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Enzyme
Properties
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Basics | Properties
| Coenzymes
and Cofactors
| Isoenzymes |
I
proteins I electrolytes |
basis for catalysis I
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.
POLYELECTROLYTES:
Proteins they have many ionizable groups. Amino acids components
are ionizable: such as carboxyl, and basic amine.
For example, look at 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]
 CATALYST:
As Dr. Paul Boyer, previously at the University
of Wisconsin, summarizedso well (and we have edited
a little):
*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
enzymes do in reactions?
*Make bonds
*Break bonds
*Bring reactants together
What happens?
*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. |
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The BASIS of
CATALYSIS
Consider a very simple reaction
such as: S
 P
S may conceivably be a substance
which can be kept in the laboratory, perhaps unchanged,
for long periods of time.
For the reaction S P 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 (see figure above right).
The energy needed to reach the activated state is
called activation energy
or Ea.
Thus
the basis of catalysis is
the lowering of Ea, enabling the reaction to go more easily.
Why
not just apply heat to make the reaction go?
Heat can help (see middle graph
to the right). But neither the enzymes (nor our bodies)
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.
Are enzymes better than heat?
YES! Enzymes are more efficient,
even WITHOUT added heat! (See the third
graph on the right.)
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From
Robert J. Huskey's Activation
Energy and Enzymes I [University of Virginia]
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DEPARTMENT
OF BIOCHEMISTRY AND MOLECULAR BIOLOGY, 185
South Orange Avenue, Newark, NJ 07103-2714.
Phone: 973-972-4750.
FAX: 973-972-5594. For information, contact Dr. Kumar:
kumarsu@umdnj.edu
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