Most of the enzymes for the implementation of catalytic activity require 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 the protein and cofactor is called the holoenzyme, and only the protein part is called the enzyme. The cofactor, which is constantly part of the enzyme and is connected with it by covalent bonds, is called the prosthetic group.
In a broader sense, a cofactor is an additional group of any complex protein that supports 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 by chemical nature are divided into 2 large groups:
- divalent metal ions (zinc, magnesium, potassium, copper, manganese, iron, etc.);
- Coenzymes are organic non-protein compounds.
In turn, coenzymes are divided into vitamins with their derivatives and compounds of a non-vitamin nature. The latter include:
- UDP glucose;
- nucleotides;
- metalloporphyrins;
- FAD, NAD + , NADF + ;
- 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 by chemical means.
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, which means that it is not a coenzyme. In a generalized interpretation, coenzyme is a special case of 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
Cofactors of enzymes can perform various functions, including:
- participation in the formation and stabilization of the tertiary and quaternary conformations;
- stabilization of the substrate or catalytic center, ensuring complementarity 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 associated with cofactors.