Multiple Rubisco forms in proteobacteria: Their functional significance in relation to CO₂ acquisition by the CBB cycle

Rubisco is the predominant enzymatic mechanism in the biosphere by which autotrophic bacteria, algae, and terrestrial plants fix CO₂ into organic biomass via the Calvin-Benson-Basham reductive pentose phosphate pathway. Rubisco is not a perfect catalyst, suffering from low turnover rates, a low affinity for its CO₂ substrate, and a competitive inhibition by O₂ as an alternative substrate. As a consequence of changing environmental conditions over the past 3.5 billion years, with decreasing CO₂ and increasing O₂ in the atmosphere, Rubisco has evolved into multiple enzymatic forms with a range of kinetic properties, as well as co-evolving with CO₂-concentrating mechanisms to cope with the different environmental contexts in which it must operate. The most dramatic evidence of this is the occurrence of multiple forms of Rubisco within autotrophic proteobacteria, where Forms II, IC, IBc, IAc, and IAq can be found either singly or in multiple combinations within a particular bacterial genome. Over the past few years there has been increasing availability of genomic sequence data for bacteria and this has allowed us to gain more extensive insights into the functional significance of this diversification. This paper is focused on summarizing what is known about the diversity of Rubisco forms, their kinetic properties, development of bacterial CO₂-concentrating mechanisms, and correlations with metabolic flexibility and inorganic carbon environments in which proteobacteria perform various types of obligate and facultative chemo- and photoautotrophic CO₂ fixation.