Rubisco or Ribulose Bisphosphate Carboxylase

In this article, we will discuss about the Rubisco-universality occurring enzyme of all photosynthesizing plant.

Rubisco or ribulose bisphosphate carboxylase/oxygenase is universally occurring en­zyme of all photosynthesizing plants except few photosynthetic bacteria. In eukaryotes, it is exclusively found in chloroplast stroma and is so abundant that it represents about 40% of the total soluble protein of most leaves.

According to Ellis (1979) it is by far the most abundant protein on earth. Andrews and Lorimer (1987) have estimated that the average human requires about 44 kg of rubisco just to obtain food through photosynthesis. Rubisco catalyses the biological fixation of 10¹¹ tonnes of CO₂ from the atmosphere annually.

The enzyme rubisco has two functions:

(i) Mainly in the carboxylation of ribulose 1, 5- bisphosphate (RuBP) leading to the formation of 3-phosphogylceric acid (3PGA) in dark reaction of photosynthesis (see Calvin Cycle).

(ii) Also in the oxygenation of ribulose 1, 5-bisphosphate (RuBP) leading to the formation of glycolate and 3-PGA in photorespiration. When rubisco catalyses this reaction, it is known as ribulose bisphosphate oxygenase.

Ribulose bisphosphate carboxylase/oxygenase has common substrate, ribulose 1, 5- bisphosphate (RuBP). However, the affinity of this enzyme for CO₂ is sufficiently high in comparison to O₂ that ensures rapid carboxylation at low concentrations of CO₂ found in photosynthesizing cells. But, oxygen fixation in all C₃ plants can occur because O₂ concentration in leaves or algal cells is much higher than that of CO₂, (atmospheric concentrations of O₂ averaging about 21% by volume and those of CO₂, 0.03%).

Rubisco is found in two functionally analogous forms which differ in structure, distri­bution and their sensitivity to oxygen. In most oxygenic phototrophs (blue-gree algae, chlo­roplasts and many photosynthetic bacteria) this enzyme is made up of 8 small(S) subunits each of 14kDa and 8 larger (L) subunits each of 55 kDa i.e. a molecular weight of about 560 kDa for complete enzyme (L₈S₈). In some purple non-sulphur bacteria rubisco consists of two large subunits each of 50 kDa. In eukaryotic algal Dinoflagellates rubisco is also believed to be dimeric.

In most eukaryotes, the genes that code for smaller subunits of rubisco are found in nuclear genome while those encoding larger subunits occur in chloroplast genome. How­ever, the assembly of rubisco from its smaller and larger subunits takes place in chloro­plast. In brown and red algae the genes encoding small and large subunits of rubisco are found in chloroplast genome. In prokaryotes, gene expression and synthesis of rubisco takes place in cytosol. Special proteins called chaperonins are believed to be involved in the assem­bly of L₈S₈ type of rubisco.