Comparing Photosynthetic RC

Comparing Photosynthetic RC

1. COMPARE PSI + PSII PRIMARY DONORS

   • P700
     • Primary donor (PSI) has stronger excitation coupling due to shorter distances between cofactors
       • Absorbs longer, lower energy wavelengths (red-shifted)
         • Stronger exciton coupling = red-shift
           • Deeper exciton trap
     • More negative redox potential
       • More delocalisation, stabilises + charge over whole molecule
     • Stronger delocalisation + exception coupling
     • Stronger overlap between primary donor special pair
       • Mg-Mg 0.3A vs 8.1A in PSII
   • P680
     • More positive redox potential
       • More spatially separated special pair, so less stabilising delocalisation
         • Not enough on its own
           • Lack of H bonds to conjugated system, so can't be this !!!
           • Non-polar environment, makes destablises positive charge
           • Long term electrostatic effects (further away): orientation of Mn cluster and dipole a-helices, + side of helix can face P680+ destabilising it
     • Weaker exciton coupling so less red shifter
       • Absorbs higher energy wavelength = max 680nm

2. SUMMARY OF ROLE OF PROTEIN-COFACTOR + COFACTOR-COFACTOR INTERACTIONS IN FINE TUNING REDOX AND SPECTRAL PROPERTIES OF PRIMARY DONORS

   • 1. Exciton coupling: depending on the geometry, it can lead to a red-shifted absorption maximum of P (exciton trap)
     • PSI MORE (red-shifted)
     • 2. Delocalisation of pie-electrons over a (bacterio)chlorophyll dimer: stabilises positive charge leading to decrease of P/P+ redox potential
       • PSI MORE (more negative potential)
       • 3. H-bonds increase P/P+ potential, if in pie-electron system by destabilising oxidised state
         • None in more positive PSII though ?
         • 4. Hydrophobic environment: destabilisation of charged states, increases redox potential
           • PSII

3. COMPARE RC COMPLEXES

   • Purple bacterial L/M subunits from Type II-RC have same topology as D1/D2 subunits from PSII
     • Topology: the orientation of proteins in the lipid membrane
     • Topology also similar in PSI and PSII
       • Structural similarities are good indication of evolution
         • Twilight zone: <25% sequence identity
   • Characteristic motif of 5 TMH homo or heterodimers that enclose cofactors for ETC = reaction centre domains
     • Type-I RC (PSI) has antenna binding domains covalently linked to RC domains (belong to same subunit)
       • PSII has separate protein subunits
     • Sequence similarity between RC domains in type-II and type-I RC is quite low
   • Antenna binding domains = 6 TMHs that bind Chl and carotenoid of core antenna system
     • Share higher sequence similarities but not found in purple bacteria
   • In PSI PsaA and PsaB are identical = homodimeric
     • Type-II heterodimeric

4. In chloroplasts of eukaryotes, PSI and PSII form larger structures than in cyanobacteria

   • The PSI + II supercomplexes found in plants + algae contain a large core which is very similar to cyanobacteria PSI + PSII
     • Attached to these cores are LHCI + LHCII
     • Structure of these supercomplexes are dynamic to changing light conditions