Reversible hydration of carbon dioxide (Homo sapiens)
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Description
Carbonic anhydrases reversibly catalyze the hydration of carbon dioxide and directly produce bicarbonate and protons, bypassing the formation of carbonic acid (reviewed in Lindskog 1997, Breton 2001, Esbaugh and Tufts 2006, Boron 2010, Gilmour 2010). Carbonic anhydrase deprotonates water to yield a zinc-hydroxyl group and a proton which is transferred to external buffer molecules via histidine or glutamate residues in carbonic anhydrase. The hydroxyl group reacts with carbon dioxide in the active site to yield bicarbonate. A water molecule displaces the bicarbonate and the reaction cycle begins again. There are currently 12 known active carbonic anhydrases in humans.
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CA4 has high catalytic activity. CA9, CA12, and CA14 have moderate activity. CA4 is anchored to the extracellular face of the plasma membrane by glycosylphosphatidylinositol. CA9, CA12, and CA14 are single-pass transmembrane proteins. CA4 is found on the extracellular face of capillaries in kidney, lung, and muscle where it maintains the gradient of carbon dioxide between tissue and blood. CA9 and CA12 are found on basolateral membranes of epithelia. CA9 is inducible by Hypoxia-inducible factor 1 alpha (HIF1alpha) and acidifies the extracellular environment of tumors. In rodents CA15 is membrane anchored and has low activity; in primates CA15 is a pseudogene.
CA2 and CA7 have high catalytic activity, CA1 has low activity (10% of the activity of CA2), and CA3 has very low activity (1% of the activity of CA2). CA1 and CA2 are found in erythrocytes. CA2 is also found in kidney, lung, and white muscle where it facilitates diffusion of carbon dioxide. CA3 is found in red muscle where it participates in resistance against oxidative stress.
CA4 has high catalytic activity. CA9, CA12, and CA14 have moderate activity. CA4 is anchored to the extracellular face of the plasma membrane by glycosylphosphatidylinositol. CA9, CA12, and CA14 are single-pass transmembrane proteins. CA4 is found on the extracellular face of capillaries in kidney, lung, and muscle where it maintains the gradient of carbon dioxide between tissue and blood. CA9 and CA12 are found on basolateral membranes of epithelia. CA9 is inducible by Hypoxia-inducible factor 1 alpha (HIF1alpha) and acidifies the extracellular environment of tumors. In rodents CA15 is membrane anchored and has low activity; in primates CA15 is a pseudogene.
CA2 and CA7 have high catalytic activity, CA1 has low activity (10% of the activity of CA2), and CA3 has very low activity (1% of the activity of CA2). CA1 and CA2 are found in erythrocytes. CA2 is also found in kidney, lung, and white muscle where it facilitates diffusion of carbon dioxide. CA3 is found in red muscle where it participates in resistance against oxidative stress.