Sensing inorganic carbon: CO2 and HCO3−

Abstract

Enzymes and transporters that catalyse reactions involving inorganic carbon are well characterized with respect to the species of inorganic carbon (CO 2 or HCO 3 − ) with which they interact. There is less information on the species recognized by proteins that sense inorganic carbon. In this issue of the Biochemical Journal, Hammer and colleagues show conclusively that cyanobacterial adenylyl cyclases are activated by CO 2 and not HCO 3 − , as was believed previously. While in some circumstances a similar in vivo regulatory outcome is achieved from sensing HCO 3 − as from sensing CO 2 , there are cases in which the outcomes are significantly different. The most striking example is where a compartment lacks carbonic anhydrase yet supports large metabolic fluxes of inorganic carbon species so that CO 2 and HCO 3 − are not at equilibrium. Other examples involve changes in pH, or temperature, of a compartment containing an equilibrium mixture of CO 2 and HCO 3 − .

Connexin hemichannel-mediated CO2-dependent release of ATP in the medulla oblongata contributes to central respiratory chemosensitivity

“…5 and ), suggesting that changes in intracellular [HCO 3 − ] or pH are not the proximal stimulus for ATP release. Although there is evidence for a CO 2 /HCO 3 − ‐sensitive adenylate cyclase (Mittag et al 1993; Raven, 2006), SQ22536, a blocker of adenylate cyclase, had no effect on the amplitude of CO 2 ‐evoked ATP release (Fig. 5 and ).…”

Stimulation of Mammalian G-protein-responsive Adenylyl Cyclases by Carbon Dioxide

“…Two Class IIIb ACs, Slr1991 of Synechocystis PCC 6803 and CyaB1 of Anabaena PCC 7120, have been proven to respond to CO 2 and not HCO 3 Ϫ , giving rise to the idea of AC as a true gas-sensing molecule (22,23). The finding that Class IIIb ACs respond to CO 2 and not HCO 3 Ϫ necessitates an examination of the assumption that G-protein-regulated ACs and related prokaryotic enzymes do not respond to C i .…”

Compartmentation of defensive compound synthesis in a red alga

“…Among these are faster enzyme reaction rates when enzymes of the pathway are more closely packed and path‐lengths for diffusion of intermediate compounds is decreased, retention of leaky and/or volatile intermediates, and separation of toxic intermediates from metabolism in the rest of the cell (Ovádi and Saks , Kerfeld et al. , Raven , Rae et al. , Chowdhury et al.…”

“…Various reasons have been proposed for the occurrence of certain biochemical pathways in organelles, or in microcompartments of bacteria including Cyanobacteria, with a limited number of other pathways. Among these are faster enzyme reaction rates when enzymes of the pathway are more closely packed and path-lengths for diffusion of intermediate compounds is decreased, retention of leaky and/or volatile intermediates, and separation of toxic intermediates from metabolism in the rest of the cell (Ov adi and Saks 2004, Kerfeld et al 2005, Raven 2006, Rae et al 2013, Chowdhury et al 2014, see Table 1). There are potential reasons for locating end products of the synthesis of feeding deterrents and anti-fouling compounds in vacuoles, since they might be toxic to the producing organism as well as the "competitor" organism.…”