117
CHAPTER FOUR
Cellular Metabolism
into water and oxygen, an action that helps prevent accumu-
lation of hydrogen peroxide, which damages cells.
The action of the enzyme catalase is obvious when using hydrogen
peroxide to cleanse a wound. Injured cells release catalase, and when
hydrogen peroxide contacts them, bubbles of oxygen are set free. The
resulting foam removes debris from inaccessible parts of the wound.
Each enzyme must be able to “recognize” its specific
substrate. This ability to identify a substrate depends upon
the shape of an enzyme molecule. That is, each enzyme’s
polypeptide chain twists and coils into a unique three-
dimensional conformation that F
ts the particular shape of its
substrate molecule.
RECONNECT
To Chapter 2, Proteins, page 65.
During an enzyme-catalyzed reaction, regions of the
enzyme molecule called
active sites
temporarily combine with
portions of the substrate, forming an enzyme-substrate com-
plex. This interaction strains chemical bonds in the substrate
in a way that makes a particular chemical reaction more likely
to occur. When it does, the enzyme is released in its original
form, able to bind another substrate molecule
(f g. 4.4)
. Many
enzyme-catalyzed reactions are reversible and in some cases
the same enzyme catalyzes both directions.
Enzyme catalysis can be summarized as follows:
Enzyme-
Substrate+Enzyme
substrate
Product+Enzyme
complex
(unchanged)
The speed of an enzyme-catalyzed reaction depends
partly on the number of enzyme and substrate molecules
in the cell. The reaction occurs more rapidly if the concen-
tration of the enzyme or the concentration of the substrate
increases. The efF
ciency of different types of enzymes var-
ies greatly. Some enzymes can process only a few substrate
molecules per second, whereas others can handle as many
as hundreds of thousands.
Cellular metabolism includes hundreds of different
chemical reactions, each controlled by a specific type of
PRACTICE
1
What are the general functions of anabolism and catabolism?
2
What type of molecule is formed by the anabolism of
monosaccharides? Of glycerol and fatty acids? Of amino acids?
3
Distinguish between dehydration synthesis and hydrolysis.
4.3
CONTROL OF METABOLIC
REACTIONS
Different types of cells may conduct specialized metabolic
processes, but all cells perform certain basic reactions, such
as the buildup and breakdown of carbohydrates, lipids, pro-
teins, and nucleic acids. These common reactions include
hundreds of very speciF
c chemical changes that must occur
in particular sequences. Enzymes control the rates of these
metabolic reactions.
Enzyme Action
Like other chemical reactions, metabolic reactions require
energy
(activation energy)
before they proceed. This is
why in laboratory experiments heat is used to increase the
rates of chemical reactions. Heat energy increases the rate
at which molecules move and the frequency of molecular
collisions. These collisions increase the likelihood of inter-
actions among the electrons of the molecules that can form
new chemical bonds. The temperature conditions in cells are
usually too mild to adequately promote the reactions of life.
Enzymes make these reactions possible.
Most enzymes are globular proteins that catalyze spe-
ciF c chemical reactions in cells by lowering the activation
energy required to start these reactions. Enzymes can speed
metabolic reactions by a factor of a million or more.
Enzymes are required in small amounts, because as they
work, they are not consumed and can, therefore, function
repeatedly. Each enzyme is speciF c, acting only on a par-
ticular molecule, called its
substrate
(sub
stra
ˉt). ±or exam-
ple, the substrate of an enzyme called catalase (found in the
peroxisomes of liver and kidney cells) is hydrogen perox-
ide, a toxic by-product of certain metabolic reactions. This
enzyme’s only function is to decompose hydrogen peroxide
FIGURE 4.3
Peptide bonds link amino acids.
When dehydration synthesis unites two amino acid molecules, a peptide bond forms between a
carbon atom and a nitrogen atom, resulting in a dipeptide molecule (arrows to the right). In the reverse reaction, hydrolysis, a dipeptide molecule is
broken down into two amino acids (arrows to the left).
Amino acid
N
H
H
CC
H
R
Dipeptide molecule
+
+
Peptide
bond
Amino acid
N
H
H
O
CC
H
H
H
R
H
O
N
H
H
O
CC
H
R
H
O
N
H
CC O
H
R
H
O
O
N
H
H
CC
H
R
N
H
CC O
H
R
H
O
O
H
2
O
Water
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