Chapter 5 - δ-Carbonic anhydrases

Abstract

One-third of the CO2 released in the atmosphere by anthropogenic activities is absorbed by the oceans determining a dramatic change in seawater chemistry. In fact, CO2 reacts with water to form carbonic acid (H2CO3), which being unstable dissociates into hydrogen ions (H+) and bicarbonate (HCO3−). Diatoms are a crucial component of the biological carbon pump that exports carbon into the ocean, contributing significantly to the long-term sequestration of atmospheric CO2. Diatom genome sequencing reveals the presence of different systems for HCO3− uptake, most of which are phylogenetically affiliated with those found in metazoans. In Phaeodactylum tricornutum, a plastid-targeted β-CA suggests that the localization and the role of carbonic anhydrases in diatoms are central to primary carbon metabolism. This chapter focuses on the δ-CAs (TweCA) identified in the genome of the marine diatom Thalassiosira weissflogii, demonstrating that TweCA is catalytically active for the hydration of CO2 but not for the hydrolysis of p-nitrophenyl acetate. Besides, the phylogenetic analysis showed that the δ-CAs are phylogenetically correlated more with the α-CAs than the other CA classes. Moreover, inhibition profiles of sulfonamides, inorganic anions, and the mono- and dithiocarbamates showed distinctive features for this enzyme providing new insights into the role of δ-CA in the algal carbon-concentrating mechanism.