The Role of Enzymes in Photosynthesis
Plants use an array of enzymes to convert sunlight into usable energy, and Rubisco is one of the most important. It makes up about half of the protein in chloroplasts and consists of eight identical large subunits. Both large and small subunits are encoded by mRNA, transcribed from chloroplast DNA and the nucleus. Together, they catalyze a reaction that converts CO2 into a five-carbon molecule.
The active site of RuBisCO
The active site of RuBisCO, the main photosynthesis enzyme, is remarkably diverse. The team found a connection between the charge distribution on its active site and the rate of CO2 fixation. By optimizing the RuBisCO carbon dioxide-fixing activity, the researchers could increase photosynthesis rates in plants, increase food supply, and lower emissions. The study, led by Associate Professor Hiroki Ashida, Professor Emere Akiho Yokota, and Associate Professor Eiichi Mizohata, has implications for crop improvement, a new metric in plant physiology, and genetic engineering.
In photosynthesis, the RuBisCO enzymes fix atmospheric CO2 and produce glucose essential for growth in most photosynthetic organisms. The RuBisCO enzymes are slow to produce new products, and a competing reaction with atmospheric oxygen slows their metabolism. Therefore, they are very costly to the organism, and their efficiency is largely determined by the rate of photorespiration, which occurs when plants lose their carbon dioxide-fixing capacity.
RuBisCo enzymes in photosynthesis
The RuBisCo enzymes in photosynthesis catalyze the reaction of ribulose-1,5-bisphosphate with molecular oxygen and carbon dioxide. The RuBP catalytically breaks the C-C bond and generates an unstable intermediate, 3-phosphoglycerate. Two additional steps follow this reaction: the first uses ATP to phosphorylate twelve PGA molecules, and the second step uses NADPH electrons to reduce the 12 BPG molecules. The final product of light-dependent photosynthesis is glyceraldehyde-3-phosphate.
Because of its poor activity, RuBisCO is one of the most abundant proteins on Earth. Plants overcome this by storing huge amounts of RuBisCO inside their cells. Consequently, the RuBisCO enzymes are a major contributor to photosynthesis. However, the kinetics of RuBisCO has varied greatly. For this reason, RuBisCO kinetics have yet to be completely clarified.
One of four GAPC-type GAPDHs
OsGAPDH is one of four GAPC-type GAPDHs. This heterotetramer contains four identical subunits stabilized by hydrogen bonds. The monomer consists of the cofactor-binding domain (residues 1-153) and the catalytic domain (residues 154-317). These domains have a Rossmann fold, with five a-helices on each side of a b-sheet.
GAPDH plays multiple roles in photosynthesis, ranging from its role in catalysis to its role in protein interactions. It is thought that GAPDH binds to AU-rich elements (AREs) found in eukaryotic messenger RNAs. By binding to AREs, proteins can alter the stability of mRNAs, thereby affecting their expression levels. For example, the mRNA of the endothelin ET-1 contains an AU-rich element (ARE) in its 3′-UTR. This enables GAPDH to unwind the mRNA and favor faster degradation by ribonucleases.
While the role of plant GAPDH is still under debate, some studies suggest that it serves as a signaling hub. The enzyme senses metabolic and redox imbalances in the cell. Combined, these signals can trigger multiple cellular responses, ranging from improved growth to the induction of PCD. This requires effective regulatory systems and redox codes. Fortunately, GAPDH enzymes are a vital part of plant photosynthesis.
The structure of A2B2-GAPDH was obtained using electron density maps of all the individual subunits. These maps revealed that the oxidized CTE negatively affects the recognition of NADP. Additionally, the R-axis-related B subunit had two residues that did not interact with the oxidized CTE, contributing to the downregulation of NADPH-dependent activity in oxidized A2B2-GAPDH.
The Rubisco activase enzyme
A transgenic plant containing a smaller isoform of the Rubisco activase enzyme can maintain its active form in light. It also modulates its activity, enabling it to function like a wild-type plant. While it is unknown exactly how the enzyme affects photosynthesis, it is thought to play an important role in regulating redox potential and the CO2/ATP ratio.
The active enzyme in the leaf is the Rubisco, which initiates photosynthetic carbon metabolism. During this process, CO2 is incorporated into ribulose 1,5-bisphosphate (RuBP), which combines with atmospheric CO2 to form 3-phosphoglyceric acid. The Rubisco activase enzyme helps the plant recover from inhibitory sugar-phosphate derivatives that bind to its active site.
This study was conducted by evaluating the effect of the Rca proteins on Rubisco activation. Using the Carmo-Silva and Salvucci assay, we measured the activity of Rubisco activation and ATP hydrolysis using the recombinant proteins Rca 2b and Rca 2c. The recombinant Rca was added to a solution containing 100 mM Tricine-NaOH pH 8.0, ten mM MgCl2, 50 mM DL-dithiothreitol, and 0.1 mM EDTA.
ATP hydrolysis and activity of Rubisco
A study of the ATP hydrolysis and activity of Rubisco activase revealed that Cys residues at positions 411 and 392 were essential for the low ATP hydrolysis and high activity of the 46-kDa isoform. This study also demonstrated that redox regulation of the larger isoform modulates the activity of both isoforms. Further research is needed to determine how redox regulation of Rubisco activase affects the activity of both isoforms in the plant.
RCA is a molecule that uses energy from ATP hydrolysis to promote the release of inhibitors from the active sites of Rubisco and ATP in the cellular respiration process. The three RCA isoforms (Rca 2b, Rca 2b, and Rca 1b) hydrolyze ATP at comparable rates. R 2a and 2b promote Rubisco activation, but Rca 1b is the predominant Rca isoform.
Magnesium is a macronutrient
Plants need Mg for photosynthesis to transport carbohydrates to active tissues. Hence, if plants lack Mg, they are less able to perform photosynthesis. In addition, a deficiency of Mg in plants induces a major change in photosynthesis, the accumulation of carbon in the source leaves. Therefore, plants need ample Mg to do photosynthesis and reduce crop stress.
The use of foliar MgSO4 in crops can provide adequate amounts of Mg to alleviate Mg deficiency symptoms. However, it is not always sufficient. Magnesium deficiency affects crop yield and quality. This is particularly important in intensive cropping systems, which rely on N, P, and K as their main fertilizers. Magnesium deficiency can occur in highly weathered soils due to potential leaching and aluminum interaction.
Plants also need Mg to transport carbohydrates from green leaves to active tissues. Magnesium deficiency impairs the growth of shoots and roots. Plants also need Mg to function correctly in various enzymes. As a secondary macronutrient, magnesium is indispensable in photosynthesis. However, magnesium toxicity in plants can cause other nutrient deficiencies. As such, the need for magnesium is extremely high.
While Mg is crucial for chlorophyll formation, it is also involved in other functions. Magnesium acts as a cofactor and allosteric modulator for 300 enzymes, including the photophosphorylation and fixation of CO2. Mg is essential in protein biosynthesis and plays a crucial role in ribosomes responsible for protein synthesis. In Mg-deficient plants, this process is severely inhibited, and the concentration of precursor amino acids increases.
Effects of environmental stress on photosynthesis
Heat stress directly affects the function of many plant enzymes, including those involved in photosynthesis. Plant cells exposed to high temperatures have less time to recover, and their photosynthesis is inhibited before other cellular functions. The primary targets of heat stress in photosynthesis are the enzymes Photosystem II (PSII) and ribulose-1,5-bisphosphate carboxylase (Rubisco).
Environmental stress affects many biological, biochemical, and molecular processes in plants. One of the processes severely affected by stress is photosynthesis, the most basic physiological process of green plants. Environmental stress affects photosynthesis at all phases. The mechanisms involved in photosynthesis involve several components, including enzymes that catalyze oxygen production. Damage to any of these components can reduce the plant’s photosynthetic capacity.
The effects of environmental stress
The effects of environmental stress on photosynthesis vary across species. Heat stress has an important effect on chlorophyll production in leaf tissue. It can also affect the quantum yield of photosystem II. Researchers have found that changes in non-stomatal factors cause a decrease in Pn and Ci. For example, high-temperature treatments reduced the enzyme Gs and Ci activity, while high temperatures decreased Pn and Ci simultaneously.
Heat stress also promotes the production of reactive oxygen species, which damage plant cells. To counteract this effect, plants produce enzymatic scavengers called antioxidants. The most important of these is superoxide dismutase (SOD), which catalyzes the dismutation of O2 to H2O2. Other antioxidant enzymes are also present in the plant.