Initiation FactorsInitiation factors are proteins that enable ribosomes to attach to mRNA. These factors can be produced when certain proteins are needed. For example, the eggs of many organisms contain mRNA that is not needed until after fertilization. At this time, an initiation factor is activated.Posttranslational ControlThese mechanisms act after the protein has been produced.Protein ActivationSome proteins are not active when they are first formed. They must undergo modification such as folding, enzymatic cleavage, or bond formation.Example: Proinsulin is a precursor to the hormone insulin. It must be cleaved into 2 polypeptide chains and then some amino acids must be removed to form insulin.Many proteins are activated by adding phosphate groups. They can be inactivated by removing phosphate groups. For example, kinases activate by adding phosphate groups and phosphodiesterase inactivates by removing the phosphate groups.
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