Most chemical reactions do not occur spontaneously in nature, since the amount of free energy called activation energy necessary for this is too great.


 

Enzymes are biological catalysts.  They play a major role in metabolic pathways by lowering the activation energy necessary for chemical reactions to occur.

Enzymes are generally named based on the type of substrate they act upon, followed by the suffix -ase.

The primary structure of any enzyme is composed of protein.  Some enzymes, called simple enzymes, are completely protein.  Others, called conjugated enzymes, are composed of a protein portion called the apoenzyme and one or more nonprotein portions. There are two main categories of such substances:

Cofactors are inorganic metal ions such as iron, magnesium or zinc.

Coenzymes are organic compounds composed of vitamins or vitamin derivatives.  Examples include NAD+, NADP+ and FAD+, all of which serve in the transfer of electrons and hydrogens released by catabolic pathways.

Both cofactors and coenzymes help to complete the structure of a conjugated enzyme, forming a holoenzyme.

Some RNA molecules also have enzyme function.  These special substances, called ribozymes, serve to remove pieces of nonfunctional messenger RNA (mRNA) called introns and splice the functional portions, called exons, together.

Enzymes operate by binding to a substrate or substrates temporarily, lowering the activation energy necessary for a reaction to take place.  The catalytic (active) site is the portion of the enzyme where substrate binding occurs.

Each enzyme is specific for one or more substrate types.  Two different theories about how enzyme-substrate binding exist.  In the induced-fit (key and lock) model (a), the active site of an enzyme has a shape specific to the substrate.  In the space-filling (conformal) model (b), the holoenzyme changes its shape to bind to the substrate(s).
 

Factors affecting enzyme activity include temperature, pH and substrate concentration.  High temperatures and high or low pH can cause enzymes to denature or lose their shape, thus their activity since they can no longer bind to a substrate.  Enzyme activity will increase until saturation is reached, where all enzyme molecules are bound to substrate.

Heavy metal compounds such as mercury and silver nitrate can also block enzyme activity by binding to sulfhydryl groups that hold the structure of the protein portion of the enzyme together.

Enzyme activity can be blocked by competitive inhibition.  This occurs when a substance having a similar chemical composition to the usual substrate competes for the enzyme's catalytic site.  One example is the activity of the antimicrobial compound sulfonamide that competes with the binding site for the substance para-aminobenzoic acid (PABA) some bacteria use in the synthesis of folic acid compounds necessary for the synthesis of nucleic acids.
 


 

Enzyme activity can be regulated in several ways:

On a genetic level, enzymes are controlled by special DNA pathways called operons that can be activated or shut down due to the presence or absence of the substrate.

In allosteric inhibition (a), an enzyme having a secondary active site called an allosteric site closes the active site, preventing the formation of the enzyme-substrate complex.

In allosteric activation (b) the substrate binds to the allosteric site, allowing the active site to open.


 

Allosteric enzymes can be used to regulate enzymatic pathways.  In a negative-feedback pathway, the end product of a pathway binds to the allosteric site of the first enzyme, closing its active site and blocking the rest of the reactions.