52
UNIT 1
Organization of the Body
2
digestive enzymes produced in the pancreas are activated in the
small intestine, where they actually do their work. If they were
produced in active form, the pancreas would digest itself.
Sometimes, enzymes are inactivated immediately aFer they
have performed their catalytic function. Tis is true of enzymes
that promote blood clot formation when the wall of a blood
vessel is damaged. Once clotting is triggered, those enzymes
are inactivated. Otherwise, you would have blood vessels full of
solid blood instead of one protective clot. (Eek!)
Enzyme Action
How do enzymes perform their catalytic
role? Every chemical reaction requires that a certain amount
of energy, called
activation energy
, be absorbed to prime the
reaction. Te activation energy is the amount of energy needed
to break the bonds of the reactants so that they can rearrange
themselves and become the product. It is present when kinetic
energy pushes the reactants to an energy level where their ran-
dom collisions are forceful enough to ensure interaction. Activa-
tion energy is needed regardless of whether the overall reaction
is ultimately energy absorbing or energy releasing.
One way to increase kinetic energy is to increase the temper-
ature, but higher temperatures denature proteins. (Tis is why a
high fever can be a serious event.) Enzymes allow reactions to
occur at normal body temperature by decreasing the amount of
activation energy required
(Figure 2.20)
.
Exactly how do enzymes accomplish this remarkable feat? Te
answer is not fully understood. However, we know that, due to
structural and electrostatic factors, they decrease the randomness
of reactions by binding to the reacting molecules temporarily and
presenting them to each other in the proper position for chemical
interaction (bond making or breaking) to occur.
Tree basic steps appear to be involved in enzyme action
(Figure 2.21)
.
1
Substrate(s) bind to the enzyme’s active site, temporarily
forming an enzyme-substrate complex.
Substrate binding
causes the active site to change shape so that the substrate
and the active site fit together precisely, and in an orienta-
tion that favors reaction. Whether the shape of the substrate
reactions. More specifically, enzymes can be thought of as chemi-
cal traffic cops that keep our metabolic pathways flowing. Enzymes
cannot force chemical reactions to occur between molecules that
would not otherwise react. Tey can only increase the speed of
reaction, and they do so by staggering amounts—from 100,000 to
over 1 billion times the rate of an uncatalyzed reaction. Without
enzymes, biochemical reactions proceed so slowly that for practi-
cal purposes they do not occur at all.
Characteristics of Enzymes
Some enzymes are purely protein.
In other cases, the functional enzyme consists of two parts, col-
lectively called a
holoenzyme
: an
apoenzyme
(the protein por-
tion) and a
cofactor
. Depending on the enzyme, the cofactor
may be an ion of a metal element such as copper or iron, or an
organic molecule needed to assist the reaction in some particu-
lar way. Most organic cofactors are derived from vitamins (es-
pecially the B complex vitamins). Tis type of cofactor is more
precisely called a
coenzyme
.
Each enzyme is chemically specific. Some enzymes control
only a single chemical reaction. Others exhibit a broader specif-
icity in that they can bind with molecules that differ slightly and
thus regulate a small group of related reactions. Te substance
on which an enzyme acts is called a
substrate
.
Te presence of specific enzymes determines not only which
reactions will be speeded up, but also which reactions will
occur—no enzyme, no reaction. Tis also means that unwanted
or unnecessary chemical reactions do not occur.
Most enzymes are named for the type of reaction they cata-
lyze.
Hydrolases
(hi
9
druh-lās-es) add water during hydrolysis
reactions,
oxidases
(ok
9
sĭ-dās-es) oxidize reactants by adding
oxygen or removing hydrogen, and so on. You can recognize
most enzyme names by the suffix
-ase
.
In many cases, enzymes are part of cellular membranes in a
bucket-brigade type of arrangement. Te product of one enzyme-
catalyzed reaction becomes the substrate of the neighboring en-
zyme, and so on. Some enzymes are produced in an inactive form
and must be activated in some way before they can function, of-
ten by a change in the pH of their surroundings. ±or example,
Energy
Activation
energy
required
Less activation
energy required
WITHOUT ENZYME
WITH ENZYME
Reactants
Product
Product
Reactants
Progress of reaction
Energy
Progress of reaction
Figure 2.20
Enzymes lower the activation energy required for a reaction.
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