Ribulose-1,5- bisphosphate carboxylase oxygenase (Rubisco) is the most abundant enzyme, facilitating photosynthesis by CO₂ sequesteration_. Despite being a crucial enzyme required for existence of life, it has a very low catalytic efficiency. Although Rubisco is conserved among different families, they show different substrate (CO₂) specificities and a broad spectrum of catalytic activity remaining a puzzle to enzymologists. There are two processes; carboxylation (carbon fixation) and oxygenation carried out by Rubisco. The oxygenation process reduces the efficiency of photosynthesis in the carbon fixation process by 60%. The poor efficiency of  Rubisco is due to its peculiar defect in distinguishing CO₂ from O₂, thus leading to inefficiency in CO₂ fixation. Higher plant Rubiscos have weaker specificity for CO₂  and faster activity while non-green algal Rubiscos show the inverse. Since red algal Rubisco is particularly at the high end of the spectrum in terms of substrate specificity, structural and kinetic studies may explain some of the differences. Even though the essential residues of Rubisco are conserved, the differences in affinity and catalytic activity makes red algal Rubisco significant to study in terms of the molecular origin of specificity. The main objective of the project is to understand and characterize red algal Rubisco from the seaweed Porphyra and compare the results with green algal Rubisco from seaweed Ulva, Spinach and tobacco Rubiscos.

Towards this goal: We focus on the red algal (Porphyra) Rubisco protein which is ~60% identical to the higher plant homolog and is characterized by a higher affinity to substrate/CO₂, but it still shows a lower catalytic rate. Porphyra and Ulva Rubiscos from Lyttelton have been extracted, purified and structural characterization studies are ongoing. We measured the size, shape and the stability of Rubisco using biophysical techniques such as analytical ultracentrifugation, dynamic light scattering, size exclusion chromatography, transmission electron microscopy, differential fluoroscence spectrophotometry  etc. To compare these results with well characterized Rubisco enzymes, spinach and tobacco Rubisco were also extracted, purified and characterized. Further ongoing characterization studies will provide important details of functional and structural similarities and differences between the various species.