An enzyme is a catalyst, which is a chemical agent that changes the rate of a
reaction without being consumed by the reaction. The reactant that an enzyme
acts on is called its substrate. While the enzyme is bound to the substrate, the
catalytic action converts the substrate to the product. Because of its shape,
each enzyme catalyzes a specific reaction (molecular recognition). Only a
certain region of the enzyme molecule actually binds to the substrate. This area
is called the active site and is usually a groove on the surface of the enzyme.

The shape of the active site and the shape of the substrate fits accordantly.

When the substrate enters the active site, it induces the enzyme -in a process
called induced fit- to change its shape to fit tightly around the substrate. The
induced fit is very important during the catalytic cycle when the substrate
enters the active site to form an enzyme-substrate complex. The enzymes emerge
from the reaction, however, in their original form and are ready to function
over and over again. When a reaction rearranges the atoms of molecules, the
bonds in the reactants must be broken and the new bonds in the products must be
formed. This requires energy. The energy required to break the bonds in the
reactant molecules is known as the activation energy. The bonds only break when
the molecule has absorbed enough energy (usually in the form of heat) to become
unstable. Heat speeds up a reaction, but the high temperature kills cells;
therefore, organisms must instead use a catalyst. Enzymes (mostly proteins) are
biological catalysts. Enzymes speed up reactions by lowering the barrier of
activation energy, so that the transition state is within reach at regular
temperatures. A transition state occurs when reactants are in an unstable
condition and are ready for the reaction to take place. An enzyme cannot changes
the free energy of the reaction or the spontantinuity of the reaction. It can
only accelerate the reactions that would occur normally. However, this increases
the metabolism of the cell and determines which chemical process will be going
on in the cell at a certain time. Enzymes are regulated by factors that affect
their activity. Some of these factors are environmental conditions, cofactors,
enzyme inhibitors, allosteric regulation, and cooperativity. Enzymes are
sensitive to their environment and each enzyme has environmental conditions in
which it works best. For example, a temperature change can disrupt the hydrogen
bonds, ionic bonds, and other weak interactions that stabilizes the protein
(enzyme); thus, the molecule denatures. Other environmental factors that
influences the shape of proteins is pH and salt concentration. Also, many
enzymes require cofactors, nonprotein helpers for catalytic activity. Some
cofactors are inorganic like zinc and copper; however, organic cofactors are
called coenzymes like vitamins. Another factor is allosteric regulation.

Molecules that inhibit enzyme activity may bind to an allosteric site, a
receptor site separate from the active site. Complex enzymes with two or more
polypeptide bonds may have allosteric sites where the polypeptides join. Also,
the entire unit may oscillate between two conformational states, and the
inhibitor stablizes the catalytic conformation. Cooperativity, another factor,
occurs when the induced-fit binding of a substrate molecule to one polypeptide
subunit can change the conformation such that the active sites of all subunits
are more active.