Some enzymes need to first become "activated" in order for them to work. These special enzymes are called Holoenzymes
You activate enzymes by attaching a Cofactor on to them. There are 2 types of cofactors:
1) coenzymes = these are loosely attached to the enzyme
2) prosthetic groups = these are tightly attached (ionic or covalently) to the enzyme
Enzymes which required cofactors to be functional are called apoenzyme when they are not functional and when they are functional (i.e attached to cofactors) called Holoenzyme (i.e the whole complex)
One example I can recall right out of my head is the RNA Polymerase in E.coli. It has a holoenzyme which includes the sigma factor, and when transcription initiates, the sigma factor falls off and the core enzyme works along to synthesize RNA.
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