Enzymes are proteins that consist of chains of amino acids gathered together by peptide bonds. An enzyme molecule can have one or more peptide bond or polypeptide chains. The chain of amino acids inside the polypeptide chains is characteristic for each enzyme and is believed to establish the unique three-dimensional conformation in which the chains are organized.

This conformation, which is necessary for the functionality of the enzyme, is stabilized by interactions of amino acids in several parts of the peptide chains with each other and with the surrounding medium. These interactions are somewhat weak and can be disrupted readily by high temperatures, acid or alkaline ambience, or alterations in the polarity of the medium. These changes produce an unfolding of the peptide chains (or denaturation) and a subsequent loss of enzymatic activity, solubility, and other abilities characteristic of the native enzyme.

Many enzymes have an extra, nonprotein component, called a coenzyme. This can be an organic molecule, often a vitamin by-product, a metal ion (copper and zinc for some of the enzymes in snail mucins) or an organic (often metal-containing) group.

The coenzyme, in most instances, works directly in the catalytic reaction. For example, it can serve as an intermediate carrier of a group being transferred from one substrate to another. Certain enzymes have coenzymes that are strongly bound to the protein and difficult to eliminate, while others have coenzymes that dissociate readily. When the protein moiety and the coenzyme are separated from each other, neither conserves the catalytic abilities of the former conjugated protein (the holoenzyme).

By simply mixing the protein moiety and the coenzyme together, the completely active holoenzyme can often be reconstituted. The same coenzyme can be linked with many enzymes which catalyze different activities. It is thus primarily the nature of the protein moiety rather than that of the coenzyme which commands the specificity of the reaction.

The enzyme-cofactor couple confers an active configuration, commonly presenting an active site into which the substance (substrate) involved in the reaction can fit. Many enzymes are specific to one substance. If a competing molecule blocks the active site or changes its shape, the enzyme's activity is inhibited. If the enzyme's configuration is destroyed its activity is lost.

Enzymes are classified by the kind of reaction they catalyze: (1) oxidation-reduction, (2) transfer of a chemical group, (3) hydrolysis, (4) elimination or addition of a chemical radical, (5) isomerization, and (6) binding together of substrate units (polymerization).

Enzymes catalyze all aspects of cell metabolism, including the digestion of aliments, in which large nutrient molecules (even proteins, carbohydrates, and fats) are broken down into tinier molecules; the conservation and changing of chemical energy; and the construction of cellular materials and structures.

The fermentation of wine, leavening of bread, curdling of milk into cheese, and brewing of beer are all enzymatic reactions. The uses of enzymes in medicine involve destroying disease-causing microorganisms, the treatment of injuries, and diagnosing some diseases.

Thanks to enzymatic processes, science has been able to derive new treatments that can help repair damaged skin.