lecture 30 - Electron Transport and Carbon Fixation in Chloroplasts

Question 1

What is the first component of the redox chain?

Answer 1

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

Question 2

What is the second component of the redox chain?

Answer 2

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)

Question 3

What is the third component of the redox chain?

Answer 3

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)

Question 4

What is the fourth component of the redox chain?

Answer 4

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

Question 5

Describe Lateral Heterogeneity and

Plastoquinone Diffusion

Answer 5

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

Question 6

What is the fifth component of the redox chain?

Answer 6

Plastocyanin (PC)

A small, water-soluble copper-containing protein located in thyl. lumen

Question 7

What is the sixth component of the redox chain?

Answer 7

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 …

Question 8

What is the seventh component of the redox chain?

Answer 8

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

Question 9

What is the eighth component of the redox chain?

Answer 9

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

Question 10

Describe how Many of the steps are sensitive to

specific inhibitors

Answer 10

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

Question 11

What are The Useful Products of Photosynthetic e- Transport?

Answer 11

1. NADPH: Subsequently used in reduction of CO2

2. PMF: e- transport chain pumps H+ into lumen, hence ATP is synthesised

Question 12

Describe the magnitude of the PMF

Answer 12

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

Question 13

Describe Cyclic e- Transport and Variable

ATP/NADPH Production

Answer 13

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.

Question 14

Describe carbon fixation

Answer 14

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

Question 15

Describe rubisco

Answer 15

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

Question 16

look at slide on assimilation of hexoses

Answer 16

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+).