ture reaches the point that radically changes the shape of the active site, the bond
between the active site and the substrate is broken and makes the enzyme in-
effective. This is called thermal denaturation. Denatured enzymes lose their
specific three-dimensional shape, making them nonfunctional. For example, the
clear liquid portion of an egg turns to a white solid when the egg is heated.
The clear liquid is made up of proteins. Heating these proteins denatures them.Inhibitors
There are substrates that block active sites from bonding to a substrate. These sub-
stances are called inhibitors. There are two kinds of inhibitors: competitive and
noncompetitive. Acompetitive inhibitor is a substance that binds to the active site
of an enzyme, thus preventing the active site from binding with the substrate. For
example, sulfa drugs contain the chemical sulfanilamid. Sulfa drugs inhibit micro-
bial growth by fitting into the active site of an enzyme required in the conversion
of paraaminobenzoic acid (PABA) into the B vitamin folic acid. Folic acid is
needed for DNA synthesis in bacteria and thus prevents bacteria from growing. A
noncompetitive inhibitor binds to another site on the enzyme called the allosteric
site and in doing so alters the shape of the active site of the enzyme. The shape of
the active site no longer complements the corresponding site on the substrate and
therefore no binding occurs. Noncompetitive inhibitors do not bind to active sites.CARBOHYDRATE METABOLISM
Carbohydrates are the main energy source for metabolic reactions and glucose is
the most used carbohydrate in metabolism. Energy is produced by breaking
down (catabolized) glucose in a process called glycolysis, which takes place in
the cytoplasm of most cells. Glycolysis, also known as the Embden-Meyerhof
pathway, is the oxidation of glucose to pyruvic acid. In glycolysis, which
originated from the Greek word glykysmeaning “sweet” and lyseinmeaning
“loosen,” enzymes split a six-carbon sugar into two three-carbon sugars, which
are then oxidized. Oxidation releases energy and rearranges atoms to form two
molecules of pyruvic acid. It is during this process that NAD+is reduced to
NADH with a net production of two ATP molecules.
In the presence of O 2 (aerobic environment), pyruvic acid enters the bridging
pathway and becomes connected to acetyl CoA. It then enters the Krebs cycle,
which will result in the production of three NADH and, one FADH 2 molecules. In
the absence of O 2 (anaerobic environment), the NADH produced during glycoly-
sis is oxidized and an organic compound accepts the electrons. This process is
called fermentation. This pathway of fermentation results in fewer ATP molecules.(^92) CHAPTER 5 The Chemical Metabolism