lecture 30 - Electron Transport and Carbon Fixation in Chloroplasts Flashcards Preview

Molecular Biology and Biochemistry (part 2) > lecture 30 - Electron Transport and Carbon Fixation in Chloroplasts > Flashcards

Flashcards in lecture 30 - Electron Transport and Carbon Fixation in Chloroplasts Deck (16):

What is the first component of the redox chain?

1 Oxygen-evolving complex – OEC
3 proteins; associated with PS II on LUMINAL side of
Active centre: 4 tightly-bound Mn2+ ions
Catalyses reaction:
2H2O --> O2 + 4H+ + 4 e
e- are passed, one at a time via tyrosine residues, to oxidized P680+ reaction centres. (source of oxygen radicals)


What is the second component of the redox chain?

PS II Reaction Centre
Comprises a supramolecular complex: several distinct proteins binding redox chain components:
(i) P680: Chl a dimer (Chl* E'0 ~ – 0.65 V)
(ii) Pheophytin (Chl a without Mg2+) E'0 ~ – 0.55 V
(iii) 2 molecules of plastoquinone, bound to specific
proteins. PQA-tight, PQB- loose (diffuses within membrane)


What is the third component of the redox chain?

Plastoquinone (PQb)
oxidised quinone. reduced quinol

Reduction of QB is prevented by a number of
herbicides (e.g. dichlorophenyldimethylurea (DCMU))
Once reduced to plastoquinol (PQH2) the QB molecule
diffuses into
The PQ Pool: A large number of molecules of PQ,
freely dissolved in the hydrophobic portion of the
thylakoid membrane (c.f. ubiquinol in mitos)


What is the fourth component of the redox chain?

The Cytochrome b6f complex
A supramolecular complex, accepting e- from PQ comprises:
-2 spectroscopically–distinct b-type
cytochromes (cytochromes b6)
-Cytochrome f
-an Fe2S2 centre (Rieske protein)
-binds PQH2

Structurally and functionally, the cytochrome
b6f complex is very similar to Complex III
(cytochrome bc1) of mitochondria:
(i) both are inhibited by Antimycin A
(ii) both accept e- from a quinol
(iii) Cyt f is similar structurally to cyt c1
(iv) the b-type cytochrome (actually 2 haem groups bound to a single apoprotein) shows extensive
sequence homology to cyt b of mitochondria
(v) both contain a “high potential” (E’0 = + 300 mV)
Fe2S2 centre
All these factors point to a common evolutionary
origin of Complex III and the Cytochrome b6f complex


Describe Lateral Heterogeneity and
Plastoquinone Diffusion

Cyt b6f complex and PS I are in stromal lamellae; PS
II is in granal stack
Plastoquinone: A very mobile molecule, which diffuses in the plane of the membrane transfers reducing equivalents from PS II to Cyt b6f complex


What is the fifth component of the redox chain?

Plastocyanin (PC)
A small, water-soluble copper-containing protein located in thyl. lumen


What is the sixth component of the redox chain?

PS I Reaction Centre – Oxidises PC
The 3rd supramolecular complex, comprising (bound to proteins):
(i) P700: Chl a dimer
(ii) 6 additional Chls
(iii) 2 quinones,
(iv) 3 Fe4S4 centres
All help move electrons across membrane to next component …


What is the seventh component of the redox chain?

Ferredoxin (Fd)
A small protein with an Fe2S2
centre. Loosely associated with the STROMAL side of the thylakoid membrane.


What is the eighth component of the redox chain?

Ferredoxin – NADP oxidoreductase (FNR)
A flavoprotein, containing FAD
Also located on the STROMAL side of the thylakoid membrane.
Picks up 2 e- and a H+ to reduce NADP+ to NADPH


Describe how Many of the steps are sensitive to
specific inhibitors

-reduction of QB by DCMU and atrazine
-reduction of cyt f by antimycin
-reduction of Fd by paraquat


What are The Useful Products of Photosynthetic e- Transport?

1. NADPH: Subsequently used in reduction of CO2
2. PMF: e- transport chain pumps H+ into lumen, hence ATP is synthesised


Describe the magnitude of the PMF

Δψ = +20 mV
ΔpH = 3.5 units (lumen acid)
Since PMF = Δψ + 60 (pHo – pHi)
Thus PMF = + 20 + 210 = + 230 mV (lumen + ve)
Note: PMF is inverted compared with mitochondria…
and so is orientation of ATP synthase: ATP made
on outside of thylakoid membrane, in stroma
STOICHIOMETRIES: For redox chain, 6H+ / 2e-
For ATP synthase, 4H+/ATP
i.e. for each pair of e- passing through chain:
1 NADPH and 1.5 ATP are produced


Describe Cyclic e- Transport and Variable
ATP/NADPH Production

Observation: Light of wavelength >680 nm results in a PMF, but not net production of reducing equivalents.
Interpretation: PSI is excited by long wavelength light.
Electrons are recycled through ferredoxin, b6
f complex and plastocyanin b6f complex is a H+ pump
Note: Net production of NADPH is not possible because
no reductant (i.e. H2O ) is available.
But ATP can be produced.
Cyclic electron transport might provide plants with a way of producing >1.5 ATP/NADPH if demand for ATP is high.


Describe carbon fixation

The 1st reaction: catalysed by Ribulose 1,5-
bisphosphate carboxylase/oxygenase (Rubisco)
CO2 + Ribulose 1,5-bisphosphate --rubisco-->
(3 – phosphoglycerate) x 2


Describe rubisco

Reaction Energetics: ΔGO = - 52 kJ/mol, hence spontaneous
The Protein: Very low turnover rate (about 3 s–1)
hence very abundant …
THE most abundant in the world
A large, allosteric enzyme:
8 large (L) subunits ….. Mr = 55,000
8 small (S) subunits …..Mr = 13,000
Mg2+ co factor

Control: Mg2+ released from
thylakoid lumen in exchange for H+
during electron transport activates
Rubisco in stroma


look at slide on assimilation of hexoses

1. These stromal reactions are a reversal of glycolysis, except that FBPase provides a unique step.
2. These reactions mirror those of gluconeogenesis (in liver) except that Glyceraldehyde 3-phosphate
dehydrogenase is NADP+-specific (instead of NAD+).