In some proteins, the side-chain carboxylate carbon of an...

In some proteins, the side-chain carboxylate carbon of an N-terminal glutamate residue reacts with the free amino group to form a pentagonal lactam, a cyclic structure containing a $\mathrm{C}=\mathrm{O}$ group. Draw the resulting pyroglutamate residue.

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Key Concepts

Pyroglutamate Formation

Pyroglutamate, also known as 5-oxoproline, is produced when the side-chain carboxylate of a glutamate residue reacts with the N-terminal amino group, leading to the formation of a five-membered cyclic lactam. This modification results in a cyclic structure that includes a carbonyl (C=O) group, and it is a common post-translational modification in peptides and proteins.

Lactam (Cyclic Amide) Chemistry

A lactam refers to a cyclic amide, which forms when an amino group attacks a carboxyl group within the same molecule to create a closed ring structure. The formation of a pentagonal lactam, as seen in the conversion of glutamate to pyroglutamate, involves intramolecular nucleophilic attack and results in a stable ring structure with distinct chemical properties.

N-Terminal Modification

Proteins often undergo modifications at their N-terminus, which can include cyclization reactions. In this process, the free amino group at the N-terminus can react with a nearby functional group, altering the protein’s structure and properties. In the given case, the N-terminal glutamate residue undergoes a cyclization that changes its chemistry and can affect the protein’s stability and function.

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