26.2. Amino Acids and Proteins http://www.ck12.org
FIGURE 26.12
Hemoglobin is a very large protein con-
sisting of four subunits. Two of the sub-
units are identical to one another and are
called the alpha subunits (yellow). The
other two subunits are also identical to
one another and are called the beta sub-
units (gray). Hemoglobin also contains an
atom of iron in the middle of each subunit.
Each iron atom is positioned in the center
of a smaller organic molecule known as a
porphyrin, which is shown in red.the four levels of protein structure.
Enzymes
Anenzymeis a protein that acts as a biological catalyst. Recall that a catalyst is a substance that increases the rate
of a chemical reaction without itself being consumed in the reaction. Cellular processes consist of many chemical
reactions that must occur quickly in order for the cell to function properly. Each of those biochemical reactions
is catalyzed by an enzyme. The first enzyme to be isolated was discovered in 1926 by American chemist James
Sumner. The enzyme was urease, which catalyzes the hydrolytic decomposition of urea, a component of urine, into
ammonia and carbon dioxide.
H 2 NCONH 2 (aq)+H 2 O(l)
urease
→ 2NH 3 (g)+CO 2 (g)Since that time, thousands of enzymes have been isolated and had their structures determined.
Enzymes catalyze most of the chemical reactions that occur in a cell. Asubstrateis the molecule or molecules on
which the enzyme acts. In the urease catalyzed reaction above, urea is the substrate. Below (Figure26.14) is a
diagram for a typical enzymatic reaction.
The first step in the reaction is that the substrate binds to a specific part of the enzyme molecule. The binding of
the substrate is dictated by the shapes of each molecule. Side chains on the enzyme interact with the substrate in a
specific way, weakening certain bonds and locking the molecule into a particular orientation. Theactive siteis the
region of an enzyme to which the substrate binds. Enzymes typically have just one active site, which is usually a
pocket or crevice formed by the folding pattern of the protein. Because the active site of an enzyme has such a unique
shape, only one particular substrate is capable of binding to that enzyme. In other words, each enzyme catalyzes
only one chemical reaction with only one substrate. Once the enzyme/substrate complex is formed, the reaction
occurs, and the substrate is transformed into products. Finally, the product molecule or molecules are released from
the active site. Note that the enzyme is left unaffected by the reaction and is now capable of catalyzing the reaction
of another substrate molecule.