RuBisCO

The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO or just rubisco is used in the Calvin cycle to catalyze the first major step of carbon fixation. RuBisCO is thought to be the most abundant protein in the world since it is present in every plant that undergoes photosynthesis and molecular synthesis through the Calvin cycle. Moore, et al.'s summary includes the assessment that rubisco makes up 20-25% of the soluble protein in leaves and is made on the Earth at the rate of about 1000 kg/s. They estimate that every person on Earth is supported by about 44 kg of rubisco!

"RuBisCO catalyzes either the carboxylation or oxygenation of ribulose-1,5-bisphosphate (known as RuBP) with carbon dioxide or oxygen. What makes it unique and different to every other enzyme is the fact that it can survive on its own without the need of the plant so even if it is dead it remains and helps decomposition. This is due to it not being affected by temperature or pH. "(Wikipedia)

RuBisCo has a molecular weight of 490,000 Daltons and is composed of eight large subunits and eight small subunits. The large subunits are coded in the chloroplast itself, while the small units are coded in the nucleus of the cell.

Carbon-Fixing in Plants

Carbohydrates in plants are produced from carbon dioxide and water with the energy of photosynthesis. The compound RuBP is part of the Calvin cycle in which this occurs. The large enzyme rubisco catalyzes the reaction that begins the chain of fixing carbon in plants.

The catalytic reaction with RuBP and carbon dioxide through a short-lived intermediary almost instantaneously produces two molecules of glycerate 3-phosphate (3PGA).

Formation of 3-phospoglycerate (PGA)

The reaction of carbon dioxide with RuBP in the Calvin cycle that is catalyzed by rubisco produces an unstable intermediate. It reacts with water to split into two copies of 3-phospoglycerate (PGA).

Steps to BPG and G3P

The next two steps in the Calvin cycle lead to glyceraldehyde 3-phosphate (G3P or GAP). The first of these steps uses energy from ATP to phosphorylate the 12 PGA molecules to form 12 1,3-bisphosphoglycerate (BPG) molecules. The next step uses three electrons provided by NADPH to reduce the BPG to form 12 molecules of glyceraldehyde 3-phosphate (G3P or GAP).

The ATP and NADPH used in the Calvin cycle represent the high energy output of of the light dependent reactions of photosynthesis.

Calvin Cycle Output for Sucrose Synthesis

The net output of the Calvin cycle for the synthesis of sugars and other molecules is 2 G3P molecules out of the 12 produced in the cycle. The source of energy for this synthesis is from the light reactions.

Synthesis of RuBP for the Calvin Cycle

The final step in the Calvin cycle is to resynthesize the six molecules of RuBP used in the carbon-fixing reaction catalyzed by rubisco. The source of energy for this synthesis is ATP from the light reactions.

RuBP

Rribulose 1,5-biphosphate

RuBP (ribulose 1,5-biphosphate) is a five-carbon sugar which reacts with CO₂ in the first step of the Calvin cycle for fixing carbon in photosynthetic systems. That reaction is catalyzed by the large enzyme rubisco . The catalytic reaction with RuBP and carbon dioxide through a short-lived intermediary almost instantaneously produces two molecules of glycerate 3-phosphate (3PGA).