Structure and Function of the Reaction Centre – Light Harvesting 1 Core Complexes from Purple Photosynthetic Bacteria

Abstract

Reaction centre-light harvesting 1 core complex is a fundamental unit in photosynthetic bacterium. It is the place where light energy is collected and used to power photosynthetic redox reaction, leading to the synthesis of ATP ultimately. The reaction centre is surrounded by elliptical LH1 complex. The subunit of the LH1 ring is a heterodimer of α-, β-polypeptide pair, to which pigment molecules, BChl a or BChl b and carotenoid are non-covalently bonded. There are at least three different types of the RC-LH1 core complexes found in photosynthetic bacteria so far. The core complex from Rps. palustris is a monomer. Its LH1 ring consists of 15 pairs of α/β-polypeptide with an extra protein ‘W’ located between two α-polypeptides, forming an incomplete ring. The gap of the LH1 ring was proposed as a gate to facilitate quinone/quinol exchange between reaction centre and cytochrome bc1 complex. A dimeric core complex was found in PufX-containing species, such as Rba. sphaerides. Two RCs are associated by 28 α/β-apoprotein pairs and two pufX proteins, forming an S-shaped RC-LH1-PufX core complex. The pufX protein causes incomplete LH1 ring and dimerization of the core complex. Monomeric RC-LH1 from Tch. tepidum has a complete elliptical LH1 ring that is composed of 16 pair α/β-apoprotein pairs without pufX-like protein. Sixteen Ca2+ are coordinated on C-terminal region of the α/β-polypeptide to stabilize the core complex and cause BChl a Qy absorption redshift to 915 nm. Carotenoid, spirilloxthanin contacts with α/β-apoproteins intimately to form an inter subunit interaction within the core complex, providing a further stability of the complex.