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184 Metabolism

Amino acid biosynthesis have to be supplied in food. For example,
animal metabolism is no longer capable of
carrying out de-novo synthesis of the aro-
A. Symbiotic nitrogen fixation
matic amino acids (tyrosine is only non-es-
Practically unlimited quantities of elementary sential because it can be formed from phenyl-
nitrogen (N 2 ) are present in the atmosphere. alanine when there is an adequate supply
However, before it can enter the natural nitro- available). The branched-chain amino acids
gen cycle, it has to be reduced to NH 3 and (valine, leucine, isoleucine, and threonine) as
incorporated into amino acids (“fixed”). Only well as methionine and lysine,also belongto
a few species of bacteria and bluegreen algae the essential amino acids. Histidine and argi-
are capable of fixing atmospheric nitrogen. nine are essential in rats; whether the same
These exist freely in the soil, or in symbiosis applies in humans is still a matter of debate. A
with plants. The symbiosis between bacteria supply of these amino acids in food appears to
of the genus Rhizobium and legumes be essential at least during growth.
(Fabales)—such as clover, beans, and peas—is The nutritional value of proteins (see
of particular economic importance. These p. 360) is decisively dependent on their es-
plants are high in protein and are therefore sential amino acid content. Vegetable pro-
nutritionally valuable. teins—e. g., those from cereals—are low in ly-
In symbiosis with Fabales,bacteria liveas sine and methionine, while animal proteins
bacteroids in root nodules inside the plant contain all the amino acids in balanced pro-
cells. The plant supplies the bacteroids with portions. As mentioned earlier, however,
nutrients, but it also benefits from the fixed there are also plants that provide high-value
nitrogen that the symbionts make available. protein. These include the soy bean, one of the
The N 2 -fixing enzyme used by the bacteria plants that is supplied with NH 3 by symbiotic
is nitrogenase. It consists of two components: N 2 fixers (A).
an Fe protein that contains an [Fe 4 S 4 ]cluster Non-essential amino acids are those that
as a redox system (see p. 106), accepts elec- arise by transamination from 2-oxoacids in
trons from ferredoxin, and donates them to the intermediary metabolism. These belong
the second component, the Fe–Mo protein. to the glutamate family (Glu, Gln, Pro, Arg,
This molybdenum-containing protein trans- derived from 2-oxoglutarate), the aspartate
fers the electrons to N 2 and thus, via various family (only Asp and Asn in this group, de-
intermediate steps, produces ammonia (NH 3 ). rived from oxaloacetate), and alanine,which
Some of the reducing equivalents are trans- can be formed by transamination from pyru-
+
ferred in a side-reaction to H . In addition to vate. The amino acids in the serine family (Ser,
NH 3 , hydrogen is therefore always produced Gly, Cys) and histidine,which arise from in-
as well. termediates of glycolysis, can also be synthe-
sized by the human body.

B. Amino acid biosynthesis: overview
The proteinogenic amino acids (see p. 60) can
be divided into five families in relation to
their biosynthesis. The members of each fam-
ily are derived from common precursors,
which are all produced in the tricarboxylic
acid cycle or in catabolic carbohydrate metab-
olism. An overview of the biosynthetic path-
ways is shown here; further details are given
on pp. 412 and 413.
Plants and microorganisms are able to syn-
thesize all of the amino acids from scratch, but
during the course of evolution, mammals
have lost the ability to synthesize approxi-
mately half of the 20 proteinogenic amino
acids. These essential amino acids therefore

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