What is usually the origin of a non-standard amino acid residue found in a polypeptide chain?

The origin is a posttranslational modification of a standard aminoacid

Explanation:

There are 20 standard amino acids that can be found commonly in proteins. A non-standard amino acid is produced from an standard amino acid which suffered a chemical modification after it was incorporated in a protein chain, which is ussually called posttraslational modification. For example, γ-carboxyglutamic acid (Gla) is a non-standard amino acid that is formed from the standard amino acid glutamic acid (Glu). The γ-proton of glutamic acid is substacted and CO₂ is added. The reaction requires vitamin K as cofactor and it is catalyzed by a carboxylase. Gla has the ability of binding calcium and play an important role in blood coagulation.