Poster Presentation The 43rd Lorne Conference on Protein Structure and Function 2018

The Evolution of a Quantum Coherent Light Harvesting Apparatus: Selection for Efficiency or Merely a Spandrel. (#223)

Harry W Rathbone 1 , Katharine Michie 1 , Paul MG Curmi 1
  1. University of New South Wales, Sydney, NSW, Australia

The inception of studies into non-trivial quantum effects in biology has been fraught with criticism and zeal from various factions. It has become increasingly apparent that non-classical effects are present in biological sensing and energy transduction mechanisms. In the single-celled algal phylum Cryptophyta, light harvesting antennae proteins have been demonstrated to bear quantum coherent effects through optical spectroscopies. Underlying quantum processes in this system produce excited states with coherence times much longer than the thermal environment would (semi-classically) allow, increasing the probability on energy capture.


This effect is produced largely by structural means where a central pair of chromophores are pushed into steric contact by the surrounding protein. This puts them in a strong coupling regime generating a coherent quantum system; a superposition of states. The family of proteins where this occurs, the phycobiliproteins, are evolutionary derivatives of the antennae proteins in the red alga phycobilisome. After a series of endocytotic events, the phycobilisome was deconstructed and the cryptophyte light harvesting antennae were distributed between chloroplast laminations filling the lumen. The effect this has had on light harvesting efficiency is unknown. Furthermore, during the evolution of this system a second set of antennae proteins have been generated which open a solvent filled cavity and decouple the central chromophores. This protein family forms a dimer of dimers.  Comparisons of crystal structures highlights a highly conserved globin fold on a repeated β-subunit and two shorter α-subunits which stabilise the fold and the quaternary structure. A diverse range of these α-subunits have been discovered crystallographically and genomically. Spectroscopically, differences in α-subunits produce a marked change in coherence properties and may generate differences in chromophorylation. The heterogeneity in coherent/incoherent forms suggests that quantum effects may be required for efficient intra-protein optical downconversion during certain steps in energy funnelling toward the photosynthetic reaction centre.