Organic Chemistry

Section 25.1 Overall View of Metabolism 1035

Sir Frederick G. Hopkins

(1861–1947)was born in England.

His finding that one sample of a

protein supported life while another

did not led him to conclude that the

former contained a trace amount of a

substance essential to life. His

hypothesis later became known as the

“vitamin concept,” for which he

received a share of the 1929 Nobel

Prize in physiology or medicine. He

also originated the concept of

essential amino acids.

VITAMIN

Christiaan Eijkman (1858–1930) was a member

of a medical team that was sent to the East Indies

to study beriberi in 1886. At that time, all diseases were thought

to be caused by microorganisms. When the microorganism that

caused beriberi could not be found, the team left the East In-

dies. Eijkman stayed behind to become the director of a new

bacteriological laboratory. In 1896, he accidentally discovered

the cause of beriberi when he noticed that chickens used in the

laboratory had developed symptoms characteristic of the dis-

ease. He found that the symptoms had developed when a cook

had started feeding the chickens rice meant for hospital pa-

tients. The symptoms disappeared when a new cook resumed

feeding chicken feed to the chickens. Later it was recognized

that thiamine (vitamin ) is present in rice hulls but not in

polished rice. For this work, Eijkman shared the 1929 Nobel

Prize in physiology or medicine with Frederick Hopkins.

B 1

B 1

Christiaan Eijkman Sir Frederick Hopkins

All the water-soluble vitamins except vitamin C function as coenzymes. In spite of
its name, vitamin C is not actually a vitamin because it is required in fairly high
amounts and most mammals are able to synthesize it (Section 22.19). Humans and
guinea pigs cannot synthesize it, however, so it must be included in their diets. We
have seen that vitamin C and vitamin E are radical inhibitors and therefore are antiox-
idants. Vitamin C traps radicals formed in aqueous environments, whereas vitamin E
traps radicals formed in nonpolar environments (Section 9.8).
It is impossible to overdose on water-soluble vitamins because the body can readily
eliminate any excess. You can, however, overdose on water-insoluble vitamins be-
cause they are noteasily eliminated by the body and can accumulate in cell mem-
branes and other nonpolar components of the body. For example, excess vitamin D
causes calcification of soft tissues. The kidneys are particularly susceptible to calci-
fication, which eventually leads to kidney failure. Vitamin D is formed in the skin as
a result of a photochemical reaction caused by the ultraviolet rays from the sun
(Section 29.6).

25.1 Overall View of Metabolism

The reactions that living organisms carry out to obtain the energy they need and to
synthesize the compounds they require are collectively known as metabolism. The
process of metabolism can be divided into two parts:catabolismand anabolism.
Catabolic reactionsbreak down complex nutrient molecules to provide energy and
simple precursor molecules for synthesis. Anabolic reactionsrequire energy and result
in the synthesis of complex biomolecules from simpler precursor molecules.

catabolism:

anabolism:

It is important to remember that almost every reaction that occurs in a living system
is catalyzed by an enzyme. The enzyme holds the reactants and any necessary cofac-
tors in place, so that the reacting functional groups and catalytic groups are oriented in
a way that will cause a specific well-defined chemical reaction (Section 24.8). In some
cases, it may be helpful to see where a particular enzyme-catalyzed reaction discussed
in this chapter fits into the overall metabolic scheme, so we will start by taking an
overall view of metabolism.

simple molecules+ energy ¡complex molecules

complex molecules¡simple molecules+ energy