Division of Mikrobiologie

Tetrapyrroles and phages

Phycobilisome
Fig. 1: Composition and structure of a cyanobacterial phycobilisome.

The color variety of cyanobacteria, red algae and cryptophytes depends on the composition of their light harvesting complexes that differs from organism to organism. These light harvesting complexes consist primarily of the proteins phycocyanin (blue) and phycoerythrin (red) and form the so-called phycobilisome.  

Phycobilisomes enable the organisms to collect light of additional wavelengths, which can’t cannot be absorbed by chlorophyll (green gap). The color of these phycobiliproteins is based on the covalently bound chromophores, the so-called phycobilins. Phycobilins are linear tetrapyrroles resulting originating from the cleavage of the circular molecule heme. Heme oxygenases catalyze this cleavage, resulting in the product biliverdin IXα. Biliverdin IXα serves then as precursor for all known phycobilins.

Fig. 2: Reaction mechanism of heme oxygenases with the resulting product biliverdin IXα.

Subsequently, biliverdin functions as the substrate for the protein class of ferredoxin-dependent bilin reductases (FDBR). The resulting bilins phycocyanobilin (PCB) and phycoerythrobilin (PEB) are the most abundant ones. The FDBR PcyA catalyzes the synthesis of PCB, whereas the PEB-synthesis gets catalyzed byrequires the action of both, PebA and PebB. There, biliverdin gets is reduced by PebA to dihydrobiliverdin (DHBV). Afterwards, PebA passes on the DHBV to PebB which synthesizes PEB. The newly discovered FDBRs PebS and PcyA PcyX are of a viral origin (cyanophages) and are able to catalyze the reduction as a single enzyme.

Fig. 3: Synthesis of phycobilins catalyzed by ferredoxin-dependent bilin reductases.

In our research we are interested in the biosynthesis of the different phycobilins. We investigate the reaction mechanisms and the structural properties that are essential for the enzyme activities. The research of the viral enzymes is a new field of our group and allows an insight in the evolution of the tetrapyrrole synthesis and its role in the ecosystem. The FDBRs PebS and PcyX of cyanophages belong to the viral auxiliary metabolic genes (AMGs), which are expressed in their hosts. In this project we investigate the influence of these AMGs on the photosynthesis and the light-harvesting complex assembly to gain a detailed understanding of the influence of AMGs.