Enzyme

An enzyme is a biological catalyst, almost always a protein, that speeds up a specific chemical reaction without being consumed by it. Reactions that would take years at body temperature happen in milliseconds because an enzyme lowers the energy required.

Specificity is the defining feature. Each enzyme has an active site shaped for a particular substrate, which is why the body needs thousands of different ones, and why a single missing enzyme can cause a whole metabolic disease.

They are regulated rather than simply present. Activity changes with temperature, pH, the concentration of substrate and product, and with control mechanisms including phosphorylation, which is how hormones such as insulin exert their effects inside cells.

They matter for training because adaptation is largely enzymatic. Endurance training increases the concentration of oxidative enzymes in muscle, which is a substantial part of why a trained muscle produces more energy aerobically at the same pace.

They matter for nutrition because digestion is enzymatic. Amylase, protease and lipase break food into absorbable pieces, and lactose intolerance is simply reduced production of one of them.

The supplement version is where care is needed. Digestive enzyme supplements have clear uses in genuine insufficiency, such as pancreatic disease or lactase deficiency, and much weaker support for general bloating or for improving nutrient absorption in healthy people. Enzymes taken by mouth for other purposes are usually digested like the proteins they are.

In practice, enzymes are the reason training changes what your muscle can do, and the reason most enzyme supplements do not do what they claim.