Abstract
The catalytic performance of the major CO2-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity. Natural diversity in the catalytic properties of Rubisco indicates possibilities for improvement. Oceanic phytoplankton contain some of the most efficient Rubisco enzymes, and diatoms in particular are responsible for a significant proportion of total marine primary production as well as being a major source of CO2 sequestration in polar cold waters. Until now, the biochemical properties and three-dimensional structures of Rubisco from diatoms were unknown. Here, diatoms from arctic waters were collected, cultivated, and analyzed for their CO2-fixing capability. We characterized the kinetic properties of five and determined the crystal structures of four Rubiscos selected for their high CO2-fixing efficiency. The DNA sequences of the rbcL and rbcS genes of the selected diatoms were similar, reflecting their close phylogenetic relationship. The Vmax and Km for the oxygenase and carboxylase activities at 25 °C and the specificity factors (Sc/o) at 15, 25, and 35 °C were determined. The Sc/o values were high, approaching those of mono- and dicot plants, thus exhibiting good selectivity for CO2 relative to O2 Structurally, diatom Rubiscos belong to form I C/D, containing small subunits characterized by a short βA-βB loop and a C-terminal extension that forms a β-hairpin structure (βE-βF loop). Of note, the diatom Rubiscos featured a number of posttranslational modifications of the large subunit, including 4-hydroxyproline, β-hydroxyleucine, hydroxylated and nitrosylated cysteine, mono- and dihydroxylated lysine, and trimethylated lysine. Our studies suggest adaptation toward achieving efficient CO2 fixation in arctic diatom Rubiscos.
Highlights
The catalytic performance of the major CO2-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity
Tom Rubiscos featured a number of posttranslational modifications of the large subunit, including 4-hydroxyproline, -hydroxyleucine, hydroxylated and nitrosylated cysteine, monoand dihydroxylated lysine, and trimethylated lysine
Our studies suggest adaptation toward achieving efficient CO2 fixation in arctic diatom Rubiscos
Summary
The catalytic performance of the major CO2-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity. Oceanic phytoplankton contain some of the most efficient Rubisco enzymes, and diatoms in particular are responsible for a significant proportion of total marine primary production as well as being a major source of CO2 sequestration in polar cold waters. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) is the principal enzyme responsible for the assimilation of CO2 into the biosphere It catalyzes the primary photosynthetic CO2 reduction reaction, the carboxylation of D-ribulose 1,5-bisphosphate (RuBP) by CO2. Oceanic contribution to the global net primary production (NPP) has been estimated to 45–50 Pg of carbon/year [2], which amounts to about 45% of the total NPP This is remarkable, considering the low availability of free dissolved CO2 and its slow diffusion in sea water, but is connected to the overall fast growth rates of phytoplankton, often leading to a ϾϾ50% increase in biomass per day [3].
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