EFB530 Plant Physiology

Photosynthesis-electron transport

Overview

Its probably easiest to understand photosynthetic electron transport by looking at it from three different angles:

• as a series of chemical (redox) reactions that accomplish the oxidation of H₂O (producing O₂) and the reduction of NADP⁺ to NADPH using photons of light for energy
• as a physical structure of multisubunit protein complexes and other electron carrying molecules that bind cofactors and pigments to capture light and accomplish the reactions described above
• as an associated set of reactions that accomplish the transport and concentration of H⁺ in the lumen; this concentration gradient provides energy to drive ATP synthesis by the ATP synthase protein

CLICK HERE FOR A DIAGRAM DEPICTING PHOTOSYNTHETIC ELECTRON TRANSPORT

Redox reactions

Reaction centers-PS II, PS I

• Sequence of events involves redox chemical reactions
• =reactions involving the transfer of electrons to/from redox active components of the photosynthetic complexes

The ability of a chemical species to donate or receive an electron

• defined by redox potential (midpoint potential=E_m); higher values represent greater ability to oxidize something
• this is expressed relative to hydrogen
• Reductant=gives electrons (reduces something)
• Oxidant=receives electrons (oxidizes something)
• As redox potential (oxidizing potential) increases, energy level decreases

PS II - reaction center chlorophyll is very different from antenna chlorophylls

P₆₈₀

• when P₆₈₀ receives light energy (from the antenna pigments) it generates a redox active chemical species
• has a peak absorbance at 680 nm
• = P₆₈₀^*, which is a weak reductant

P₆₈₀ + hv -> P₆₈₀^* when light is red light or shorter wavelength

• P₆₈₀^* donates that electron to another component of PS II, then eventually to cyt b₆/f complex, then to PS I - losing some energy along the way

P₆₈₀^* -> P₆₈₀⁺ + e^-

P₆₈₀⁺ is a very unstable component

• P₆₈₀⁺ - one of the strongest oxidants in biology
• P₆₈₀⁺ is reduced by an electron pulled from the splitting of water, 2H₂O -> 4H⁺ + 4e^- + O₂
• gets P₆₈₀⁺ back to its stable, ground state P₆₈₀

PS I

• reaction center chlorophyll=P₇₀₀ , similar to P₆₈₀, but with peak absorbance at 700 nm
• generates a redox active species= P₇₀₀^*, which is a very strong reductant (one of the strongest in biology)
• P₇₀₀^* reduces other components of PS I
• then eventually reduces NADP⁺ to NADPH

Physical structure

4 protein complexes together perform photosynthesis

• PS II, cyt b₆-f, PS I (in series), CF₀-CF₁ ATPase

PS electron transport is accomplished by three membrane bound, protein complexes, while ATP synthesis is performed by the fourth complex

More detail about the physical structure of these protein complexes & electron carriers

PS II

• reaction center = 2 core proteins (D1 and D2) that contain all of the electron carriers
• other peripheral proteins that act in light harvesting

P₆₈₀ -> Pheophytin -> Q_A -> Q_B -> (PQ->PQH₂)

• Note: PQ (plastoquinone) is reduced by 2 electrons to form PQH₂ (plastoquinol)
• PS II is very similar to photosynthetic bacterial reaction center, which has been crystalized, 3D structure determined by x-ray crystallography (by Michel and Deisenhofer, 1988 Nobel Prize)

Water is oxidized by a portion of PS II called the oxygen evolving complex (OEC) on the lumen side of the thylakoid

• oxidation of H₂O to release O₂ must involve the stable transfer of 4 electrons
• these 4 electrons are held and released to P680 one at a time by 4 atoms of Mn bound to the OEC proteins
• the intermediate steps of the reaction, when the Mn complex is holding 4, 3, 2, 1, or 0 electrons are described as the S-states (S₀, S₁, S₂, S₃, S₄)
• this reaction also releases 4 H⁺ (protons)
• the oxidation of 2 H₂O is necessary to produce stable O₂ rather than reactive oxygen radicals (1/2 O₂) 

Plastoquinone (PQ) is a mobile electron carrier (soluble in the membrane)

• can move between protein complexes
• also a 1 electron->2 electron gate
• Note: reduction of PQ to PQH₂ by PS II, then oxidation of PQH₂ to PQ by the cyt b₆/f complex "pumps" H⁺ from the stroma to the lumen

PQH₂ donates electrons to the cyt b₆/f complex

• multi-subunit complex- two types of cytochromes, and an Fe-S protein
• cytochromes are proteins that have heme cofactors
• heme=similar structure as chl, Fe atom bound
• Fe-S protein has 2 Fe and 2 S bound to the protein

cyt b₆/f reduces (donates electrons) to another mobile carrier protein

• plastocyanin (PC)=hydrophilic, soluble protein in the lumen
• PC carries one electron
• protein with a copper (Cu) atom
• purified protein looks blue (blue-copper protein)

PC will donate an electron to P₇₀₀⁺, reducing it back to P₇₀₀ (its stable ground state)

PS I

• reaction center has three polypeptides (PsaA, PsaB, and PsaC) that hold all of the electron transport components
• 2 large proteins that form a dimer, and a third small one
• also other peripheral proteins in the complex

On the dimer of proteins:

• P₇₀₀ = special chlorophyll pair
• A₀ = special chlorophyll
• A₁ = quinone
• F_X = Fe-S complex

On the third small protein:

• F_A/F_B = both Fe-S complexes

PS I reduces ferredoxin (another Fe-S protein) soluble in the stroma

• ferredoxin associates with ferredoxin-NADP reductase to reduce NADP⁺ to NADPH
• Note: the reduction of NADP⁺ to NADPH requires 2 electrons and uses H⁺ from the stroma

Cyclic electron flow:

• can occur when ferredoxin reacts with the cyt b₆-f complex instead of with NADP⁺
• get transport of H⁺ into the lumen to generate ATP, but no NADPH, no O₂

Some herbicides block electron transport:

• DCMU (diuron)-blocks Q_B from accepting electrons
• methyl viologen (Paraquat)- interrupts electron transfer from PS I to NADP⁺

Other concepts:

• charge separation-back reactions
• overload-singlet oxygen, other radicals
• 1 electron->2 electron gates
• mobile electron carriers
• short-distance photochemistry is very insensitive to temperature

H⁺ pumping and ATP synthesis:

This whole process includes the concentration of H⁺ in the lumen of the thylakoid

ATP synthesis operates by a chemiosmotic mechanism (Mitchell, early 1960's)

• high conc. of H⁺ in lumen, relative to stroma
• concentration and electrical charge differences across a membrane represent potential energy
• H⁺ flow out through the ATP synthase, down an electrochemical gradient
• this energy is captured during the flow of H⁺ through the ATP synthase to produce ATP from ADP on the stromal side
• ATP synthase=multi subunit, Cf₀/Cf₁
• ~4 H⁺ must flow through the ATP synthase to drive production of 1 ATP
• Jagendorf (1967)=isolated thylakoids equilibrated at pH 4, then transferred to pH 8, made ATP from ADP with no light
  

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