Date of Award
Cyanobacteria are gram-negative oxygen-evolving photosynthetic microorganisms found in aquatic and terrestrial environments. Cyanobacteria utilize a large light harvesting complex, called a phycobilisome (PBS), that is comprised of pigmented proteins. These pigmented proteins, called phycobiliproteins (PBP), include allophycocyanin (APC), which composes the PBS core, phycocyanin (PC) and phycoerythrin (PE), which compose the PBS rods that extend from the core. PBP allows cyanobacteria to absorb visible light energy and transfer this energy to photosystem complexes to enhance photosynthesis. PBP are comprised of α- and β-subunits that have specific cysteine residues (C), where heme-derived bilin chromophores are added. The enzymes that facilitate the attachment of bilins are called lyases. Some lyases not only mediate attachment but can also induce the isomerization of the bilin molecules to alter the double bond conjugation.
In this study, the amino acids that confer the ability to isomerize bilin are investigated in the bilin lyases MpeW and MpeQ from Synechococcus sp. A15-62. These enzymes were selected because they both attach the PEB bilin to MpeA at C83, however MpeQ isomerizes the PEB to PUB. To identify the residues that contribute to isomerization, point mutations were created in these enzymes and studied using a heterologous co-expression system in E. coli. Biochemical and molecular techniques were used to identify the key residues in lyase and isomerase function. It was determined that positions 224, 319, 320, 323, and 348 played a role in bilin isomerization, with position 319 being the most critical for isomerization to occur. Positions 318, 285, and 353 were suggested to be important for lyase activity in MpeQ, but these findings need to be further studied.
Lastly, the putative bilin lyase MpeU was characterized in this work. Previously described enzymes in the same family, CpeF and MpeV, were shown to doubly ligate a bilin to the C48/59 equivalent C residues of b-PE subunits. The CpeF enzyme, from Fremyella diplosiphon, doubly ligates the PEB bilin on CpeB; MpeV attaches and isomerizes PEB to PUB to CpeB and MpeB in Synechococcus sp. RS9916. The function of mpeU from Synechococcus spp. RS9916 (encodes both mpeV and mpeU) and A15-62 (encodes only mpeU) was explored using recombinant protein expression of MpeU, CpeS, CpeZ and the PEI and PEII subunits. The results show that MpeU is a PEB lyase isomerase that can attach and isomerize PEB to PUB at C50/61 of MpeB in RS9916 whereas MpeU from A15-62 acts as a lyase-isomerase for both MpeB and CpeB.
Joseph, Kes Lynn, "Characterization of the Structure and Function of Enzymes Required for Phycoerythrin II Biosynthesis in Marine Synechococcus species" (2023). University of New Orleans Theses and Dissertations. 3118.
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