Introduction to Carbonic Anhydrase.

Carbonic anhydrases (CAs) form a widely distributed superfamily of metalloenzymes, characterized by considerable structural diversity. These enzymes catalyze the reversible conversion of carbon dioxide into bicarbonate and protons. While they all perform this fundamental reaction, CAs show notable structural plasticity, with eight classes (α, β, γ, δ, ζ, η, θ, and ι) that have evolved independently to optimize CO₂ hydration under a range of cellular, physiological, and ecological conditions. Their catalytic efficiency plays a crucial role in several biological processes, including acid-base balance, respiratory gas exchange, renal bicarbonate reabsorption, metabolic regulation, and photosynthetic carbon fixation. Isoform-specific expression allows CAs to act in multiple cellular contexts. They contribute to signaling pathways, help maintain pH balance, function within tissue-specific environments. When these enzymes are not properly regulated, they can be associated with a variety of health issues, including renal tubular acidosis, osteopetrosis, neurological disorders, and cancer progression. Evolution has shaped enzymes in different species to have similar active sites, flexible metal use, and varied protein assemblies, all helping them adapt to challenges like limited metal ions availability and changing CO₂ levels. CAs are not just important in biology. These metalloenzymes are also key targets for treating cancer, eye conditions, and infections. Besides, they show great promise as enzymes for capturing CO₂ and for other biotechnological applications. This chapter offers a thorough introduction to CAs, exploring their biology, how their structures and functions have evolved, and their potential applications, showing how these enzymes link the worlds of science, medicine, and the environment.