 and PS2 excitation
  2. preventing LHC-absorbed excitation energy from reaching reaction centres by re-radiating it as heat (non-photochemical quenching/NPQ)
125
Q

alternative fates of excited chlorophyll in LHC

A
  1. normal photosythesis
  2. formation of triplet excited chlorophyll during photosynthesis
    triplet excited chlorophyll -> singlet oxygen production and photo-oxidative stress
  3. fluorescence emission
  4. dissipation as heat
126
Q

LHC

A

light harvesting complexes

127
Q

why is triplet chlorophyll dangerous

A

interacts with oxygen to form singlet oxygen which is highly electrophilic and causes damage to thylakoid compartment and reaction centre components
- oxidises highly unsaturated fatty acid in thylakoid membrane to produce lipid radicals and hydroperoxides

128
Q

method of visualising singlet oxygen production

A

singlet oxygen sensor green
- fluorescent probe which permeates into chlorophyll and reacts specifically to singlet oxygen when produced
- high light increases SOSG oxidation
- DCMU blcoks Qa plastoquinone binding site of PSII and increases SOSG oxidation

129
Q

why does blocking the Qa binding site of PSII with DCMU increase SOSG oxidation?

A
  • halts electron transfer
  • this causes build up of triplet and singlet chlorophyll
130
Q

how to measure NPQ and why it is done this way

A
  1. illuminate with blue light and measure red fluorescence at PSII specific wavelength
  2. illuminate with white light pulsing at fixed frequency and measure at same PSII specific wavelength
  • modulated fluorimetry allows measurements when leaves are photosynthesising under normal illumination
131
Q

what does red fluorescence in modulated fluorometry of NPQ mean

A

excited singlet chlorophyll returns to ground state by the emission of red fluorescence

132
Q

how and why is the Fm signal generated

A

how
- fluorescence signal emitted after leaves treated with high intensity light flash
why
- enables calculation of Fv/Fm which is proportional to quantum efficiency of photosynthesis

133
Q

why is Fv/Fm a good measure of the state/ efficiency of photosynthetic apparatus

A

decreases under conditions that cause damage to photosynthesis

134
Q

what is the purpose of photochemical quenching

A

gives an estimate of the redox state of plastoquinone PSII 2e- acceptor and ergo used to estimate photosynthetic e- transfer rate

135
Q

why do plants grown in lower light intensities emit more chlorophyll fluorescence when exposed to high light

A

lower photosynthetic capacity so a larger proportion of the incident light is emitted as fluorescence.
these plants also generate higher rates of NPQ

136
Q

nigericin

A
  • uncoupling agent
  • prevents build up of trans-thylakoid membrane pH gradient
  • this greatly decreases NPQ when light is on
137
Q

NPQ is dependent on

A

development of pH gradient across thylakoid membrane

138
Q

xanthophyll cycle mechanism

A
  • violaxanthin de-epoxidase (VDE) converts violaxanthin (V) to zeaxanthin (Z)
  • VDE activated by low pH, using ascorbic acid as reductant
  • less polar zeaxanthin associates with LHCs where it absorbs excitation energy from chlorophyll and efficiently re-radiates it as heat
  • In low light, lumen pH increases, VDE activity decreases, and the conversion of Z to V by zeaxanthin epoxidase
139
Q

PsbS protein mechanism

A
  • associates with PSII and LHC and facilitates structural changes when protonated
140
Q

how does qE change the structural arrangement of PSII-LHCII and how does this contribute to energy release

A
  • normal pH, V present and LHC in trimers
  • when high light present, acidification of thylakoid lumen triggers V->Z, trimers of LHC aggregate
  • in aggregated state, excitation energy of chlorophylls excites Z
  • Z structure favours the ability to lose excitation energy as heat
141
Q

what did chlamydomonas carotenoid npq1 and lor1 show?

A

both pigments requirements for NPQ and survival of cells in high light
mutant npq1 grew okay,
while lor1 didn’t and was pale
double mutant completely bleached by high light

142
Q

how does NPQ limit rate of photosynthesis during crop production?

A

time taken for NPQ to relax after exposure to high light
NPQ diverts excitation energy away from photosystem two reaction centre, while it is operating the rate of photosynthesis efficiency of photosynthesis will be lower

143
Q

how was expedited NPQ relaxation engineered

A
  • speed up the rate of NPQ relaxation by increasing the zeaxanthin epoxidase (ZEP) activity
  • removes zeaxanthin more rapidly in low light
  • however also necessary to balance the increase in ZEP activity ith violaxanthin de-epoxidase and PsbS expression.
  • overall strategy: transgenic plants in which all three proteins expressed at higher level.
144
Q

how was expedited NPQ relaxation engineered

A
  • speed up the rate of NPQ relaxation by increasing the zeaxanthin epoxidase (ZEP) activity
  • removes zeaxanthin more rapidly in low light
  • however also necessary to balance the increase in ZEP activity ith violaxanthin de-epoxidase and PsbS expression.
  • overall strategy: transgenic plants in which all three proteins expressed at higher level.
  • advantage only in naturally fluctuating light, didn’t effect plants grown in labs under constant light intensity
145
Q

cytoplasmic carotenoids may shield chloroplasts from

A

excess light

146
Q

how is balancing of excitation achieved by state transitions?

A
  • PSII is over-excited, protein kinase STN7 is activated and phosphorylates LHCII
  • change in surface charge induces LCHII movement to towards PSI
  • Excess excitation of PSI decreases STN7 activity allowing dephosphorylation and migration back to PSII
147
Q

what is the importance of balancing excitation between photosystems via state transitions

A

If photosystem two becomes overexcited compared to photosystem one there will be an imbalance in electron transport potentially leading to photoinhibition.

148
Q

favoured hypothesis of state transitions STN7 activation

A
  • redox state of plastoquinone acts as the sensor and activates STN7 when it becomes more reduced.
  • plastoquinone (PQ) lies between PSII and PSI so its redox state can measure their relative activity.
149
Q

key evidence for favoured hypothesis of state transitions STN7 activation

A
  • DCMU blocks electron transport before plastoquinone
  • DBMIB blocks electron transport after plastoquinone
  • these inhibitors have opposite effects on the redox state of plastoquinone and they affect state transitions correspondingly
150
Q

how does plastoquinone act as a signal besides during state transition

A
  • acts as a signal to activate gene expression in the nucleus
  • proteins involved in longer term acclimation to high light can be synthesised and transported into the chloroplasts
151
Q

how do arabidopsis mutant studies show cyclic e- transport has a photoprotective role

A
  • pgr5 mutants have increased sensitivity to fluctuating light
  • NDH-dependent route involves three distinct NADPH dehydrogenases.
  • Knocking out all three of these genes in tobacco causes it to be more sensitive to high temperature disruption of photosynthesis.
  • deficiency of pgr5 leads to degradation of photosystem one
152
Q

4 alternative e- sinks for photosynthetic e- transport

A

Flavodoxins: flavin mononucleotide- FMN proteins act additionally to ferredoxin. Found in bacteria and some algae but not flowering plants.

Flavodiiron proteins: group of diiron/FMN oxidases. accept electrons from PSI, reducing oxygen to water. Occurs in cyanobacteria, algae, mosses and ferns but not flowering plants.

Mehler reaction: transfer of e- from PSI to oxygen forming hydrogen peroxide. PSI reaction centre may act as e- donor.

Alternative oxidase (AOX): mitochondrial diiron oxidase accepting e- from Complex 1. Reduces oxygen to water, bypassing proton transport and ATP synthesis. strong evidence that leaf mitochondria can oxidise excess reductant exported from chloroplasts and imported into mitochondria as NADH. AOX mutants are susceptible to photoinhibition in chloroplasts. NADH consumption is also aided by activation of uncoupling proteins.

153
Q

flavodoxin as alternative low potential PSI e- acceptors

A
  • Flavodoxins can act additionally to ferredoxin (2Fe2S) in cyanobacteria, some algal groups and mosses.
  • Thought to have evolved in response to Fe deficiency following appearance of oxygen from cyanobacterial photosynthesis.
  • Flds contain a flavin mononucleotide (FMN) electron carrier instead of FeS, rendering them less suspectable to damage by oxygen and reactive oxygen species.
  • They have a slower turnover than Fd but are functional when expressed in plants, being able to interact with FNR to reduce NADP+.
  • Fe deficiency (which is common in the ocean) induces algal Fld, decreasing the Fe requirement for photosynthesis
  • Under severe stress Fld can be inactivated by oxidation of the FeS centre and Fe loss. Plants expressing cyanobacterial Fld maintain photosynthesis better under various stresses additionally to Fe deficiency.
154
Q

flavodiiron proteins provide an additional PSI e- acceptor to ferredoxin

A
  • in bacteria, algae and vascular plants
  • evidence they accept electrons from PS1 and reduce oxygen to water.
  • Evidence based on mutants in various flavodiiron (Flv) proteins: decreased formation of H218O from supplied 18O2 measured by mass spectrometry; greater reduction of the PSI reaction centre chlorophyll (P700) measured by P700 absorbance.
155
Q

3 ways antioxidant systems works

A
  • removal of ROS
  • prevention of radical formation
  • repair damaged molecules
156
Q

oxidative damage

A

ROS can result in further reactions that result in production of free radicals and damage to biomolecules

157
Q

ROS

A

electronically excited oxygen species

158
Q

superoxide/hydrogen peroxide + iron sulphur proteins =

A

release of iron and inactivation

BIO3093 energy metabolism (1 decks)

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