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NAME: AFON, AGNES ADEOLA
MATRIC NO: 14/SCI03/002
COURSE CODE: BCH 301
DEPARTMENT: BIOCHEMISTRY
Amino acid synthesis is the set of biochemical processes (metabolic pathways) by which the various
amino acids are produced from other compounds. The substrates for these processes are various
compounds in the organism's diet or growth media. Not all organisms are able to synthesise all amino
acids. Humans are excellent example of this, since humans can only synthesise 11 of the 20 standard
amino acids (a.k.a. non-essential amino acid), and in time of accelerated growth, histidine, can be
considered an essential amino acid.
A fundamental problem for biological systems is to obtain nitrogen in an easily usable form. This
problem is solved by certain microorganisms capable of reducing the inert N≡N molecule (nitrogen gas)
to two molecules of ammonia in one of the most remarkable reactions in biochemistry. Ammonia is the
source of nitrogen for all the amino acids. The carbon backbones come from the glycolytic pathway, the
pentose phosphate pathway, or the citric acid cycle.
In amino acid production, one encounters an important problem in biosynthesis, namely
stereochemical control. Because all amino acids except glycine are chiral, biosynthetic pathways must
generate the correct isomer with high fidelity. In each of the 19 pathways for the generation of chiral
amino acids, the stereochemistry at the α-carbon atom is established by a transamination reaction that
involves pyridoxal phosphate. Almost all the transaminases that catalyze these reactions descend from a
common ancestor, illustrating once again that effective solutions to biochemical problems are retained
throughout evolution.
Biosynthetic pathways are often highly regulated such that building-blocks are synthesized only
when supplies are low. Very often, a high concentration of the final product of a pathway inhibits the
activity of enzymes that function early in the pathway. Often present are allosteric enzymes capable of
sensing and responding to concentrations of regulatory species. These enzymes are similar in functional
properties to aspartate transcarbamoylase and its regulators. Feedback and allosteric mechanisms
ensure that all twenty amino acids are maintained in sufficient amounts for protein synthesis and other
processes.
We turn now to the biosynthesis of essential amino acids. These amino acids are synthesized by
plants and microorganisms, and those in the human diet are ultimately derived primarily from plants.
The essential amino acids are formed by much more complex routes than are the nonessential amino
acids. The pathways for the synthesis of aromatic amino acids in bacteria have been selected for
discussion here because they are well understood and exemplify recurring mechanistic motifs.
Phenylalanine, tyrosine, and tryptophan are synthesized by a common pathway in E. coli (Figure
24.16). The initial step is the condensation of phosphoenolpyruvate (a glycolytic intermediate) with
erythrose 4-phosphate (a pentose phosphate pathway intermediate). The resulting seven-carbon openchain sugar is oxidized, loses its phosphoryl group, and cyclizes to 3-dehydroquinate. Dehydration then
yields 3-dehydroshikimate, which is reduced by NADPH to shikimate. Phosphorylation of shikimate by
ATP gives shikimate 3-phosphate, which condenses with a second molecule of phosphoenolpyruvate.
This 5-enolpyruvyl intermediate loses its phosphoryl group, yielding chorismate, the common precursor
of all three aromatic amino acids. The importance of this pathway is revealed by the effectiveness of
glyphosate (Roundup), a broad-spectrum herbicide. This compound inhibits the enzyme that produces
5-enolpyruvylshikimate 3-phosphate and, hence, blocks aromatic amino acid biosynthesis in plants.
Because animals lack this enzyme, the herbicide is fairly nontoxic.