50 MHR • Unit 1 Metabolic Processes
Table 2.3 shows vitamins necessary to the
formation of specific coenzymes.
Deficiencies in any of these vitamins can affect
the enzymatic reactions in cells. For example, lack
of niacin may result in a lack of NAD+(nicotinamide
adenine dinucleotide), which can affect enzymatic
reactions in cellular respiration. Niacin deficiency
can cause a skin disease called pellagra. At one time
this disease was often mistaken for leprosy, but in
the early 1900s American researcher Dr. Joseph
Goldberger determined that pellagra is caused by
a nutritional deficiency. To treat the disease, he
recommended a diet that included meat, milk, fish,
or a small portion of dried brewer’s yeast.Both coenzymes NAD+and FAD (flavin adenine
dinucleotide) serve as electron acceptors in redox
reactions. They carry electrons from one active site
to another. Once the electrons have been released,
the coenzymes return to the original enzyme for
another complement of electrons.
The NAD+coenzyme takes the energy from the
oxidation of nutritive molecules digested by
animals to form NADH, a molecule with more
chemical energy. NADH is then oxidized into
NAD+again in order to collect more electrons.
NAD+is the principal carrier of electrons in the
oxidation of molecules that are used as an energy
source in the cell. For example, NAD+accepts
electrons from the products of the breakdown of
glucose in one stage of cellular metabolism, and
then transports them to a metabolic pathway that
reduces oxygen to water. During such reactions,
NAD+accepts two electrons, but only one hydrogen
ion, as shown in the following equation:
NAD++H++2e−→NADH
When NADH is oxidized back into NAD+, energy
is released. Similar in function to NAD+, NADP+
(NAD+plus an additional phosphate group) is a
coenzyme in photosynthetic reactions.Enzymes and Coenzymes for
Human Health and Industry
Enzymes and coenzymes have proven useful in
medical and industrial applications. Medical
researchers have been conducting tests using NADH
on patients with Alzheimer’s disease or Chronic
Fatigue Syndrome (CFS). In a study conducted
in the 1990s at Georgetown University Medical
Center, CFS patients who received injections
of NADH experienced only one quarter of the
symptoms experienced by patients who were given
a placebo (a substance with no medical value).
At the end of the twentieth century, six out of
10 individuals who were taking NADH used it to
improve their energy level; two out of 10 used it
to control Alzheimer’s symptoms; and one out of
10 took it to relieve CFS.
Enzymes are also used in the process of DNA
fingerprinting, which you will learn more about in
Chapter 9. DNA fingerprinting has been used in a
variety of circumstances, including paternity tests,
murder trials, and identifying people. In one step
of the DNA fingerprinting process, special enzymes
called restriction enzymesare used to cut the
DNA at specific places. DNA restriction enzymes
recognize short, specific sequences of DNA bases
and make breaks in the sugar–phosphate backbone
of the DNA molecule in the region of the recognized
sequence. Without these enzymes, the process of
DNA fingerprinting would be much more involved.
DNA fingerprinting also uses a process called PCR,
polymerase chain reaction, which you will learn
about in Chapter 9.Deposits of the protein fragment called beta-amyloid are
always found in large amounts in the brains of Alzheimer’s
patients. Many scientists believe it is these protein deposits
that damage brain cells, thereby producing memory loss
and other symptoms of the disease. The beta-secretase
enzyme is responsible for creating these deposits of beta-
amyloid protein. This is accomplished by breaking a larger
molecule called amyloid precursor protein (APP). Another
enzyme, called gamma-secretase, also contributes to
breaking APP, but beta-secretase is the first to act on APP.
Scientists believe that drugs created to inhibit the action of
beta-secretase will reduce the number of beta-amyloid
deposits in the brains of Alzheimer’s patients.BIO FACT
Vitamin Coenzyme
niacin
(riboflavin)
(thiamine)
pantothenic acid
(cobalamin)(nicotinamide adenine dinucleotide)
FAD (flavin adenine dinucleotide)
thiamine pyrophosphate
Coenzyme A (CoA)
coenzymesTable 2.3
Coenzymes associated with some common vitamins