Enzymes

Enzymes are biological catalysts.  They are proteins that fold into particular conformations such that they can help speed up very particular chemical reactions.  For biochemical reactions, the reactant is typically called the substrate.  The substrate is converted into the product.  The mechanisms for many enzymes are very similar.  The substrate(s) and the enzyme bind into a complex.  The physical location on the enzyme in which the substrate binds is called the "active site".  Once bound, this complex can then weaken particular bonds in the substrate such that chemistry occurs to form the product.  The product is weakly bound to the substrate such that it now dissociates and the enzyme is free to bind another substrate molecule.

The active sites in enzymes can be very specific such that the enzyme will only catalyze a very specific reaction for a very specific molecule.   Typically there is equilibrium between the bound complex and the free substrate and enzyme such that the binding could be reversible.  In contrast, once the product is formed the backward reaction typically will never happen.

Substrate + Enzyme ↔ Complex → Product.

The activity of many enzymes can be blocked by molecules which mimic the substrate but don't do any chemistry.  These molecules then effectively "turn off" the enzyme by blocking the active site and preventing binding of the substrate.  Many pharmaceutical drugs operate in this way.  Such molecules are typically called inhibitors as they inhibit the activity of the enzyme.

Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions. Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semi-metallic elements boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports.

In general, catalytic action is a chemical reaction between the catalyst and a reactant, forming chemical intermediates that are able to react more readily with each other or with another reactant, to form the desired end product. During the reaction between the chemical intermediates and the reactants, the catalyst is regenerated. The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Typical of these reactions are acid–base reactions, oxidation–reduction reactions, formation of coordination complexes, and formation of free radicals. With solid catalysts the reaction mechanism is strongly influenced by surface properties and electronic or crystal structures. Certain solid catalysts, called polyfunctional catalysts, are capable of more than one mode of interaction with the reactants; bifunctional catalysts are used extensively for reforming reactions in the petroleum industry.

Catalyzed reactions form the basis of many industrial chemical processes. Catalyst manufacture is itself a rapidly growing industrial process.