In biochemistry, activation, specifically called bioactivation, is where enzymes or other biologically active molecules acquire the ability to perform their biological function, such as inactive proenzymes being converted into active enzymes that are able to catalyze their substrates reactions into products. Bioactivation may also refer to the process where inactive prodrugs are converted into their active metabolites, or the toxication' of protoxins into actual toxins.
An enzyme may be reversibly or irreversibly bioactivated. A major mechanism of irreversible bioactivation is where a piece of a protein is cut off by cleavage, producing an enzyme that will then stay active. A major mechanism of reversible bioactivation is substrate presentation where an enzyme translocates near its substrate. Another reversible reaction is where a cofactor binds to an enzyme, which then remains active while the cofactor is bound, and stops being active when the cofactor is removed.
In protein synthesis, amino acids are carried by transfer RNA (tRNA) molecules and added to a growing polypeptide chain on the ribosome. In order to transfer the amino acids to the ribosome, tRNAs must first be covalently bonded to the amino acid through their 3' CCA terminal. This binding is catalyzed by aminoacyl-tRNA synthetase, and requires a molecule of ATP. The amino acid bound to the tRNA is called an aminoacyl-tRNA, and is considered the activated molecule in protein translation. Once activated, the aminoacyl-tRNA may move to the ribosome and add the amino acid to the growing polypeptide chain.