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Amino Acid Metabolism 2:
Amino acid biosynthesis, amino acids are
metabolites, metabolic genetic disorders
Bioc 460 Spring 2008 - Lecture 39 (Miesfeld)
Roundup Ready soybeans
are resistant to glyphosate
and are sold by Monsanto
Albinism is caused by
mutations in the tyrosinase
gene which is required for
pigment biosynthesis
It was thought at one time
that Count Dracula may have
suffered from porphyria
Key Concepts in Amino Acid Metabolism
•
Plants and bacteria synthesize all twenty amino acids, whereas, most other
organisms obtain at least some of the required amino acids from their diet.
Humans can only synthesize about half of the twenty amino acids.
•
In general, the more complex amino acids are essential amino acids in humans
as they require enzymes that have been lost from the human genome over
evolutionary time. Phenylalanine is an essential amino acid, but it is also the
metabolic precursor to tyrosine through the action of phenylalanine hydroxylase.
•
The herbicide glyphosate (RoundUp) is an inhibitor of the Shikimate Pathway
responsible for the synthesis of aromatic amino acids. RoundUp Ready
transgenic plants are resistant to the herbicide and are used in agriculture.
•
Numerous genetic diseases caused by defects in amino acid metabolic
pathways have been identified. These include phenylketonuria, albinism and
porphyrias. Genetic diseases can be due to recessive or dominant mutations.
Overview of Amino Acid Biosynthesis
The carbon skeletons of all twenty amino acids are derived from just
seven metabolic intermediates, that together, are found in three
metabolic pathways. These include:
1) three glycolytic pathway intermediates; 3-phosphoglycerate,
phosphoenolypyruvate, and pyruvate,
2) two pentose phosphate pathway intermediates; ribose 5phosphate and erythrose 4-phosphate
3) two citrate cycle intermediates; -ketoglutarate and
oxaloacetate.
Note that while plants and bacteria are capable of synthesizing all
twenty of the amino acids, most animals are much more restricted in
their ability to synthesize amino acids de novo because they lack many
of the required enzymes.
Overview of Amino
Acid Biosynthesis
Based on the principles of nutritional
biochemistry, it was determined that humans
require ten of the twenty amino acids in their
diet in order to thrive. These ten amino acids
are called essential amino acids, whereas,
the other ten amino acids which humans can
synthesize on their own, are called
nonessential amino acids.
Overview of Amino Acid Biosynthesis
Arginine is listed as an essential amino acid because humans
require arginine in their diet to support rapid growth during childhood
and pregnancy. However, arginine is actually generated from
argininosuccinate in the urea cycle, which means that a small
amount of this "essential" amino acid is made available for protein
synthesis through this route.
Tyrosine is also highlighted because this conditional nonessential
amino acid is made in humans from the essential amino acid
phenylalanine by the enzyme phenylalanine hydroxylase.
Therefore, as long as we have enough phenylalanine in our diets we
can generate tyrosine, although in fact, much of the tyrosine in our
bodies actually comes directly from dietary tyrosine.
Alanine and aspartate are
nonessential amino acids
because humans can make
them from pyruvate and
oxaloacetate, respectively, using
transamination reactions.
In contrast, essential amino
acids, such as tryptophan and
methionine, must be obtained
from the diet because humans
lack the enzymes necessary to
synthesize them de novo.
In general, the structures of the
essential amino acids are more
complex than the nonessential
amino acids which is reflected in
the number of enzymatic
reactions required for synthesis.
Since amino acid side groups have
different chemical properties, the
abundance of individual amino
acids in proteins is not uniform.
For example, lysine, alanine, valine,
isoleucine, glycine and glutamic
acid are the most common amino
acids in proteins, whereas, cysteine,
tyrosine, histidine and methionine
are relatively rare.
Because of these differences, it is
critical that metabolic flux through
various amino acid biosynthetic
pathways be tightly regulated by
feedback inhibition to provide the
required proportions of each amino
acid in response to cellular needs.
Overview of Amino Acid Biosynthesis
The biosynthesis of three nonessential amino acids (alanine, aspartate and
asparagine), and six essential amino acids (methionine, threonine, lysine,
isoleucine, valine and leucine) in E. coli involves two interconnected pathways
utilizing pyruvate and oxaloacetate. The metabolic intermediate -ketobutyrate
links the oxaloacetate and pyruvate pathways together because the carbon
skeleton of isoleucine is derived from both -ketobutyrate and pyruvate.
Aromatic amino acids are
synthesized in plants, fungi,
and bacteria by a the
shikimate pathway which
requires the formation of a C10
compound called chorismate
that is the precursor to the
three aromatic amino acids
tryptophan, tyrosine and
phenylalanine.
One of the most widely used
herbicides is glyphosate, the
active ingredient in
Roundup®. Glyphosate is a
competitive inhibitor of the
enzyme EPSP synthase
which is required to convert
shikimate 3-phosphate to
EPSP. Since animals do not
have the shikimate pathway
enzymes, Roundup is an
animal safe herbicide.
Do you think
glyphosate works
faster in the summer
or winter? Explain.
Roundup Ready Crops are Glyphosate-Resistant
Monsanto developed glyphosate-resistant crop plants so that farmers could
spray their transgenic crops with Roundup (also made by Monsanto) and kill
weeds that reduce crop yields without harming the crop plants. The first
glyphosate-resistant crop plant developed was a strain of soybeans marketed
as Roundup Ready® soybeans.
In order to protect its investment in the
Roundup Ready crop plants, Monsanto
sells seeds to the farmers that are
sterile so that the transgenic plants
cannot be propagated. The farmer
must buy seeds from Monsanto.
Moreover, with well over 80% of the
soybeans planted in the United States
being the Roundup Ready variety, the
company has a robust market for its
many Roundup herbicide products.
Amino Acids as Metabolic Precursors
The bulk of amino acids recovered from
protein turnover, or obtained from the
diet or de novo synthesis, are used to
support ongoing protein synthesis in
cells.
However, because of the nitrogen
content of amino acids (the -amino
group), they are also used as metabolic
precursors for numerous biomolecules,
including heme groups (hemoglobin
and cytochromes), nucleotide bases
(purines and pyrimidines) and a variety
of signaling molecules
(neurotransmitters, hormones, nitric
oxide).
For example, the prosthetic group of
hemoglobin, myoglobin and cytochromes
is heme, a porphyrin ring containing iron.
Amino Acids as Metabolic Precursors
Tyrosine is the precursor to several important molecules in metabolic
signaling and neurotransmission, including epinephrine and dopamine.
Tyrosine is oxidized by the enzyme tyrosine hydroxylase in a reaction
requiring the enzyme cofactor tetrahydrobiopterin to form
dihydroxyphenylalanine (L-DOPA), a metabolic precursor to dopamine.
Amino Acids as Metabolic Precursors
Tyrosine is also the precursor to
pigment molecules called
melanins that are produced from
dopaquinone.
The two primary melanins are
eumelanins, which are dark
pigments having a brown or black
color, and pheomelanins that
have red or yellow color.
The yellow color of pheomelanin
pigments comes from the sulfur in
cysteine that is combined with
dopaquinone.
Amino Acids as Metabolic Precursors
Melanocytes are cells that produce melanins, and depending on the
ratio of eumelanin and pheomelanin pigments, one can have either
dark hair or light hair depending in the distribution of melanin-filled
granules along the hair shaft.
Natural loss of hair color occurs as a result of aging when melanin
production in human melanocytes located near the base of hair
follicles shuts down and these defective cells are not replaced as
they normally are in younger individuals. Gray hair can be colored by
treating it with a mixture of hydrogen peroxide and an ammonia
based solution containing artificial pigments.
Inborn Errors of Metabolism: Genetic Disease
A genetic defect in the gene encoding phenylalanine hydroxylase is
responsible for the metabolic disease phenylketonuria (PKU).
Tyrosine is used not
only for protein
synthesis, but as
described above,
tyrosine is also the
precursor for
neurotransmitter
dopamine, as well as,
skin pigments
(melanins) and
epinephrine.
Disease symptoms in
untreated individuals with
PKU include severe mental
retardation, stunted growth
and dental problems. The
clinical symptoms of PKU
are caused by the
accumulation of
phenylalanine in the
blood that is 30-50 times
higher than normal.
This high level of
phenylalanine leads to the
production of
phenylalanine metabolites
such as phenylpyruvate,
phenylacetate and
phenyllactate, all of which
are associated with the
observed neurological and
developmental problems.
NutraSweet contains a phenylalanine derivative
Since the symptoms of PKU are
caused by excess phenylalanine and
its metabolites, and humans require
phenylalanine in their diets (it is an
essential amino acid), PKU
treatment involves careful monitoring
of phenylalanine intake to provide
just enough for protein synthesis
without causing phenylalanine
accumulation.
Phenylketonuriacs also have to be
careful to avoid processed foods and
beverages containing the food
additive aspartame (aspartylphenylalanine methyl ester).
PKU is an autosomal recessive genetic disease
The phenylalanine hydroxylase gene is located on chromosome 12 making
it an autosomal recessive genetic disease. An autosomal genetic
disease is one in which the mutation is located on one of the 22 autosomal
chromosomes (all chromosomes except the X or Y chromosome).
The probability that two PKU carriers
will have a child with the disease is 25%
based on simple Mendelian genetics.
Since the frequency of PKU carriers in
the general population is ~2% (~1 in
50), then the probability that a baby will
be born with PKU by random chance is
0.02 x 0.02 x 0.25 = 0.0001, or 1 in
10,000, which is close to the observed
frequency of 1 in 15,000.
What would the probability be of
having a PKU afflicted child if the
mother was normal (PP) and the
father was a carrier (Pp)?
Type 1 albinism is an autosomal recessive
genetic mutation in the tyrosinase gene
A deficiency in tyrosinase will result
in loss of hair and skin pigments
which explains the albino
phenotype.
Interestingly, individuals with
phenylketonuria can have light skin
and hair at birth because of low
levels of tyrosine.
However, phenylketonuriacs are
not albinos because they obtain
sufficient amounts of tyrosine in
their diets to support melanin
biosynthesis.
Why is PKU treatable, but albinism is not, even though both
are the result of genetic mutations in enzymes?
Congenital porphyrias effect heme biosynthesis
and can be the result of dominant mutations
Numerous metabolic disease
affecting heme biosynthesis
have been linked directly to
enzymes in the heme
biosynthetic pathway.
These diseases are
characterized by the
accumulation of heme
precursors in the blood and
liver and are collectively called
porphyrias because they
inhibit porphyrin ring
synthesis.
Congenital porphyrias effect heme biosynthesis
and can be the result of dominant mutations
Unlike recessive genetic diseases
in which both copies of the gene
(maternal and paternal) are
defective, dominant genetic
diseases cause symptoms when
only one of the gene copies is
mutated.
The inheritance pattern of a
dominant genetic disease is such
that each child has a 50% chance
of being afflicted if one parent
carries the mutation and the other
parent is normal (100% of the
children will get the disease if
both parents carry the mutation).
How might a mutant protein cause a dominant (gain of function) phenotype?
Congenital porphyrias effect heme biosynthesis
and can be the result of dominant mutations
It is thought that King George III of England may have suffered from
acute intermittent porphyria based on descriptions of his health that
were written at the time of the American Revolution.
Most notably, he suffered from
excruciating stomach pain and
episodes of delirium that may
have been due to arsenic
exposure, one of the
environmental factors known to
aggravate porphyria symptoms
in individuals carrying the
defective gene.
Congenital porphyrias effect heme biosynthesis
and can be the result of dominant mutations
A more rare form of porphyria is congenital erythropoietic porphyria that
results from recessive mutations in the gene encoding uroporphyrinogen III
cosynthase. This disease is characterized by the accumulation of
uroporphyrinogen metabolites that are excreted in the urine giving it a red color.
One of the metabolites is uroporphyrinogen I which builds up in the teeth
causing them to turn reddish brown and fluoresce under ultraviolet light.
These individuals need to avoid
excess exposure to sunlight
which can result in the formation of
painful blisters. The symptoms of
congenital erythropoietic porphyria,
combined with the common
medieval practice of drinking
animal blood as a treatment for
human ailments, led to the proposal
that rare occurrences of porphyria
might contribute to the legend of
the "vampires."
The Legend of the Vampires Lives on in the
Imaginative Minds of Biochemists Everywhere