Cofactors, Coenzymes, and Prosthetic Groups
Enzymes are specialized proteins that act as catalysts to get complex chemical reactions started. All chemical reactions require some energy to start. The energy required to get a reaction going is called its activation energy. Many of the simpler chemical reactions require so little energy that they can occur at relatively low temperatures without any outside influence. When heat is added, the additional heat energy is often enough to get many other reactions going. More complex reactions require some other stimulus, or catalyst to begin. Enzymes act as catalysts for chemical reactions by changing one or more of the reactants, called the substrate, in a way that lowers the activation energy enough for the reaction to begin. Some reactions won’t occur without their specific enzymes. With some others, the enzyme isn’t necessary, but makes it easier. Unlike the reactants, enzymes aren’t consumed during the reaction.
Sometimes the enzyme alone isn’t enough to get a reaction going. Cofactors are small molecules that an enzyme needs in order to work. Such enzymes are called apoenzymes without their cofactors, but complete, catalytically active enzymes are called holoenzymes. Cofactors can either be organic or inorganic, with inorganic metal ions being the most common. Cofactors bind to and are released from an enzyme’s active site, just like a substrate.
Coenzymes & Prosthetic Groups
Coenzymes are the organic cofactors. These organic molecules, most of which are vitamins or vitamin derivatives, make up part of the active sites of their protein enzymes. Since the coenzyme makes up a part of the enzyme’s active site, the enzyme can’t work on the substrate without it. Cosubstrates are coenzymes that are bound to the enzyme temporarily, will be released at some point, and will likely bind again later. A coenzyme that is very tightly bound and will not be released is called a prosthetic group.
All chemical reactions require some energy, called activation energy, to begin. Enzymes are specialized proteins that catalyze the more complex reactions by lowering that activation energy. They do this by altering one of the reactants, called the substrate, so that it can react more readily with the other reactant(s). Enzymes engage with substrate reactants through their active sites, but sometimes they need a little help.
A cofactor is any non-protein molecule that enables an otherwise inactive enzyme to catalyze a reaction. They work by binding to their enzyme’s active site and modifying it so that the target substrate will bind as well. Most cofactors are inorganic metal ions. If a cofactor is an organic molecule, like a vitamin, it’s called a coenzyme. Coenzymes are further divided into cosubstrates, which are transient coenzymes that bind temporarily, and prosthetic groups, which are permanently bound to the enzyme.
It is important to note that different sources give slightly different definitions for cofactors, coenzymes, and prosthetic groups. For example, some consider tightly bound organic molecules as prosthetic groups, but don’t classify them as coenzymes. Others define all non-protein organic molecules that aid in enzymatic activity as coenzymes, and count prosthetic groups as a sub-category of coenzymes. Others only refer to inorganic molecules and metal ions as cofactors, and don’t count coenzymes and prosthetic groups as sub-categories. There have been suggestions for more standardized definitions for each word, but the terms are still often used loosely.