Conjugation reactions are characterized by a number of criteria, namely:
1. The substrate is coupled to an endogenous conjugating molecule or moiety (sometimes called an 'endocon'), producing a metabolite known as a conjugate.
2. This endogenous molecule or moiety is highly polar (hydrophilic) and has a molecular weight (MW) in the 100-300 Da range.
3. Conjugation reactions are catalyzed by enzymes known as transferases.
4. They involve a cofactor that binds to the enzyme in close proximity to the substrate and carries the endogenous molecule or moiety to be transferred.
The first criterion is an essential one, and it is the criterion of decision. The other criteria are neither sufficient nor necessary to define conjugation reactions. They are not sufficient, since for example in hydrogenation reactions (i.e., typical reactions of functionalization), the hydride is also transferred from a cofactor (NADPH or NADH). And they are not necessary, since they show some important exceptions, which will become apparent below.
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