The Posttranslational modification reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Posttranslational modification

Posttranslational modification means the chemical modification of a protein after its translation. It is one of the later steps in protein biosynthesis for many proteins.

Posttranslational modification may involve the formation of disulfide bridges and attachment of any of a number of biochemical functional groups, such as acetate, phosphate, various lipids and carbohydrates. Enzymes may also remove one or more amino acids from the amino end of the polypeptide chain, leaving a protein consisting of two polypeptide chains connected by disulfide bonds. In other cases, two or more polypeptide chains that are synthesized separately may associate to become subunits of a protein with quaternary structure.

A protein is a chain composed of a long sequence of 20 possible amino acids, also called a polypeptide. Some posttranslational modification extends the range of possible functions a protein can have by introducing other chemical groups into the makeup of a protein (e.g., carbohydrate chains). Other modifications like phosphorylation are part of common mechanisms for controlling the behavior of a protein.

Types of posttranslational modifications include:

• phosphorylation, the addition of a phosphate group, usual to serine, tyrosine, threonine or histidine
• acetylation, the addition of an acetyl group, usually at the N-terminus of the protein
• farnesylation, the addition of a farnesyl group
• glycosylation, the addition of a glycosyl group to either asparagine, hydroxylysine, serine, or threonine, resulting in a glycoprotein

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