Carbonic Anhydrase: An Efficient Enzyme with Possible Global Implications

As the global atmospheric emissions of carbon dioxide (CO2) and other greenhouse gases continue to grow to record-setting levels, so do the demands for an efficient and inexpensive carbon sequestration system. Concurrently, the first-world dependence on crude oil and natural gas provokes concerns for long-term availability and emphasizes the need for alternative fuel sources. At the forefront of both of these research areas are a family of enzymes known as the carbonic anhydrases (CAs), which reversibly catalyze the hydration of CO2 into bicarbonate. CAs are among the fastest enzymes known, which have a maximum catalytic efficiency approaching the diffusion limit of 108？M？1s？1. As such, CAs are being utilized in various industrial and research settings to help lower CO2 atmospheric emissions and promote biofuel production. This review will highlight some of the recent accomplishments in these areas along with a discussion on their current limitations. 1. Introduction The atmospheric concentrations of greenhouse gases such as carbon dioxide (CO2), methane, chlorofluorocarbons, and nitrous oxides have been increasing accordingly due to human-induced activities [1]. CO2 is the most abundant greenhouse gas, being produced primarily by the burning of fossil fuels such as coal, oil, and natural gas. The atmospheric concentration of CO2 has increased since preindustrial era from ~280？ppm [2] to 400？ppm in 2013 [3]. According to Antarctic ice core extractions, these levels are significantly higher than those at any time during the past 800,000 years [4–7]. Less direct geological evidence, based on boron-isotope ratios in ancient planktonic foraminifer shells, suggests that such high CO2 atmospheric concentrations were last seen about 20 million years ago [8]. Since 1896, a trend has been associated with elevated CO2 levels in the atmosphere and an increase in the average global temperature [9]. In ~100 years (1906–2005), the average global temperature increased by 0.7 ± 0.2°C, compared to a relatively constant average over the previous two thousand years [10]. This rise in CO2 levels is linked not only to surface temperature increases (with rising sea levels, melting of the glacier, and polar ice caps) but also to increased ocean acidity [11–13]. Fossil fuel burning accounts for 75% of the elevated atmospheric CO2 levels from human activity over the past 20 years, with the remainder being associated primarily with deforestation [14]. 2. Carbonic Anhydrase The need for development of an efficient and inexpensive carbon sequestration system and the drive for an