Download Enzymes I – What Is an Enzyme?

Enzymes I – What Is an Enzyme?
Slide 2
As we saw in the previous lesson on thermodynamics, chemical reactions
do not only involve structural changes in molecules, they also involve changes in the
amount of energy of reactants and products. Chemical reactions releasing energy, for
example, are termed exergonic reactions, while chemical reactions requiring a net input
of energy are termed endergonic reactions. Regardless of whether a chemical reaction is
endergonic or exergonic, however, virtually all reactions require some initial input of
energy in order to proceed. This initial energy, called the activation energy, is required to
bring reactants to a less stable, or more reactive, transition state.
Activation energy can greatly affect the rate at which a chemical reaction proceeds. For
example, one important chemical reaction that takes place in your cells involves the
removal of phosphate groups from a type of molecule called a phosphate monoester. It is
estimated that without any aid, this reaction would occur about once every one trillion
years. In large part this is due to a relatively high activation energy which makes it very
difficult for the reaction to proceed on its own. As we know, however, organisms depend
on the constant and rapid chemical reactions occurring in their bodies and cells. So how
is life able to exist with the rates of important chemical reactions potentially so slow? The
answer is enzymes. Enzymes are biological catalysts that greatly increase the rates of
chemical reactions by reducing the activation energy that is required for the reaction. In
the case of the reaction just mentioned, the presence of the enzyme phosphatase, which
catalyzes the removal of phosphate groups from phosphate monoesters, decreases the
reaction time from one trillion years to a mere ten milliseconds!
Slide 3
So what exactly are enzymes? Enzymes are most often proteins, although
some other biological molecules, such as RNA, can have catalytic activity. Enzymes
contain specific binding sites, called active sites, where chemical reactions are catalyzed.
Active sites are usually relatively small parts of the enzyme, consisting of five to twenty
amino acids. A given enzyme may contain one to several active sites. The shape of the
active site, which is determined largely by the enzyme’s primary structure, is a specific
shape which fits a specific type of molecule. The rest of the enzyme acts almost like
scaffolding, supporting the active site and allowing it to maintain its proper shape.
Because different types of enzymes maintain different shapes and different active sites,
most enzymes are highly specific to a particular substrate. This means that most enzymes
can only catalyze one type of chemical reaction.
Slide 4
In a chemical reaction catalyzed by an enzyme, the reactants, or substrates,
bind to the active site of the enzyme. In general, this can occur in one of two ways. The
traditional description of substrate-active site interactions is called ‘lock and key.’ In this
case, the enzyme is the ‘lock,’ and the substrate is the ‘key.’ For each enzyme, or lock,
typically only one substrate, or key, will have the correct shape, and so be able to bind the
active site. Most biological enzymes can be described by the lock and key mechanism.
Other enzymes and their active sites, however, change shape when binding substrate
molecules. This mechanism of substrate binding is called ‘induced fit.’ In induced fit
interactions, the change in the shape of the enzyme caused by substrate binding often
results in increased efficiency of the enzyme.
Slide 5
Before moving on to the next lesson, let’s take a moment to summarize
what we have learned about enzymes. First, enzymes are biological catalysts. Enzymes
act just as any other catalyst does – they increase the rate of a chemical reaction by
lowering the activation energy. Note here that only the activation energy and rate of
reaction are affected by enzyme activity. The overall change in free energy between
reactants and products is not affected by an enzyme (you may want to review slide 2 to
emphasize this concept). Since the change in free energy is not affected, the equilibrium
point of a reaction is also not affected by the presence of an enzyme.
Most often, enzymes are proteins. Their structure, particularly the structure of their active
site or sites, determines which type of molecule they can bind, and so which type of
reaction they can catalyze. Most enzymes are highly specific, and may only bind a
particular substrate.
In the following lesson, we will probe enzymes in a little more detail. For instance, how
do enzymes affect substrate molecules that bind to their active sites? What factors affect
how well enzymes work? And, do enzymes work by themselves, or in conjunction with
other molecules?