A non-enzymatic reaction refers to a chemical reaction that occurs without the need for participation (or catalysis) of an enzyme. While enzymes act as biological catalysts that lower the activation energy needed for biochemical reactions to occur, they do not prevent the parallel occurrence of non-enzymatic reactions. In terms of thermodynamic principles, the same reaction characteristics apply for both non-enzymatic and enzymatic reactions alike, be it that every enzymatic reaction can also occur in principle non-enzymatically. Non-enzymatic reactions have been essential in shaping the evolution of metabolic pathways, and are retained as part of many modern metabolic networks. Non-enzymatic reactions have long provided the necessary groundwork for the evolutionary selection of enzymes, given that the occurrence of non-enzymatic reactions across the metabolic landscape is not prevented by the presence of enzymes. Three classes of non-enzymatic reactions dominate the modern metabolic landscape: Class I, Class II, and Class III.
Class I Non-Enzymatic Reactions Class I reactions are grouped by their broad chemical reactivity and low specificity for metabolic targets. These include glycation reactions, Maillard reactions (e.g. in food science), oxidation reactions (e.g. driven by reactive oxygen species), and non-enzymatic, covalent modifications of lipids and proteins (e.g. alkylation, acylation). Descriptions of such reactions are detailed within the table below:
Class II Non-Enzymatic Reactions Class II reactions are highly specific and occur purely non-enzymatically, with a well known example of such being the maturation of vitamin D3 (see below). Most class II reactions are spontaneous, and proceed without the need for a catalyst or atypical energy source.
Class III Non-Enzymatic Reactions Class III non-enzymatic reactions occur in parallel to enzymatic functions and are widely observed throughout metabolism, as many metabolic pathways descend from the evolutionary groundwork laid by promiscuous or non-enzymatic precursors. Typically, the parallel enzymatic reaction exists to prevent the formation of unwanted secondary products, which would be generated by the non-enzymatic process (e.g. negative catalysis). Class III non-enzymatic reactions occur in parallel with all six major enzymatic classes (ie., oxidoreductases, transferases, hydrolyses, lyases, isomerases, ligases). Examples of such reactions are detailed within the table below:
See Also Enzyme Catalysis Metabolic Pathways
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