Unenzymes

Unenzymes is a term that appears only sporadically in biochemical literature. It is not part of the standard taxonomy of enzymes, and its meaning varies between sources. In a narrow sense, unenzymes refer to substances or molecular systems that accelerate chemical reactions but do not fit the conventional definition of enzymes as proteins with a well-defined active site formed by specific catalytic residues. The label is often used to highlight catalytic activity that lacks a classic enzyme architecture or to describe catalysts that operate without a fixed active site.

Interpretations of the concept include non-protein catalysts with biological relevance, metal ion– or cofactor-mediated catalysis, or RNA-based catalysts that lack a protein component. Some discussions emphasize unenzymes as entities where catalysis arises primarily from the chemical environment, surfaces, or assembled cofactors rather than from a traditional enzyme scaffold. Because the term is not standardized, it can overlap with concepts such as metalloenzymes, ribozymes, or small-molecule catalysts.

Critics argue that unenzyme is a vague label that can confuse rather than clarify, and many scientists prefer to use established categories based on mechanism and molecular composition. In current usage, the term is mainly of historical or speculative interest and does not replace conventional classifications.

See also: enzyme, ribozyme, metalloenzyme, enzyme mimics, catalysis.