Review Aquaporins and membrane diffusion of CO 2 in living organisms ☆ Ralf Kaldenhoff ⁎, Lei Kai, Norbert Uehlein Department of Biology, Applied Plant Sciences, Technische Universität Darmstadt, Schnittspahnstrasse 10, D-64287 Darmstadt, Germany abstract article info Article history: Received 2 July 2013 Received in revised form 26 September 2013 Accepted 29 September 2013 Available online 17 October 2013 Keywords: Aquaporin Membrane permeability for CO 2 Gas exchange Aquaporin structure-function CO 2 conductance of aquaporins Background: Determination of CO 2 diffusion rates in living cells revealed inconsistencies with existing models about the mechanisms of membrane gas transport. Mainly, these discrepancies exist in the determined CO 2 diffusion rates of bio-membranes, which were orders of magnitudes below those for pure lipid bilayers or theoretical considerations as well as in the observation that membrane insertion of specific aquaporins was rescuing high CO 2 transport rates. This effect was confirmed by functional aquaporin protein analysis in heterologous expression systems as well as in bacteria, plants and partly in mammals. Scope of Review: This review summarizes the arguments in favor of and against aquaporin facilitated membrane diffusion of CO 2 and reports about its importance for the physiology of living organisms. Major Conclusions: Most likely, the aquaporin tetramer forming an additional fifth pore is required for CO 2 diffusion facilitation. Aquaporin tetramer formation, membrane integration and disintegration could provide a mechanism for regulation of cellular CO 2 exchange. The physiological importance of aquaporin mediated CO 2 membrane diffusion could be shown for plants and cyanobacteria and partly for mammals. General Significance: Taking the mentioned results into account, consequences for our current picture of cell membrane transport emerge. It appears that in some or many instances, membranes might not be as permeable as it was suggested by current bio-membrane models, opening an additional way of controlling the cellular influx or efflux of volatile substances like CO 2 . This article is part of a Special Issue entitled Aquaporins. © 2013 Elsevier B.V. All rights reserved. 1. The debate about membrane CO 2 diffusion and aquaporins The gas exchange between cells and between cells and atmosphere is one of the preconditions for an intact physiology in almost all living organisms. For the majority of organisms, oxygen and CO 2 are the most significant gases with this regard. While in animals, plants, fungi and many bacteria oxygen is required as an electron acceptor for respiration, CO 2 is the product of catalytic reactions and exchanged with the atmosphere. In plants, CO 2 is also the substrate for sugar synthesis initiated and catalyzed during the reductive pentose-phosphate cycle, also known as the Calvin–Benson cycle. For this process, CO 2 diffuses in the opposite direction from atmosphere into the cells respectively organelles. Even though the diffusion and exchange of both gases might be of equal importance, this review will focus on the CO 2 diffusion and a possible involvement of aquaporins in that process due to the fact that there are currently more experimental data available to this regard. Despite this availability of information, the experimental data obtained for the diffusion of CO 2 through biomembranes are inconsistent even contradictory. Accordingly, the experts are split into one faction stating that CO 2 diffusion through membranes is so rapid that any membrane protein would reduce the flow. Consequently, a protein facilitating CO 2 membrane diffusion is not necessary and not existing. Measurements indicating that CO 2 membrane fluxes increase if certain proteins inserted were suspected to be incorrect because data was provided from experiments with possible technical problems. For example, if gas diffusion was monitored by a device such as a stopped flow spectro- photometer with a detection lag time larger than a theoretical calculated value for the diffusion time, the figures must reflect an artifact because the gas has diffused before the measurement begins. Nonetheless, scientists from the other faction claim that the measured CO 2 diffusion rates were by orders of magnitudes lower than the theoretical ones. If this holds true, proteins facilitating gas respectively CO 2 transport are mandatory for high diffusion rates and these in turn are one of the preconditions for smoothly running physiology processes. In this essay, we try to depict the arguments of both sides. However, results from our own studies revealed lower CO 2 permeability of biomembranes than estimated from theoretical considerations and a significant role of aquaporins in the diffusion of CO 2 . Regarding this, it is clear that the authors favor the point of view of the latter faction. 2. Reasons for and against Support for a significant role of aquaporins in gas diffusion came from the analysis of aquaporin function in heterologous expression systems [1–4]. Aquaporin membrane-insertion increased the cellular Biochimica et Biophysica Acta 1840 (2014) 1592–1595 ☆ This article is part of a Special Issue entitled Aquaporins. ⁎ Corresponding author. Tel.: +49 6151 16 3805. E-mail address: kaldenhoff@bio.tu-darmstadt.de (R. Kaldenhoff). 0304-4165/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bbagen.2013.09.037 Contents lists available at ScienceDirect Biochimica et Biophysica Acta journal homepage: www.elsevier.com/locate/bbagen