Carbamate
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The carbamate functional group is formed when a carbon dioxide molecule reacts with the amino terminus of a peptide chain or an amino group of an amino acid, adding a COO− group to it and releasing a cation (H+ ion).
Biochemical Structure
R R O-
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N-H + O=C=O <——> N-C + H+
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H H O
"R" stands for the atoms attached to the other end of the nitrogen molecule of the amino group. Note that the COO− group is a resonance structure, (apologies for the poor representation of the C=O double bond), so the single bonds both show a degree of double bond character, and the charge is delocalised over the two oxygen atoms. This reaction is reversible (with reaction constant k << 0 in the above reaction), as the N-C bond is highly labile.
Some Occurrences of Carbamate Groups in Nature
In Haemoglobin, carbamate groups are formed when carbon dioxide molecules bond with the amino termini of the globin chains. This helps to stabilise the protein when it becomes deoxyhaemoglobin and increases the likelihood of the release of remaining oxygen molecules bound to the protein.
Ribulose 1,5-biphosphate carboxylase/oxygenase (the enzyme required to fix a carbon dioxide molecule at the start of the Calvin Cycle) also requires the formation of a carbamate to function. At the active site of the enzyme, a Mg2+ ion is bound to a glutamate residue, an aspartate residue and a lysine carbamate, which hold the ion in place. 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 so therefore able to bind the Mg2+ ion.
A group of insecticides also contain the carbamate functional group (See links below).