The role of carbonic anhydrase in the CO2-concentrating mechanism of cyanobacteria.
Published: 2004
Language: eng
Number of pages: 284
Description: Heterologous expression of other putative cyanobacterial CAs and characterization of directed cyanobacterial mutants in these proteins failed to indicate that enzymatic activity was associated with any of these putative CAs. Examination of carboxysomal polypeptides from certain marine cyanobacteria and chemolithoautotrophic bacteria resulted in the discovery of epsilon-CA, a novel fourth class of the enzyme. The expression of epsilon-CAs among several of the dominant primary producers in the world's oceans suggests that this carboxysomal CA may also be essential for autotrophic CO2 fixation and may influence the bio-geochemical cycling of carbon on a global scale.Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the interconversion of CO2 and HCO3-. This enzyme family is comprised of three distinct classes (alpha, beta, and gamma), is and distributed throughout all domains of life, where it is involved in many fundamental biological processes, including photosynthesis. In cyanobacteria, a CO2-concentrating mechanism (CCM) operates to elevate the CO 2 concentration around the primary carbon-fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), thereby enhancing photosynthetic performance. The operation of the CCM involves the light-dependent transport of inorganic carbon (Ci) from the medium and the generation of CO2 within the RuBisCO-containing carboxysome, both of which involve CA activity. However, the identity of the proteins involved in these activities and the specific roles they play in the CCM are largely undetermined.This study provides a detailed examination of the CA activity and its involvement in the cyanobacterial CCM. Antibodies raised against the beta-CA from the freshwater cyanobacterium, Synechocystis sp. strain PCC6803, revealed that the enzyme was localized almost exclusively to the carboxysome and was consequently re-designated CcaA (c&barbelow;arboxysomal c&barbelow;arbonic a&barbelow;nhydrase protein A&barbelow;). A fully-segregated mutant deficient in CcaA expression was generated that could not survive on air and required elevated levels of CO2 for growth. This mutant retained the ability to transport Ci and accumulate a large intracellular pool, but was incapable of efficiently utilizing the Ci for carbon fixation. This suggests that CcaA functions in the CCM by catalyzing the dehydration of intracellular HCO3 - to CO2 for fixation by the carboxysomal RuBisCO, and is consequently essential for efficient photosynthesis.
Notes: Adviser:  George S. Espie.

Thesis (Ph.D.)--University of Toronto, 2004.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 65-10, Section: B, page: 4936.