Enzyme Biochemistry

An enzyme is defined as a macromolecule that catalyzes a biochemical reaction. In this type of chemical reaction, the starting molecules are called substrates. The enzyme interacts with a substrate, converting it into a new product. Most enzymes are named by combining the name of the substrate with the -ase suffix (e.g., protease, urease). Nearly all metabolic reactions inside the body rely on enzymes in order to make the reactions proceed quickly enough to be useful.

Chemicals called activators can enhance enzyme activity, while inhibitors decrease enzyme activity. The study of enzymes is termed enzymology.

There are six broad categories used to classify enzymes:

oxidoreductases - involved in electron transfer
hydrolases - cleave the substrate by hydrolysis (uptaking a water molecule)
isomerases - transfer a group in a molecule to form an isomer
ligases (or synthetases) - couple the breakdown of a pyrophosphate bond in a nucleotide to the formation of new chemical bonds
oxidoreductases - act in electron transfer
transferases - transfer a chemical group from one molecule to another

How Enzymes Work

Enzymes work by lowering the activation energy needed to make a chemical reaction occur. Like other catalysts, enzymes change the equilibrium of a reaction, but they aren't consumed in the process. While most catalysts can act on a number of different types of reactions, a key feature of an enzyme is that it is specific.

In other words, an enzyme that catalyzes one reaction won't have any effect on a different reaction.

Most enzymes are globular proteins that are much larger than the substrate with which they interact. They range in size from 62 amino acids to more than 2,500 amino acid residues, but only a portion of their structure is involved in catalysis.

The enzyme has what is called an active site, which contains one or more binding sites that orient the substrate in the correct configuration and also a catalytic site, which is the part of the molecule that lowers activation energy. The remainder of an enzyme's structure acts primarily to present the active site to the substrate in the best way. There may also be allosteric site, where an activator or inhibitor can bind to cause a conformation change that affects the enzyme activity.

Some enzymes require an additional chemical, called a cofactor, for catalysis to occur. The cofactor could be a metal ion or an organic molecule, such as a vitamin. Cofactors may bind loosely or tightly to enzymes. Tightly-bound cofactors are called prosthetic groups.
Two explanations of how enzymes interact with substrates are the "lock and key" model, proposed by Emil Fischer in 1894, and the induced fit model, which is a modification of the lock and key model that was proposed by Daniel Koshland in 1958. In the lock and key model, the enzyme and the substrate have three-dimensional shapes that fit each other. The induced fit model proposes enzyme molecules can change their shape, depending on the interaction with the substrate.

In this model, the enzyme and sometimes the substrate change shape as they interact until the active site is fully bound.