The important enzyme of $C_3$ cycle is RuBP carboxylase oxygenase which catalyses reaction of carboxylation of ribulose bis-phosphate, which is 5-carbon compound. to form PGA, the first stable product in $\mathrm{C}_3$ cycle.

In $\mathrm{C}_4$ cycle, the important enzyme is phosphoenol pyruvate carboxylase (PEP carboxylase) which help in fixing $\mathrm{CO}_2$ to form oxaloecetate (4-carbon compound), the first stable product of dark reaction is $\mathrm{C}_4$ cycle.

RuBisCo, now termed as RuBP carboxylase-oxygenase, is widely occurring enzyme because RuBisCo is used in the Calvin cycle to catalyse 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.

It makes about $20-25 \%$ of the soluble protein in leaves and is made on the earth at the rate of about $1000 \mathrm{~kg} / \mathrm{s}$. It is estimated that every person on earth is supported by about 44 kg of RuBisCo.

Photorespiration is associated with $\mathrm{C}_3$ cycle, where plant lose $\mathrm{CO}_2$ fixation because of the increase in concentrate ion of $\mathrm{O}_2$ and change the nature of activity of RuBP carboxylase-oxygenase.

$\mathrm{C}_4$ plants have evolved a mechanism to avoid loss of $\mathrm{CO}_2$. There is not a direct contact of RuBP carboxylase-oxygenase as $\mathrm{C}_3$ cycle operates in bundle sheath cells (where both temperature and oxygen level low).

$\mathrm{CO}_2$ fixation is done by another enzyme PEP carboxylase in mesophyll cells and oxaloacetate is formed which is converted to malic acid and transported to bundle sheath cells.

There, it gives off $\mathrm{CO}_2$ which is used in Calvin cycle, operating in bundle sheath cells of $\mathrm{C}_4$ plants.