Phosphorylation is the addition of a phosphate (PO43-) group to a protein or other organic molecule. Phosphorylation turns many protein enzymes on and off, thereby altering their function and activity.

Protein phosphorylation in particular plays a significant role in a wide range of cellular processes. Its prominent role in biochemistry is the subject of a very large body of research (as of March 2012, the Medline database returns nearly 200,000 articles on the subject, largely on protein phosphorylation).

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