Carbamates

A carbamate is an organic compound derived from carbamic acid (NH₂COOH). A carbamate group, carbamate ester (e.g., ethyl carbamate), and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes.

Carbamic acids are derived from amines:

Carbamic acid is about as acidic as acetic acid. Ionization of a proton gives the carbamate anion, the conjugate base of carbamic acid:

Carbamates also arise via hydrolysis of chloroformamides and subsequent esterification:

Carbamates may be formed from the Curtius rearrangement, where isocyanates formed are reacted with an alcohol.

Although most of this article concerns organic carbamates, the inorganic salt ammonium carbamate is produced on a large scale as an intermediate in the production of the commodity chemical urea from ammonia and carbon dioxide.

N-terminal amino groups of valine residues in the α- and β-chains of deoxyhemoglobin exist as carbamates. They help to stabilise the protein, when it becomes deoxyhemoglobin and increases the likelihood of the release of remaining oxygen molecules bound to the protein. This stabilizing effect should not be confused with the Bohr effect (an indirect effect caused by carbon dioxide).

The ε-amino groups of the lysine residues in urease and phosphotriesterase also feature carbamate. The carbamate derived from aminoimidazole is an intermediate in the biosynthesis of inosine. Carbamoyl phosphate is generated from carboxyphosphate rather than CO₂.

Perhaps the most important carbamate is the one involved in the capture of CO₂ by plants since this process is necessary for their growth. The enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase fixes a molecule of carbon dioxide as phosphoglycerate in the Calvin cycle. At the active site of the enzyme, a Mg²⁺ ion is bound to glutamate and aspartate residues as well as a lysine carbamate. The carbamate is formed when an uncharged lysine side chain near the ion reacts with a carbon dioxide molecule from the air (not the substrate carbon dioxide molecule), which then renders it charged, and, therefore, able to bind the Mg²⁺ ion.

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