Carboxysomal

Carboxysomal refers to specialized intracellular microcompartments found in many bacteria and archaea. These structures are also known as carboxysomes. They are proteinaceous shells that encapsulate the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) along with carbonic anhydrase. The primary function of carboxysomes is to concentrate carbon dioxide and bicarbonate around RuBisCO, thereby increasing the efficiency of carbon fixation during photosynthesis or chemoautotrophy. This is particularly important in environments where CO2 concentrations might be limiting.

The carboxysomal shell is composed of proteins called carboxysomal proteins, which assemble into a semi-permeable membrane. This shell acts as a barrier, allowing the diffusion of CO2 and bicarbonate while excluding other molecules. Inside the carboxysome, carbonic anhydrase converts bicarbonate into CO2, ensuring a high local concentration of the substrate for RuBisCO. RuBisCO then catalyzes the fixation of CO2 onto ribulose-1,5-bisphosphate, a key step in the Calvin cycle.

Carboxysomes are found in a wide variety of autotrophic microorganisms, including cyanobacteria, proteobacteria, and archaea. The specific composition and structure of carboxysomes can vary between different species, but their fundamental role in enhancing carbon fixation remains consistent. The presence and organization of carboxysomes are crucial for the survival and growth of these organisms in diverse ecological niches. The study of carboxysomes contributes to our understanding of microbial metabolism and the global carbon cycle.