Most of the enzymes for the implementation of catalytic activity needs auxiliary elements - cofactors. These substances are non-protein in nature and are not always a structural part of the enzyme molecule. The functional complex of a protein and a cofactor is called a holoenzyme, and only the protein part is called an apoenzyme. A cofactor that is permanently part of an enzyme and is linked to it by covalent bonds is called a prosthetic group.
In a broader sense, a cofactor is an additional group in any complex protein that maintains it in a functional state. In enzyme proteins, cofactors can be directly involved in the catalysis reaction.
Characteristics and types of cofactors
Cofactors are low molecular weight substances that are chemically divided into 2 large groups:
- ions of divalent metals (zinc, magnesium,potassium, copper, manganese, iron, etc.);
- coenzymes - organic non-protein compounds.
In turn, coenzymes are divided into vitamins with their derivatives and compounds of non-vitamin nature. The latter include:
- UDP-glucose;
- nucleotides;
- metalloporphyrins;
- FAD, OVER+, NADP+;
- glutathione;
- ubiquinone;
- S-adenosylmethionine.
Cofactors can form both strong covalent and weak bonds with enzymes. Some groups interact with the polypeptide part so strongly that it is difficult to separate them even chemically.
Difficulties in defining the concepts of "cofactor" and "coenzyme"
In a narrow sense, cofactors are metal ions, and coenzymes are groups of organic nature. If we consider these elements from the point of view of their functional significance, then the cofactor does not participate in the catalysis reaction, and therefore is not a coenzyme. In a generalized interpretation, a coenzyme is a special case of a cofactor.
Such a number of interpretations is due to the fact that in modern biochemistry these terms are ambiguous concepts that do not have a universal definition.
The biological role of cofactors
Enzyme cofactors can perform a variety of functions, including:
- participation in the formation and stabilization of tertiary and quaternary conformations;
- stabilization of the substrate or catalytic center, ensuringcomplementarity between them;
- participation in catalysis as an additional substrate;
- regulation of enzymatic activity;
- participation in redox reactions.
Regardless of the mechanism of action and the chemical nature of the cofactor, in its absence, the enzyme cannot carry out catalytic activity. However, there is a small group of enzymes whose functioning is not related to cofactors.
