BioWeb
Glossary of biochemical terms
Enzyme Properties
I proteins I catalyst I activation energy I
As Proteins: As proteins, enzymes possess properties in common with other proteins.  They are large, with molecular weights from 10,000 to well over a million.  For instance, ribonuclease has a molecular weight of 12,700. Enzymes are labile (unstable).  Thus they may suffer subtle changes in their structure, called denaturation. This in turn can cause them to lose activity.  Hence, they have to be handled carefully.  Adverse conditions of temperature, pH and salt concentration are some of the factors that cause inactivation.
        All enzymes are polyelectrolytes i.e., they contain many ionizable groups. Amino acids components are ionizable: such as carboxyl, and basic amine. Consider amino acids such as Asp (asparagine), Glu (glutamine), His (histidine), Arg (arginine), and Lys (lysine).  As the pH changes, these groups become more or less ionized, so the net charge on the enzyme depends upon pH.  At low pH, the charge is positive, while at alkaline pH, it is negative.  While the charge on the entire molecule is important, it is particularly crucial that the catalytic site bears the correct charge. [Check out Dr. Kirk McMichael's discussion of this on his Amino Acid Structure Page, with diagrams: from Washington State University]
The Enzyme as a Special Protein: As Dr. Paul Boyer, at the University of Wisconsin, summarizes so well (and we have edited a little):
They act as a CATALYST
   *Catalysts facilitate the process of chemical reactions 
   *The figure to the right shows how much energy is required to produce products from the reactants
           -Note the reduction in energy necessary if enzyme is present 
           -The difference is in amount of "activation energy"  -- sort of like conserving energy? 
   *What can they do in reactions? 
           -Make bonds 
           -Break bonds
           -Bring reactants together 
   *How does it happen?
           -Extreme specificity of reactants and what is done with them reduces errors of metabolism
           -The fit between enzyme and reactants is often likened to a lock and key
The Basis of catalysis: Consider a very simple reaction  such as:   SP
    S may conceivably be a substance which can be kept in the laboratory, apparently unchanged, for long periods of time. 
    For the reaction  SP to take place it may be necessary that we apply heat or in some manner activate the molecule.  In other words, the compound  S must be raised to a higher energy state (Refer to Figure to the right).  The energy needed to reach the activated state is called Ea

[Take a look at the diagram to the right.  For the original and an explanation see Dr. Robert J. Huskey's Activation Energy and Enzymes I: University of Virginia]
If the reactions require heat why Don't we Just Apply Heat in the Body?
It Would Help (see middle graph to the right)! But neither we nor the enzymes are very stable at high temperatures. In fact most vertebrate enzymes are denatured at temperatures above 40oc. This denaturation (inactivation of the enzyme) is a result of breaking of H bonds and some electrostatic interactions. These bonds are very important in maintaining the active structure (conformation) of the enzyme.

It Turns Out That Enzymes are More Efficient, even WITHOUT added Heat!
[Take a look at the diagram to the right.  For the original and an explanation see Dr. Robert J. Huskey's Activation Energy and Enzymes II: University of Virginia].