ENZYMES Proteins  Elmhurst College
Lock and Key Theory Coenzyme - NAD+ Enzyme Regulation  Chemistry Department
Carboxypeptidase Enzyme Inhibitors  Virtual ChemBook

Role of Enzymes in Biochemical Reactions

Introduction - Enzyme Characteristics:

A living system controls its activity through enzymes. An enzyme is a protein molecule that is a biological catalyst with three characteristics. First, the basic function of an enzyme is to increase the rate of a reaction. Most cellular reactions occur about a million times faster than they would in the absence of an enzyme. Second, most enzymes act specifically with only one reactant (called a substrate) to produce products. The third and most remarkable characteristic is that enzymes are regulated from a state of low activity to high activity and vice versa. Gradually, you will appreciate that the individuality of a living cell is due in large part to the unique set of some 3,000 enzymes that it is genetically programmed to produce. If even one enzyme is missing or defective, the results can be disastrous.

Much of the information about enzymes has been made possible because they can be isolated from cells and made to work in a test tube environment. Extensive work has also been done with X-Ray diffraction techniques to elucidate the three-dimensional structure of some enzymes.

The ribbon and backbone form of carboxypeptidase is shown on the left. The substrate is shown in magenta.

 Enzyme Parts List:

The activity of an enzyme depends, at the minimum, on a specific protein chain. In many cases, the enzyme consists of the protein and a combination of one or more parts called cofactors. This enzyme complex is usually simply referred to simply as the enzyme.

Apoenzyme: The polypeptide or protein part of the enzyme is called the apoenzyme and may be inactive in its original synthesized structure. The inactive form of the apoenzyme is known as a proenzyme or zymogen. The proenzyme may contain several extra amino acids in the protein which are removed, and allows the final specific tertiary structure to be formed before it is activated as an apoenzyme.

Cofactors: A cofactor is a non-protein substance which may be organic, and called a coenzyme. The coenzyme is often derived from a vitamin with specific examples discussed later.

Another type of cofactor is an inorganic metal ion called a metal ion activator. The inorganic metal ions may be bonded through coordinate covalent bonds. The major reason for the nutritional requirement for minerals is to supply such metal ions as Zn+2, Mg+2, Mn+2, Fe+2, Cu+2, K+1, and Na+1 for use in enzymes as cofactors.

Final Enzyme: The type of association between the cofactor and the apoenzymes varies. In some cases, the bonds are rather loose and both come together only during the course of a reaction. In other cases, they are firmly bound together by covalent bonds. The activating role of a cofactor is to either: activate the protein by changing its geometric shape, or by actually participating in the overall reaction.

The overall enzyme contains a specific geometric shape called the active site where the reaction takes place. The molecule acted upon is called the substrate.

Carboxypeptidase - Chime in new window

QUES: Using a diagram and in your own words, describe the various parts of the enzyme, i.e. What is an enzyme?

Quiz: Inactive protein enzyme?
Organic nonprotein part of enzyme?  
 Protein part of enzyme?  


IEC Classification of Enzymes

 Group Name

 Type of Reaction Catalyzed
 Oxidases or Dehydrogenases  Oxidation-reduction reactions
 Transferases Transfer of functional groups
 Hydrolases Hydrolysis reactions
 Lyases Addition to double bonds or its reverse
 Isomerases Isomerization reactions
 Ligases or Synthetases Formation of bonds with ATP cleavage

Enzyme Nomenclature and Classification:

Enzymes are commonly named by adding a suffix "-ase" to the root name of the substrate molecule it is acting upon. For example, Lipase catalyzes the hydrolysis of a lipid triglyceride. Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

A few enzymes discovered before this naming system was devised are known by common names. Examples are pepsin, trypsin, and chymotrypsin which catalyzes the hydrolysis of proteins.

The latest systematic nomenclature system known as the International Enzyme Commission (IEC) system is based upon the type of reaction catalyzed. There are six broad groups of enzymes in this system as shown in table on the left.

For example, when using this system, "urease" becomes "urea amidohydrolase." Do not be overly concerned about enzyme names, but be able to recognize a substance as an enzyme by its "-ase" ending. Some types of reactions which are being catalyzed will be self evident.