RuBP carboxylase and oxygenase has dual nature. It has affinity for both $\mathrm{CO}_2$ and $\mathrm{O}_2$ but has more affinity for $\mathrm{CO}_2$ than $\mathrm{O}_2$. Thus, the concentrations of two determines which of the two will bind to the enzyme.
Consider the following two situations
(i) In a normal condition when $\mathrm{CO}_2$ and $\mathrm{O}_2$ concentrations are normal, it acts as carboxylase and fix $\mathrm{CO}_2$ by combining with ribulose bisphosphate and $\mathrm{C}_3$ cycle operates normally, producing glucose molecule as an first product of photosynthesis.
(ii) If $\mathrm{O}_2$ concentration goes up and $\mathrm{CO}_2$ goes down, it starts acting as an oxygenase enzyme and $\mathrm{C}_2$ cycle (photorespiration) starts where RuBP binds with $\mathrm{O}_2$ to from phosphoglycolate.
(iii) $\mathrm{C}_4$ plants have mechanisms to increase the concentration of $\mathrm{CO}_2$ at enzyme site, and increasing the intracellular concentration of $\mathrm{CO}_2$. Thus, here RuB is Co acts as carboxylase, minimising the affect of oxygenase.
Kranz Anatomy, is the feature exhibited by $\mathrm{C}_4$ plant. These possess two types of chloroplast in their leaves. Agranal chloroplast found in bundle sheath cells whereas granal chloroplast is found in the mesophyll cells.
Bundle sheath cell perform $\mathrm{C}_3$ cycle (dark reaction) where as mesophyll cell perform $\mathrm{C}_4$ cycle. $\mathrm{C}_4$ plants are more efficient even in high $\mathrm{O}_2$ concentration and temperature as compared to $\mathrm{C}_3$ plants. Many important crop plants (monocots) show $\mathrm{C}_4$ cycle like maize, sorghum, sugarcane and millet.
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.